Download Development of a Glass Resistive Plate Chamber for the Phase

Universiteit Gent
Faculteit Wetenschappen
Vakgroep Fysica en Sterrenkunde
Development of a Glass Resistive Plate
Chamber for the Phase-2 Upgrade of the
CMS Detector at the Large Hadron
Collider
Promotor:
Dr. Michael Tytgat
Author:
Dieter Loterman
Supervisor:
Alexis Fagot
June 13, 2014
Thesis submitted in partial fulfilment of the requirements for the degree of Master of Science in Physics
and Astronomy
ii
Acknowledgments
Through the past five years of my education in Physics and Astronomy at the university of Ghent my
interest has only grown. Especially when I came in contact with the field of particle physics in the
last three years. I would like to thank Professor Dr. Ryckbosch and Dr. Michael Tytgat for giving
me the opportunity to enlarge my knowledge and experience in particle physics by letting me go on a
traineeship at CERN and by letting me carrying out my thesis on this particular subject. I am very
grateful for my supervisor Alexis Fagot who immediately answered the questions I asked and for guiding
me through the whole process of the construction of the glass RPC. In particular I would also like to
thank my fellow students Céline, Véronique and Matthias, and everyone else for their support and for
making the many days I have spent at the INW more pleasant. My gratitude also goes to Simon for
letting me help in the construction of the RPCs for CMS and to Patrick,Phillipe and Christophe for
their help in technical matters. Finally I would like to thank Nicolas for the support when I was at
CERN doing the measurements of the CMS RPC.
iii
iv
Contents
1 Introduction
2 The
2.1
2.2
2.3
1
Compact Muon Solenoid Experiment
The Standard Model . . . . . . . . . . . . .
Interaction of particles with matter . . . . .
The Compact Muon Solenoid Experiment .
2.3.1 Collider experiments . . . . . . . . .
2.3.2 Goals . . . . . . . . . . . . . . . . .
2.3.3 Coordinate System . . . . . . . . . .
2.3.4 Description of the detector . . . . .
2.3.5 Upgrade of CMS . . . . . . . . . . .
3 Resistive plate chambers
3.1 Design . . . . . . . . . . . .
3.2 Operation mode . . . . . .
3.3 Resistive plate material . .
3.4 Gas Mixture . . . . . . . . .
3.5 Performance and efficiency .
3.6 Multi-gap RPC . . . . . . .
3.7 Readout electronics . . . . .
3.8 RPCs at CMS . . . . . . . .
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25
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36
4 Design and construction of a gRPC prototype
4.1 Glass cutting and gluing . . . . . . . . . . . . .
4.2 Resistive coating . . . . . . . . . . . . . . . . .
4.2.1 Properties . . . . . . . . . . . . . . . . .
4.2.2 Painting procedure . . . . . . . . . . . .
4.3 Resistivity measurement . . . . . . . . . . . . .
4.4 Gas gap construction . . . . . . . . . . . . . . .
4.5 High voltage connection . . . . . . . . . . . . .
4.6 Readout strips . . . . . . . . . . . . . . . . . .
4.7 Detector casing . . . . . . . . . . . . . . . . . .
4.8 Multi-gap gRPC . . . . . . . . . . . . . . . . .
4.9 Possible Improvements . . . . . . . . . . . . . .
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5 Characteristics of the prototype
39
5.1 Experimental setup . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39
5.2 Dark current test . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39
5.3 Detection efficiency test . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41
v
vi
CONTENTS
6 CMS RPC characterization
45
6.1 Experimental setup . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45
6.2 Scintillator characterization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46
6.3 RPC characterization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47
7 Conclusions
53
8 Nederlandse Samenvatting
55
A CMS RPC characterization
57
List of Figures
59
List of Tables
61
Bibliography
63
Chapter 1
Introduction
Particle physics is one of the branches in physics where new discoveries are being made, trying to
understand how nature works at small scales. In order to understand the nature of subatomic particles
various experiments have been conducted to confirm theoretical explanations of particle interactions.
During the latter half of the 20th century, the Standard Model of particle physics was developed by
physicists coming from several collaborations. The theory explains how elementary particles interact
through the electromagnetic,weak and strong force and how they build up matter. The recent discovery
of the Higgs boson confirmed the successful description of the Standard Model. Nevertheless the
Standard Model does not explain dark matter and dark energy and it does not incorporate gravity
completely. In order to further explore the world of particle physics, new experiments are being build
and conducted. Some of these experiments take place at the Large Hadron Collider at the European
Center for Nuclear Research (CERN) at Geneva. The LHC collides two beams of high energetic particles
from which many new particles are produced. The higher the energy of these colliding particles, the
higher the chance that new ”exotic” particles will be created. Among the experiments that take place
at the LHC is the Compact Muon Solenoid (CMS) experiment. Several institutes from over the world
contribute to this experiment by building the detector and analyzing the data that arises from the
particle collisions. In the next chapter the Standard Model will be explained together with a description
of the CMS experiment. One of the institutes involved in the CMS experiment is the University of
Ghent. Our group contributes in the analysis of top quark physics and supersymmetry (SUSY) and in
the production of resistive plate chambers (RPCs) as part of the muon system of the CMS detector.
The operation and implementation of resistive plate chambers in the CMS experiment will be described
in chapter 3. In order to make the collisions of the particles more energetic, the LHC is upgraded
through several phases. More energetic particle collisions means that a higher luminosity is achieved
and thus the detectors must be able to cope with higher incoming particle rates. The currently installed
RPCs cannot handle these high rates and start to lose their high efficiency. New types of detectors
are being developed to solve this problem. One of these detectors that is a good candidate is a glass
RPC. Within the CMS Ghent group I started developing and constructing a prototype glass RPC
in cooperation with A. Fagot as part of his PhD thesis and F. Van Assche as part of his bachelor
project. The construction of this prototype will be described in chapter 4. Once this prototype was fully
constructed, I did some tests to determine its characteristics which is explained in chapter 5. Next to
that, I contributed in the characterization of a CMS RPC at CERN. These preliminary measurements
were necessary towards the testing of a new type of electronics used for RPCs. These new type of
electronics will further improve the performance of RPCs.
1
2
CHAPTER 1. INTRODUCTION
Chapter 2
The Compact Muon Solenoid
Experiment
The Standard Model was developed in order to describe the interactions of the fundamental particles.
To validate the Standard Model, experiments have been set up among which the Compact Muon
Solenoid is one of them. One way to comprehend the properties of these particles is to collide them at
very high energy and detect the many new particles that are created in the collision.
2.1
The Standard Model
By the beginning of the 20th century, it was found to be that matter is composed of atoms. Further
investigation of the properties of atoms showed that these constituents are not indivisible. By the
mid-thirties, it was known that the atom consists out of protons, neutrons and electrons. A few decades
later, experiments showed that these particles are part of a larger family of particles. At that time, the
Standard Model was developed trying to explain the different interactions and properties of these newly
discovered particles. Even today the Standard Model is still used and adjusted to new discoveries.
Three out of four fundamental forces are described by this theory. These include the electromagnetic
interaction, the weak interaction and the strong interaction. The gravitational force is not yet fully
encapsulated into this theory. Since its strength is much weaker in comparison to the other fundamental
forces, it is often neglected in particle physics. The electromagnetic force is responsible for the binding
of electrons in atoms. The weak force arises in radioactive decay of some particles and the strong force
in the binding of the constituents inside the atomic nucleus. The main division between particles is the
difference between bosons and fermions. Fermions have a half-integer spin and obey the Pauli-exclusion
principle which states that no two fermions can have the same quantum state. Bosons in contrast have
an integer-spin and do not obey the Pauli-exclusion principle.
ˆ Bosons
The elementary bosons are force carriers of the electromagnetic (photon), weak (W and Z bosons)
and strong interaction (gluons). Each force is characterized by a ”charge”. The electromagnetic
force is characterized by the electric charge while the strong interaction by colour and the weak
interaction by weak charge. Photons and gluons are massless while the W and Z bosons are
massive. Aside from these bosons, the Higgs boson was recently discovered[1]. It is a massive
scalar particle and explains why elementary particles (except the photon and the gluons) are
massive.
ˆ Fermions
The elementary fermions are subdivided into quarks and leptons according to the interaction they
are subjected to and thus by the charge they carry. These particles are the constituents of matter.
3
4
CHAPTER 2. THE COMPACT MUON SOLENOID EXPERIMENT
– Quarks
Until now six quarks have been discovered (up, down, strange, charm, bottom (or beauty)
and top). They make up the hadrons which are made up of two quarks (the mesons) or
three quarks (the baryons) and carry colour and electric charge and are subjected to the
strong and electroweak interaction. The quarks are further subdivided into generations. For
each generation the mass of the particles increases. Particles of the first generation do not
decay into lighter particles as opposed of those of the second and third generation. The up
and down quarks belong to the first generation, charm and strange to the second and top
and bottom to the third generation. Protons and neutrons consist of up and down quarks.
The concept of colour charge which the quarks carry leads to ”confinement”. Three colours
(red,blue and green) exist together with its ”anti-color”. Only colourless states can exist
(for instance: green and anti-green or red,blue and green). This leads to the conclusion that
single quarks can not be free, i.e they are confined since they only carry ”one” colour.
– Leptons
The leptons consist of the electron, muon and tau particle together with the neutrinos.
The charged leptons carry electric charge and interact by the electroweak interaction. The
neutrinos have a very small mass, do not decay, interact only weakly and are therefore
difficult to detect. Up to now only upper limits of their mass have been determined. The
charged leptons that are heavier than the electron, decay into lighter particles. In any
reaction involving leptons or antileptons, the number of leptons of a certain family minus
the number of the corresponding antilepton is conserved[2]. This is referred as conservation
of lepton number. Table 2.1 and table 2.2 give an overview of the elementary particles
described above.
Particle type
leptons
quarks
Particle
e−
µ−
τ−
νe
νµ
ντ
u
d
c
s
t
b
electric charge
-1
-1
-1
0
0
0
2
3
- 13
2
3
- 13
2
3
- 13
spin
1
2
1
2
1
2
1
2
1
2
1
2
1
2
1
2
1
2
1
2
1
2
1
2
mass (eV/c2 )
511*103
105*106
1.777*109
< 2.2
< 0.19 * 106
< 18.2 * 106
2.3 * 106
4.8 * 106
1.275 *109
95 *106
173.5 * 109
4.18*109
Generation
1
2
3
1
2
3
1
1
2
2
3
3
Table 2.1: Fermions in the Standard Model
Particle type
gauge bosons
Particle
g
γ
Z
W±
H
electric charge
0
0
0
±1
0
spin
1
0
1
1
0
mass (eV/c2 )
0
0
91.2*109
80.4*109
126* 109
Table 2.2: Bosons in the Standard Model
For every particle, there exist also an anti-particle that has the same mass but the opposite charge. A
particle has the property to annihilate with its own anti-particle. Despite its successful description, the
2.2. INTERACTION OF PARTICLES WITH MATTER
5
Standard Model is far from complete. It does not give a full description of gravity nor of dark matter
and dark energy. Many theories are being developed in order to explain these deficiencies. One of them
is the so called supersymmetry theorie (SUSY) and predicts the existence of supersymmetric particles
i.e. the partners of the particles in the Standard Model. These supersymmetric particles are so heavy
due to symmetry breaking that the current particle colliders are not able to detect them.
2.2
Interaction of particles with matter
When particles pass through matter they interact with the atomic electrons and lead to ionization or
excitation of the atoms. Ionizations lead to the creation of free electrons and ions while excitation leads
to the emission of photons. In addition, electrons lose energy through bremstrahlung. In order to detect
particles it is important to know how they interact with matter. For relativistic charged particles more
massive than the electron, the mean energy loss per unit path length traversed or stopping power, can
be described by the Bethe-Bloch formula [2]:
ρZ z 2
dE
=K
−
dx
A β2
ln
2mc2 γ 2 β 2
I
−β
2
(2.1)
where K is a constant equal to 30.7 keV m2 kg−1 . The formula is only valid for a limited range of
the incoming particle energy. At the lower limit the particle’s velocity becomes comparable with the
speed of the atomic electrons while at the upper limit radiative effects become more important [3]. At
the lower limit, charge exchange between particle and absorber becomes more important, reducing
the charge of the particle and consequently reducing the stopping power. The particle will lose its
energy mainly by excitation of atomic and molecular levels. In the range of a few hundred MeV, the
energy loss shows a minimum for all types of particles. These relativistic particles are often referred as
”minimum ionizing particles”. As an example, the stopping power of muons in Copper is shown in
figure 2.1. The behaviour for electrons is different due to the fact that electrons not only lose energy
through ionization but also by bremstrahlung in the nuclear Coulomb field. At low momenta the
energy loss of the particles is mainly due to excitation and ionization of the atoms. Photons lose their
energy mainly through the photoelectric effect at low energies. For higher energies the Compton effect
becomes more important and at still higher energies, enough energy is available for pair creation of
electron-positron pairs. Hadrons interact not only through the electromagnetic interaction but also by
the strong interaction. This leads to the disappearance of the incoming particle and the production
of secondary hadrons (i.e. hadronization). Muons mainly lose their energy by ionization up to a
critical energy which is defined as the energy where energy loss by ionization and by radiation are
equal. Radiative losses include bremsstrahlung, pair production and photonuclear reactions. The mean
stopping power of muons can be described according to the following formula [4]:
h−
dE
i = a(E) + b(E)E
dx
(2.2)
a(E) is the stopping power attributed to electronic energy losses, while b(E) is the stopping power due
to radiative processes.
6
CHAPTER 2. THE COMPACT MUON SOLENOID EXPERIMENT
Figure 2.1: Energy loss of muons in Cu normalised to the mass density of the target
Not all particles can be easily detected since some particles have a very short lifetime. In that
case experiments detect the decay products and try to identify the particle from the properties of
the detected particles. Particles that liberate ions and electrons in the process of ionization can be
detected by applying an electric field that collects the ions and electrons such as gas ionization chambers.
Radiative losses of particles can be detected by photosensitive detectors such as photomultiplier tubes
(PMTs).
2.3
2.3.1
The Compact Muon Solenoid Experiment
Collider experiments
To discover and investigate the properties of subatomic particles various experiments have been
developed. A collider experiment typically fires two beams of particles towards each other. The beam
consists of a bunch of hadrons or leptons. When the particles collide with enough energy many new
particles are created which decay into other particles. By reconstruction of the track of these particles
and by determination of their properties such as energy and momentum from which the mass can be
calculated, the particles they originated from can be identified. As opposed to an experiment with a
fixed target, colliding beams have several advantages. The main advantage is that the energy available
to create new particles is higher. However, colliding two bunches of particles requires very strong
focusing due to the tiny dimensions of the bunches. The most powerfull particle collider is the Large
Hadron Collider (LHC) at CERN (Conseil Européen pour la Recherche Nucléaire). It is a circular
collider and has a circumference of 27 km generating proton-proton collisions (although it can operate
with heavy lead ions). The Compact Muon Solenoid (CMS) experiment is one of the experiments that
takes place at the LHC.
2.3. THE COMPACT MUON SOLENOID EXPERIMENT
2.3.2
7
Goals
The LHC is currently being upgraded to reach a beam energy of 7 TeV and a design luminosity of 1034
cm−2 s−1 . With this upgrade it is the main goal of CMS and other collaborations to explore physics at
the TeV scale. Recently there was the discovery of the Higgs boson which was a huge breakthrough
in physics, however it is not yet clear why the Higgs boson should be light [ 5]. Supersymmetry could
explain the mass of the Higgs Boson by incorporating new supersymmetic particles. The contribution
of the masses of these particles to the Higgs mass would cancel out the contributions of the Standard
Model particles, making a light Higgs boson possible [6]. Supersymmetry would also explain the
difference between fermions and bosons and provide good candidates for dark matter. Other searches
of the LHC, include the search for new massive vector bosons and extra dimensions.
The CMS detector has four main requirements: bunch crossing identification, muon identification,
momentum measurement and triggering which will help to reconstruct the high energy proton-proton
collisions. Apart from the other particles that are detected by the CMS detector, muons appear in the
decay products of a lot of interesting particles such as the Higgs boson. Therefore it is important to
achieve a good muon identification and momentum measurement. Since many new particles are created
during each collision, triggering is important to assign each particle to the correct bunch crossing to
reconstruct the events.
2.3.3
Coordinate System
The coordinate system used by CMS has the origin centered at the collision point. The y-axis points
vertically upward, the x-axis points radially inward towards the center of the collider while the z-axis
points along the beam direction. The azimuthal angle φ lays in the x-y plane and is measured from the
x-axis and the polar angle θ is measured from the z-axis. The pseudorapidity η is related to the polar
angle and is defined as η=-ln(tan(θ/2)).
2.3.4
Description of the detector
The CMS detector is composed of various detectors brought together in a huge cylinder or barrel enclosed
by discs called endcaps. The axis of the cylinder lies according to the beam line. A superconducting
solenoid is present in the detector system to bend the charged particles. The higher the momentum of
a charged particle, the less its path is curved. Knowing the path of the particle, the momentum of the
particle can be calculated. Considering the huge strength and size of the magnet allowed some of the
detectors to be placed inside the magnet making it very compact as opposed to detectors of similar
weight.
ˆ Tracker
Starting from the inner part and moving radially outward, the first detector is a silicon tracker
which is used to reconstruct the tracks of high-energy muons, electrons, hadrons and even particles
which decay very quickly. In order to successfully construct the tracks, a high granularity design
is required. Furthermore the tracker determines the momentum of the particles as they move
through a magnetic field generated by a superconducting solenoid as indicated on figure 2.2. Since
the tracker is closest to the interaction point, the rate of incoming particles is high. The above
requirements lead to the choice of using silicon technology. Particles traveling through the tracker
initiate tiny electrical signals which are amplified and recorded.
8
CHAPTER 2. THE COMPACT MUON SOLENOID EXPERIMENT
Figure 2.2: A slice of the CMS detector[7]
ˆ Electromagnetic calorimeter
The next layer on top of the silicon tracker is an electromagnetic calorimeter, it measures
the energies deposited by photons and electrons. The electrons mainly lose their energy via
bremsstrahlung (photons) while the main energy loss mechanism for the photons is pair production
resulting in a positron and an electron. Thus both processes create even more electrons, positrons
and photons denoted as an electromagnetic shower. The calorimeter is made of lead tungstate
(PbWO4 ) crystals which scintillate (i.e. produces light) when electrons and photons pass through
it due to their energy loss. This light is recorded by silicon avalanche diodes in the barrel region
and vacuum phototriodes in the endcap region. These detectors have a fast response time and can
withstand a large amount of radiation. The lead tungstate crytals have a high density and short
radiation length. This makes a compact design and a high granularity of the electromagnetic
calorimeter possible. Between the tracker and the electromagnetic calorimeter, the preshower
detector identifies neutral pions and helps to locate the positions of the electrons and photons.
ˆ Hadronic Calorimeter
On top of the electromagnetic calorimeter is the hadronic calorimeter, it measures the energy of
hadrons passing through and consists of brass or steel (used as absorbing material) combined
with slices of scintillator. The scintillator light is wavelength-shifted and read out by hybrid
photodiodes. Due to the limited space available, an outer hadronic calorimeter is placed beyond
the magnet. At high η, a hadronic calorimeter endcap and a forward calorimeter are located.
ˆ Magnet
The hadronic calorimeter is surrounded by a superconducting solenoid generating a magnetic
field of 2T outside the magnet which is used to bend the trajectories of the particles. Particles
with high momentum have a path that is more difficult to bend and this requires a high magnetic
field. The flux of the magnetic field is returned in the iron yoke, located in the next segment.
ˆ Muon detectors
Next to the magnet are the muon detectors. the drift tubes (DT), cathode strip chambers (CSC)
and the resistive plate chambers (RPC). The muon system is divided into a central part (Barrel
detector, |η|<1.2) and forward parts (Endcap detector, 1.2<|η|<2.4). The muon detectors are
2.3. THE COMPACT MUON SOLENOID EXPERIMENT
9
used as muon identifiers, muon triggers and muon momentum measurement. The iron yoke stops
almost all particles except for the highly penetrating muons and neutrinos. The drift tubes are
used for trajectory measurements in the barrel region where the neutron induced background,
the muon rate and the residual magnetic field are low, while the cathode strip chambers for
the endcap region where the opposite is true. The resistive plate chambers have an excellent
time resolution and are installed in both the barrel and endcap region. Due to their good time
resolution they are used as triggers. They have however a worse position resolution than the
CSCs and DTs. The combined information of these three different detectors is used to reconstruct
the tracks and timing information of the muons.
– Each tube of a drift tube contains gas that ionizes when a muon passes through. The
electrons that are knocked free follow the electric field and hit a wire. The position of the
muon can be determined by calculating the drift time to the wire and by registering the
position of the electrons on the wire.
– The cathode strip chamber is a multiwire proportional chamber and consist of anode wires
crossed with copper cathode strips within a gas volume. When a muon enters electrons are
liberated from the gas inside that drift in the direction of the wires creating an avalanche.
The positive ions move in the opposite direction towards the copper cathode inducing a
charge on the strips. Since the strips and wires are perpendicular, two position coordinates
can be extracted.
– Resistive plate chambers consists of two parallel plates (one is the anode, the other the
cathode) both made of a high resistive material. Between the plates there is a gas mixture
and a high voltage is applied between the two plates. When muons pass through the gas
they create an electron avalanche which causes a discharge that is registered by electrodes.
This will be explained more in detail in section 3.1.
In figure 2.3 one quarter of the CMS detector is shown. The different detectors are placed into chambers
and are grouped according to a fixed value of the coordinate R or to a fixed value of the coordinate
z. The barrel stations are labeled with MB which stands for Muon Barrel (η≤1.2) while the endcap
the stations are labeled with ME, Muon Endcap. Likewise, the RPC stations are labeled RB for the
barrel region and RE for the endcap region. In the Muon Barrel there are 5 wheels each containing 4
stations. In the endcap the stations are divided into 3 discs perpendicular to the beam direction with
an additional disc that is installed during the first upgrade of the CMS detector. The drift tubes are
indicated in orange, the cathode strip chambers in green and the resistive plate chambers in blue.
With a design luminosity of 1034 cm−2 s−1 , 109 interactions per second occur [8] with a bunch
crossing of 25 ns. The time resolution of the detectors should be adequate enough to prevent confusion
between different bunch crossing, the so called pile-up events. Most of the events are uninteresting and
are considered as background. This large number is reduced by a factor of 107 to 100 Hz which is the
maximum rate that can be processed by the computer system. The Level-1 (L1) trigger reduces the
rate below 100 kHz by eliminating muons with low transverse momentum pT . Further reduction of the
rate is done by the High-Level-Trigger (HLT) to 100 Hz. The L1 trigger collects information of the
calorimeters and the muon system only and is partly programmed into the electronics of the detectors
placed on the trigger boards and partly in a control room. Since the bunch crossings happen every 25
ns, the information must be delivered fast enough which is not possible with the tracker system. Once
the events pass through the L1 trigger, the high level trigger uses all data with better resolution and is
software implemented. Events that surpass both triggers are stored and analyzed offline.
2.3.5
Upgrade of CMS
The LHC is currently shut down (Phase 1, Long Shutdown 1: 2013-2015) to upgrade the LHC to
reach its design luminosity of 1034 cm−2 s−1 . All the machinery is upgraded to cope with this increased
beam energy of 7 TeV and luminosity. Before the first shut down, the fourth endcap disk was not fully
completed. CSCs were only installed in a limited region (1.8<η<2.4) and RPCs were not present in the
fourth disk[9]. Currently more CSCs are installed in the region below |η|=1.8 and an additional fourth
layer of RPCs has been installed in the endcap region up to |η|=1.6. The reason for an additional
10
CHAPTER 2. THE COMPACT MUON SOLENOID EXPERIMENT
Figure 2.3: Layout of the CMS detector
fourth station is to be able to control the increased trigger rate at higher luminosity. The muons with
low transverse pT momentum that are abundantly present increase in number with higher luminosity.
An additional station should decrease incorrect measurements of muons with low pT , thus more low pT
muons will be eliminated decreasing the trigger rate. This will enable a 3-out-of-4 trigger logic in the
endcap region. Figure 2.4 shows the simulated RPC L1 Trigger performance for three and four RPC
stations in the endcap.
Towards the second Long Shutdown (Phase1, LS 2: 2017-2018), the coverage in the region 1.6
<|η|<2.1 will be further extended. Gas Electron Multipliers (GEMs) seem to be a good candidate to
sustain the high radiation environment in this region. This detector is a parallel plate chamber which
contains a metal clad polymer foil pierced with holes[10]. It contains a gas that is ionized by incoming
charged particles. An avalanche created by ionization is amplified when it passes through the holes.
The charge created by this avalanche serves as the basic signal of the GEM. GEMs have a good rate
capability and spatial resolution. The intent is to install them during LS 2 and the Phase 2 upgrade in
the ME1/1 station.
The goal of the second upgrade (Phase 2, Long Shutdown 3: start around 2020) is to reach a center
of mass energy of 14 TeV with an increased luminosity of 7*1034 cm−2 s−1 . Accordingly the name
referred will be ” High Luminosity Large Hadron Collider”. In the region 1.6 <|η|<2.1 of the fourth
station, new RPCs with improved technology need to be installed to cope with the high particle rates.
The implementation of RPCs towards the upgrades of the LHC will be discussed more in detail in
section 3.8. The upgrades will open a new range of physics to be studied. The drawback is that a higher
luminosity means a higher rate of particles hitting the detectors and thus a higher background rate
that needs to be eliminated which requires a more efficient trigger system. A higher rate of incoming
particles also means that aging effects of the detectors will play an important role.
2.3. THE COMPACT MUON SOLENOID EXPERIMENT
Figure 2.4: Simulated RPC trigger efficiency for three and four RPC stations[11]
11
12
CHAPTER 2. THE COMPACT MUON SOLENOID EXPERIMENT
Chapter 3
Resistive plate chambers
Resistive plate chambers (RPC) were developed by R. Santonico and R. Cardarelli in 1981[12]. Since
then a lot of research has been done to improve the properties of RPCs. These detectors are known
through the world of experimental particle physics for their high efficiency, good time resolution and
low cost.
3.1
Design
RPCs are gaseous detectors and consist out of two parallel plates of high resistivity. A high voltage is
applied between the two plates making one plate the anode and the other the cathode. This can be
done by applying either a high voltage to both plates or by grounding one plate and applying a high
voltage to the other plate. The gap between the plates is typically of the order of a mm and is filled
with a gas mixture. To ensure a uniform gap thickness and thus a uniform electric field, spacers are
placed between the plates. On top of the plates a coating is applied in order to be able to apply the
high voltage electric field. The coating is covered with an insulator with on top of it the readout strips.
Figure 3.1 gives a schematic view of an RPC.
Figure 3.1: Schematic view of an RPC. Indicated in green are the resistive plates together with the resistive
coating (purple). The red circle represents a spacer. The arrow shows an incoming charged particle
that initiates an avalanche (yellow). The resistive plates are shielded with an insulator (orange)
with the copper strips above (pink).
13
14
3.2
CHAPTER 3. RESISTIVE PLATE CHAMBERS
Operation mode
An RPC has a similar operation as that of a gas ionization chamber: when a charged particles passes
through the gas between the two plates it ionizes the atoms of the gas. The electrons and ions that are
liberated are accelerated by the applied electric field and collide with the gas molecules. The voltage
difference must be high enough to avoid recombination of positive ions and electrons since this effect
would give a false indication of the rate of ion formation and charge collection [13]. The charge that is
induced by the electrons on the electrodes is the useful signal of a RPC and is a rather small signal so
that amplifiers are needed. This signal is read out by the readout strips usually made of copper. An
insulation sheet is put between them to avoid electrical contact between the electrode and the readout
strips to ensure that the signal is again an induced one so that spatial resolution can be maintained. If
a single sheet would be put in electrical contact with the electrodes, the charge would spread across
the sheet and no spatial information could be acquired. The electrons have a drift speed that is larger
than that of the ions. Usually only the signal of the fast electrons is used. This requires that the time
constant τ = RC of the measuring circuit is larger than the rise time of the electronic signal induced by
the electrons but smaller than that of the ions. The advantage is that this way of measuring is efficient
for fast timing of the events. The downside is that the charge induced on the anode is dependent on
the position of the original created electron by the ionization. If the voltage is increased further the
electrons will gain enough energy to ionize other molecules and this process repeats itself until the
electrons are collected at the anode, an electron avalanche is created. This mode of operation is called
”avalanche mode”. The total charge Q generated by this multiplication process is given by the following
equation:
Q = n0 eM
(3.1)
where n0 is the number of original ion pairs created and M is the multiplication factor. In this way a
Townsend avalanche is created. The Townsend equation gives the fractional increase in the number of
electrons per unit path length:
dn
= αn
(3.2)
dx
α is the first Townsend coefficient and increases with increasing electric field. From equation 3.2 it is
apparent that the electron density grows exponentially towards the anode.
n(x) = n0 exp(αx)
(3.3)
where x denotes the distance from the anode.
By assuming this exponential growth towards the anode and by expecting that the positions of
the primary clusters follow a Poisson distribution, it has been shown that the charge induced on the
read-out strips is found to be [14]:
q=
n
X
j
qe
nj0 Mj (eη(g−x0 ) − 1)
∆Vw
ηg
j=1
(3.4)
where qe is the electron charge, g is the gap thickness, Mj is a factor that takes account of stochastic
fluctuations and η is the first effective Townsend coefficient which can be written as η=α-β where β
is the electron attachment coefficient. The electron attachment coefficient is given by the number of
ionizing encounters per unit length undergone by one electron. Vw is a weighting potential that is
equal to one for a single gap and equal to 12 for a double gap (i.e. an RPC with two gas gaps, see
further in section 3.6). The index j refers to the jth cluster created by an incoming ionizing particle.
The number nj0 no longer refers to the number of original ion pairs but refers only to the number of
liberated electrons in a cluster. The sum of this number over all clusters is equal to the number of
primary ionizations n0 as defined in equation 3.1. This takes into account that only the electrons form
the useful signal.
If the electric field is large enough, the electrons will gain enough energy to excite the neutral gas
molecules producing UV photons by deexcitation that will initiate secondary ionization. The electrons
liberated by this secondary ionization will on their turn ionize gas molecules and thus multiple
3.3. RESISTIVE PLATE MATERIAL
15
avalanches are created. The corresponding signals are much larger than in avalanche mode and thus
use of preamplifiers is not needed any more. These signals are called ”streamers” and the corresponding
operation mode ”streamer mode”. There is a transitional regime from avalanche to streamer mode
in which both the signal of a saturated avalanche and a streamer can be observed[15]. The streamer
signal follows the avalanche by a few ns and is not a direct coincidence of the avalanche but is a later
discharge stage due too the photoionization process. Due to the slow drift speed of the ions, a positive
space charge will be created which will reduce the electrical field. At a certain threshold the electrical
field will be reduced so drastically that there will be no more gas multiplication because the electrons
do not gain enough energy to liberate more electrons. When a streamer discharges the electric field
the corresponding electrodes that detect the streamer will have a dead time where they cannot detect
any other signals due to the fact that the electric field needs time to reestablish at the location of
the streamer. This is why RPC’s are coated with a resistive coating in order to keep the discharge
spot as small as possible since this reduces the gas amplification. Thus RPC’s in streamer mode have
the disadvantage to operate with a lower rate than RPC’s in avalanche mode. Historically streamer
mode was first used. Later on, the avalanche mode was discovered together with its advantage. To
obtain higher rates, the streamers need to be suppressed. A solution for this problem is to use a dense
quenching gas. Its high density increases the number of clusters originating from primary ionization
while due to its high electron affinity the formation of streamers is suppressed by absorbing the UV
photons emitted by the gas molecules. Another solution to obtain a higher rate capability is to use a
wider gap. However, a wider gap has a worse time resolution due to the fact that a wider gap means a
lower electric field and thus a lower drift velocity resulting in a worse time resolution.
3.3
Resistive plate material
The RPC rate capability is also dependent on the resistivity of the plates. In the DC model for RPCs
the rate capability is inversely proportional to the product of the plate resistivity times its thickness
[16]. Therefore the resistivity should be kept low. The problem to find appropriate material for the
resistive plates is the fact that the required resistivity is between a conductor and an insulator which
are rarely found in nature. The currently used materials as resistive plates are Bakelite and glass.
Up to now, the low production cost and resistivity of Bakelite made this material the optimum
choice as resistive plates in the RPCs for CMS. For the second phase of the upgrade of CMS (Phase 2,
2022-2023), more RPCs will be installed in the high pseudorapidity region (|η|≥1.6). With the expected
increased luminosity of the LHC it is expected that the rates of incoming particles are very high (10
kHz/cm2 ). The current Bakelite RPCs cannot handle these high rates due to aging effects and limited
rate capability. Therefore, further research was necessary to look for alternative plate materials.
A study was performed measuring various electric properties of different materials that could suit as
resistive plates[16]. Besides resistivity, permittivity of the resistive plates and gas gap is an important
property that affects the rate capability of RPCs.
The permittivity of the gas gap and plates play an important role when it comes to the distribution
of the electric field across the gaps and the material [16]. It has a big influence on the electric field
recovery time of the gas gap related to the relaxation time τ (the time it takes to dissipate the charge),
the resistive electrode stored energy and the continuity of the electric field displacement at the gas
and gap. The voltage drop should be larger at the gap than at the plates themselves. This implies
that the permittivity of the resistive plates should be bigger than the permittivity of the gas inside the
gap. The resistive electrode stored energy should be kept as small as possible to avoid damaging the
electronics. Since the energy is proportional to the resistive electrode capacity, this means that the
permittivity should be kept small as opposed to the previous requirement. These considerations make
the right choice of plate material in high rate RPCs challenging. For Soda Lime Silicate (SLS) glass,
Low resistivity silicate (LRS) glass and Bakelite the permittivity is of the same magnitude and its role
is limited concerning the distribution of the electric field.
Another problem with choosing the right material is that due to the high radiation rates, most
materials used in the resistive plates show an increase of resistivity. It is found to be that ferrite ceramic
and LRS glass show only a limited increase of resistivity after being exposed to high rates of radiation
as can be seen in figure 3.2. Bakelite and float glass show a conduction behaviour that is ionic[16].
16
CHAPTER 3. RESISTIVE PLATE CHAMBERS
Figure 3.2: Resistivity vs. transferred charge measured for several materials and different temperatures [16]
This form of conduction has an important drawback when the material is exposed to high radiation
rates. When the ions drift in the resistive plate, they cannot be easily resupplied at the side they are
drifting towards. This leads to an increase of the resistivity of the plate. This type of conduction
mechanism is different for electrons, they can be more easily resupplied at the material where the
anode is. Once the ions are collected on one side of the plate, they do not contribute any further to the
current. This behaviour is illustrated in figure 3.3. LRS glass contains oxides that shows electronic
conduction behaviour (at low enough temperatures ≤ 30°C ), therefore the aging effects are less severe
then in float glass and Bakelite. Finally, the medium resistivity of LRS glass, makes this material
Figure 3.3: Behaviour of the density current through time[16]
suitable for achieving a high rate capability [17],[18]. For these reasons LRS glass and ferrite ceramic
(although ferrite ceramic still needs to be investigated at lower temperatures) are good candidates
for plate material in high radiation environments. Table 3.1 shows the bulk resistivity of some of the
materials discussed here. Besides its conduction behaviour, LRS glass has another advantage over
Bakelite plates. Glass can be produced with very smooth surfaces, while Bakelite plates require a
linseed oil treatment and this reduces the noise rate and the number of spurious pulses.
3.4. GAS MIXTURE
17
material
Bakelite[19]
LRS glass [18]
SLS glass [16]
Bulk resistivity (Ω cm)
109
1010
1012
Table 3.1: Bulk resistivity of plate materials
3.4
Gas Mixture
Most RPCs nowadays operate in avalanche mode due to the much higher rate capability compared to
streamer mode. In order to stay in the region of avalanche mode, streamers must be suppressed while
at the same time the gas must be dense enough to create an avalanche that is large enough to produce a
detectable signal. As it turns out the composition of the gas mixture plays a key role in obtaining these
signals. The UV photons that are produced and trigger a new avalanche can be absorbed by adding
isobutane iC 4 H10 [20]. The discharge size can be reduced by capturing the outer electrons by adding
freon. A main component that is frequently used and has good properties is tetrafluorethane C2 H2 F4 .
This molecule has a high primary ionization density λ (i.e. the number of primary ionizations per
unit length) making it a good component for obtaining good detectable signals. Due to the high drift
electron velocity in C2 H2 F4 , it has good time characteristics [21]. Other advantages of tetrafluorethane
are that it is non-flammable and environmental safe. However, tetrafluorethane shows to decrease the
resistivity of linseed oil treated Bakelite plates due to chemical reactions between the linseed oil and the
freon [22]. As a third component usually SF6 is added to further suppress the presence of streamers due
to its high electron affinity as can be seen in figure 3.4. When space charge effects reduce the field, the
attachment becomes very effective assuming that the attachment cross-section is a decreasing function
of the electric field [23]. The free electrons form negative ions which are less effective in radiating UV
photons when being recombined with positive ions[24]. This high quenching capability reduces the
efficiency slightly[25] and is also visible in figure3.4. The concentration of SF6 is mainly limited due
to its high global warming potential. Instead of using tetrafluorethane as the main component Ar
is also a possible candidate since it provides good gas multiplication. However mixtures containing
tetrafluorethane show to have a large ”fast” charge (the charge induced by the electrons) and this
enables the RPCs to operate with a higher efficiency and a higher threshold of the front-end electronics
[21]. The CMS RPCs operate with the following gas mixture [26]:
ˆ 95.2% C2 H2 F4
ˆ 4.5% iC4 H10
ˆ 0.3% SF6
3.5
Performance and efficiency
The efficiency of a RPC is mainly determined by the gas multiplication. Variations in the gap width
can lead to variations in the gas multiplication. The charge distribution for narrow and wide gaps are
different. For narrow gaps the number of events with small induced charges is always greater than for
wide gaps is the same and diverges for qind →0 [14]. Figure 3.5 illustrates this behaviour. Small induced
charges require good amplifiers, low thresholds and low noise. Thus wide gaps are more efficient.
18
CHAPTER 3. RESISTIVE PLATE CHAMBERS
Figure 3.4: Effect of adding SF6 to the efficiency and streamer probability[24]
(a) Narrow gap
(b) Wide gap
Figure 3.5: Charge spectra for a narrow and wide gap[14]
In order to have a detectable signal, the gas gain or multiplication must be set high enough to
produce a large enough avalanche. Electrons that initiate an avalanche over the whole gap width will
produce the largest signals. Avalanches that originated further from the cathode might not grow large
enough to produce a detectable signal. There is a small probability that there is no primary ionization
in the first mm closest to the cathode. In that case, the chance of producing a detectable signal is
dependent on the remaining distance to which a cluster might form and develop. For a wide gap there
remains more distance to develop an avalanche that reaches the detection threshold. Wider gaps tend
to have a lower streamer probability for the same detection efficiency than narrow gaps[14] because
3.6. MULTI-GAP RPC
19
the gain can be set lower in a wider gap. However wide gaps have a worse time resolution since the
electrons have to travel a larger distance. Variations in gap width in a narrow gap are far more worse
than in a wide gap. For a narrow gap with gap width 2 mm variations in gap width of 100 µm can
already lead to a variation in gain of a factor 2500[27].
Another important parameter as discussed before is the composition of the gas mixture. This
mixture needs to be chosen so that streamers should be suppressed by adding a quenching gas but at
the same time the primary ionization density λ (i.e. the number of primary ionizations per unit length)
should be high enough. If λ is too low, there would be too few primary pairs to form a strong enough
signal.
Aging effects due to the high radiation environment show a decrease in efficiency due to an increase
in resistivity. This effect was discovered in Bakelite RPCs which were treated with linseed oil 2[ 8],[29].
The efficiency is both dependent on the temperature and pressure[30]. Changes in temperature
and/or pressure cause a change in density of the gas. For a lower temperature, the gas density increases
and this is equivalent to a larger gap width which requires a higher voltage. This means that a proper
gas flow is important to reduce the variations in the density. The effective high voltage corrected for
both temperature and pressure is defined as:
HVef f (P, T ) = HV
P0 T
P T0
(3.5)
P0 and T0 are some reference pressure and temperature values. The high voltage HV can be further
rescaled to take into account the voltage drop by the current drawn by the RPC and this gives the
high voltage across the gap[31]:
HVgap = HV − 2RI
(3.6)
R = ρd
S is the electrode resistance with S the electrode surface and d its thickness and I is the total
current drawn. The efficiency dependence on the effective high voltage can be written as a sigmoidal
response function [32]:
max
(3.7)
=
1 + e−λ(HVef f −HV50% )
where max is the efficiency for HV → ∞, HV50% is the high voltage value for which the efficiency is
the half of max is reached. The coefficient λ is proportional to the sigmoidal slope at the inflection
point. The efficiency is very low for low voltages and shows a rapid increase as the high voltage is
increased and reaches eventually a plateau. The rapid increase is due to fluctuations in the primary
ionization and the avalanche process. The operation point is usually chosen not far from the knee1
since for very high voltages streamers appear lowering the efficiency.
The rate capability of the detector increases with increasing temperature due to a lower resistivity
[31]. Time jitter arises from the fact that one does not know how many electrons initiated the avalanche.
This might have started from 10 electrons or just one electron where the number has increased during
the very beginning of the avalanche. It is expected that the time jitter is related to the time it takes
for the electrons to travel one ionization length [33].
3.6
Multi-gap RPC
A multi-gap RPC has several gaps interleaved with internal plates. A double gap consists only of two
gaps as shown in figure 3.6. For a double gap the two anodes of the gaps can be placed against each
other, adding the two signals of the gaps together. This configuration provides a higher detection
efficiency that is the logic OR combination of the separate detection efficiencies and can be operated
with a lower operating voltage. In a multi-gap the inner plates are not electrically connected, they take
the right voltage by electrostatics and by the flow of ions and electrons.
1 Defined
as the point where the efficiency reaches 95% of its maximum efficiency
20
CHAPTER 3. RESISTIVE PLATE CHAMBERS
(a) Double gap
(b) Multigap
Figure 3.6: Different gap structures
A multi-gap RPC has a better time resolution than a single gap RPC. In section 3.5 it was already
pointed out that the first mm closest to the cathode is crucial for producing a detectable signal. The
time jitter is due to the finite drift speed of the electrons in this closest mm to the cathode. For a three
layer multi-gap RPC this distance is reduced by a factor of three since the avalanches in the three gaps
are produced almost simultaneously. Accordingly the time jitter is also reduced by a factor of three
[34].
Another advantage of the multi-gap structure is related to the charge spectrum. In a single gap
RPC, the avalanches arising from individual clusters merge together [35], i.e. the positive space charge
of a previous avalanche can affect another avalanche by recombination and a reduction in growth.
This process makes the fluctuations on avalanche size worse and the most probable value of these
fluctuations is zero [35]. In a double gap RPC there are now two independent avalanches meaning that
the fluctuations are averaged with a drastic effect on the charge spectrum. The most probable value is
no longer peaked at zero meaning that more avalanches will be detected and thus a higher efficiency
can be obtained. This also means that one is able to work at higher thresholds.
Since the currents drawn in a multi-gap structure are much smaller, the power dissipated is also
much smaller. Next to that, the voltage drop in the gaps is smaller and this means that the electric
field is restored more quickly. Thus, compared to a single gap RPC, one can use a plate with larger
resistivity while obtaining the same rate capability.
The placement of the spacers in a multi-gap give this structure another advantage. The spacers
have a dielectric constant that is greater than the surrounding gas. This leads to increased electrostatic
coupling between the avalanches and the pick-up strips. Furthermore the electric field around the
spacer will be reduced due to a leakage current but this will lead to a corresponding increase of the
electric field in the other subgaps.
Aside from these advantages, a multi-gap is more difficult to construct. Since they usually have a
smaller gap width, the tolerance of the gap width is more strict. To conclude: multi-gap RPCs have a
much higher rate capability and a better time resolution than a single gap.
3.7
Readout electronics
When used in a high radiation environment like at CMS RPCs are mostly operated in avalanche
mode. Due to the relatively low electric field, the signals acquired in this mode are relatively weak and
preamplifiers need to be used. The electronics that amplify the signal of the RPCs and convert it into
a logical signal are referred to as the front-end electronics. The better the amplification of the signal,
the smaller the incoming signals may be. This means that the total charge deposited over time on the
resistive plates can be reduced and thus limiting the aging effects. Secondly a smaller deposited charge
means that the recharge time is smaller and thus the detector has a higher rate capability.
The front-end electronics for the RPCs at CMS consist of a front-end amplifier-discriminatormonostable integrated circuit, called Application Specific Integrated Circuit (ASIC) [36]. The ASIC
basically amplifies the incoming signal and changes the shape of the signal. A single channel of the
circuit of the ASIC is displayed in figure 3.7, each channel contains an amplifier, a zero-crossing
discriminator, a monostable and a differential line driver.
3.8. RPCS AT CMS
21
Figure 3.7: Diagram of one channel of the front-end circuit
The amplifier converts the input current into a voltage while a dummy amplifier is used to balance
DC output variations of the first input stage. The gain stage consists of two differential amplifiers
with different gains. One of the two amplifiers amplifies small signals while the other one takes care of
larger signals. To assign signals to specific bunch crossings, timing performance is a crucial element for
the RPC’s. This can be done by leading edge timing by means of a threshold discriminator. Leading
edge timing generates a logic output pulse whenever the incoming signal crosses a fixed discrimination
level. This method is however affected by amplitude time-walk, pulses with the same shape and time of
occurrence cross the discrimination level at a different time. To overcome this problem the zero-crossing
technique is used. This technique uses a C-R network that differentiates the input signal and produces
a bipolar pulse so that it crosses the zero where the input pulse peaks. Together with a one-shot circuit,
the output signal of the zero-threshold-discriminator is generated. This is illustrated in figure 3.8. The
pulse generated by the discriminator is transferred to a monostable. The monostable ensures that
the discriminator is not triggered by the noise coming from an afterpulse after the avalanche. This
afterpulse has a typical delay of 0 to some tens of ns. The monostable provides a pulse length of about
100 ns to prevent this noise triggering. The dead time introduced by the monostable is about 4%. The
differential driver generates the desired power for the low voltage differential signaling (LVDS) that
communicates with the read-out. The front-end electronics are mounted on a front-end board together
with some other devices. The readout strips can be connected through an adapter board with the
front-end electronics. The amplified and discriminated signals of the front-end electronics are send
through a LVDS twisted pair cables to a link board which synchronizes the signals with the LHC clock
[37]. Each link board reads 96 strips of one pseudorapidity region. After a data compression the signals
are send to splitter boards and then to the trigger boards in the CMS counting room.
3.8
RPCs at CMS
As noted before in section 2.3 CMS currently uses RPCs as muon triggers. Good time performance,
small average cluster size2 , low power consumption and good rate capability are the main requirements
of the CMS RPCs[38][39]. To achieve a high trigger efficiency, good timing resolution is necessary.
The fast response of the RPCs ensures that for each particle the correct bunch crossing is assigned.
Good momentum resolution requires low cluster sizes[38]. Especially in the forward region, the rate of
incoming particles is high (1kHz/cm2 ). Therefore good rate capability is required. These considerations
have lead to the design of double gap RPCs. These double gap RPCs use Bakelite plates of 2mm
thick with a bulk resistivity of 1010 -1011 Ωcm as resistive plates mainly due to their low cost and
resistivity. The gas gaps are 2 mm wide and are coated with a graphite coating for the HV and ground
2 The
number of adjacent fired strips for a single event
22
CHAPTER 3. RESISTIVE PLATE CHAMBERS
Figure 3.8: Illustration of the zero crossing technique:[36]
connection[40][41]. The gap design is illustrated in figure 3.9. The gas gaps and strips are wrapped in
Copper foil (Faraday cage) and Mylar foil is used as insulating foil. To cool the electronics, a system of
Copper pipes is present.
Figure 3.9: Design of the CMS gaps[42]
The Bakelite surfaces are treated with linseed oil to reduce the number of spurious pulses since this
treatment makes the surfaces more smooth. They are operated in avalanche mode due to the higher
rate capability as opposed to streamer mode. The RPCs are both installed in the barrel and endcap
region. The operation and basic design of the barrel and endcap RPCs is the same. They mainly differ
in geometry and size of the readout strips. For the endcap RPCs the top gap is split into a narrow and
wide part while the bottom gap consists of one piece to allow some space for the readout cables as
shown in figure3.10. The readout strips are placed between the gaps and are divided into 3 η segments
and are connected through coaxial cables to adapter boards[9]. These adapter boards are connected to
one of the three front-end electronics, one for each η segment. In the barrel region six layers of RPCs
are present while in the endcap region there are four discs containing each one layer of RPCs. For
the first part of the shutdown of the LHC (Phase 1, 2013-2015), RPCs were installed in an additional
fourth disk in the endcap region up to |η|≤1.6. Figure 3.11 shows the current installed RPCs (indicated
in red) together with the fourth disk. After the first shut down, the forward region (|η|>1.6) remains
largely uninstrumented and contains only CSCs. When the LHC will reach its design luminosity, the
incoming particle rate in this region will be very high 5-10 kHz/cm 2 . The current Bakelite RPCs are
3.8. RPCS AT CMS
23
Figure 3.10: Double gap structure of the Forward RPCs[42]
not suitable for this high radiation environment mainly due to aging effects. However the glass RPC’s
might be able to overcome this problem when used with LRS glass. Further improvements can be made
by using a multi-gap RPC since the deposited charges in a multi-gap structure are smaller [43]. Due to
their better time resolution, they could serve as good triggers in the high η region.
Figure 3.11: View of the CMS detector for the first upgrade: indicated in red are the currently installed
RPCs. A fourth disk of RPCs will be installed.[11]
24
CHAPTER 3. RESISTIVE PLATE CHAMBERS
Chapter 4
Design and construction of a gRPC
prototype
Glass resistive plate chambers are RPC’s constructed with glass electrodes (in this case low resistive
silicate glass). This version of the RPC is more suitable to withstand the high radiation intensity of
the LHC due to limited aging effects and resistivity of the low resistive silicate glass. Furthermore,
the LHC is upgraded to achieve an even higher luminosity. As already noted in section 3.8, there is a
proposal to install gRPC’s in the high pseudorapidity region. The first problem when constructing a
gRPC with the required dimensions of an RE/4/1 RPC for CMS is that the glass plates cannot be
produced with the same dimensions. The low resistivity silicate glass is only available in dimensions of
30x30 cm2 . Accordingly, a smaller prototype with much cheaper float glass has been constructed in
order to test the gluing of glass plates and to get familiar with the construction. The design of the
gRPC follows as much as possible the design of the current Bakelite RPCs at CMS. Figure 4.1 shows a
cross-sectional view of the double gap CMS-like gRPC. In this design the two anodes are facing each
Figure 4.1: Cross-sectional view of the double gap CMS-like gRPC
other and are read out by the same copper strips. The first step in the construction is to glue the
glass plates which is explained in section4.1. Afterward as explained in section4.2, the resistive coating
has been applied on the glass plates. Once the resistive coating has been put on top of the plates,
25
26
CHAPTER 4. DESIGN AND CONSTRUCTION OF A GRPC PROTOTYPE
some measurements have been done of the coating and are discussed in section 4.3. The next step is to
construct the gas gap and is clarified in section 4.4. The flow diagram in figure 4.2 summarizes the
different steps in the construction.
Figure 4.2: Different steps in constructing the glass resistive plate chamber
4.1
Glass cutting and gluing
The used glass plates are made of float glass in a square shape of 30x30 cm2 . These plates were cut
into four pieces as indicated in figure 4.4. Before using the glass cutter the area that needs to be cut is
lubricated with white spirit. A score is made on the surface of the glass with the glass cutter which
makes it more easier to break the glass along the score. By carefully increasing the tension on the glass,
the glass breaks along the score. Applying to much force on the glass cutter might lead to cracks in the
glass plate before it is properly cut. Oppositely it might happen that the glass does not break along
the score if too little force is used. After cutting the glass, the four glass plates were put back together
to form the original plate. Two different types of glue were tested namely Verifex UV adhesive B 690-0
[44] and Verifex UV adhesive MV 760 [45]. These glues are especially designed for glass bonding, are
easy to apply with a small needle and do not require a lot of cleaning. The latter glue [45] turned out
to be the best glue considering rigidity of the glued glass plates. First a thin layer of glue is applied
to one of the edges of the glass plates. Next, the alignment of the glass plates has been done with
the aid of a metal plate that has trenches in the shape of a cross. The edges on the glass plates are
placed above the trenches to prevent them from sticking to the metal plate. The glue was cured by UV
radiation for 30 seconds. Any excessive glue was cleaned and afterward the glue was cured again for
4 minutes. For cleaning excessive glue a cleaning product was used both for removing the glue and
for preparing a clean surface where the glue needs to be applied. A second cleaning product removes
dust and grease on the plates (referred to as the ”General Cleaner” in table 4.1). It is important that
4.2. RESISTIVE COATING
27
the glass plates are not only horizontally well aligned but also vertically. If sharp edges arise, they
will cause noise and lead to a nonuniform gap width. Furthermore holes between the edges should be
avoided. After gluing the glass plates more glue was added to avoid sharp edges. A list of the used
products can be found in table 4.1.
Item
BO 400.0
BO 02B120
BO 032.1
BO 5107911
BO 5107800
BO 51 078 00
BO 5141620
Description
Carbide glass cutter
Carbide wheels
L square
Cleaner for UV bonding
General Cleaner
Spray head for glass cleaner
Paper towels
Table 4.1: Used items for cutting and gluing the glass sheets. All items can be found on http://www.
bohle-group.com/shop/.
4.2
4.2.1
Resistive coating
Properties
The main usage of the resistive coating is to uniformly distribute the applied high voltage and to
ensure that the gap can easily recharge after an ionizing event. The resistance of a coating is usually
characterized by means of the surface resistivity, i.e. the resistance per unit surface in units of Ω/.
This quantity is independent of the surface area which makes it more easy to compare with other
coatings. When an avalanche is initiated and charge is drifting towards the glass plates, the charge will
drift through the glass and reach the coating. If the resistivity of the coating is too low, the charge
will spread. The induced charge will fire multiple readout strips thus losing spatial resolution, this
is referred as a high transparency. Furthermore this might lead to an operation mode of continuous
discharging the plates. If however the resistivity is too high, the electric field will not be uniform and
at the same time the gap will not recharge easily after an event. Since the signal on the readout strips
is an induced charge, a high resistivity would lead to a small signal. The resistive coating is chosen so
that the average number of readout strips fired is reduced to a minimum but at the same time that the
electric field is uniform leading to a uniform detection efficiency across the surface. A conductive paint
Electrodag 6017 SS [46] and a resistive paint Electrodag PM-404 [47] have been mixed to provide the
resistive coating. The conductive paint [ 46] consist of conducting carbon particles in a thermoplastic
resin, while the resistive paint [47] only consists of the latter. The mass ratio rc defined by (4.1)
between the two paints is chosen to reach a resistivity between 100 kΩ/ and 1 MΩ/ according to the
explanation below. mc denotes the mass of the conductive paint and mr the mass of the resistive paint.
mc
rc =
(4.1)
mc + mr
The ratio of the two paints and the resulting resistance is tabulated by the manufacturer. The
manufacturer provides resistivity values from 35 Ω/ to values higher than 1 GΩ/ at a thickness of
25 µm. The paint mixture shows a phase transition to a more conductive regime as the concentration
of the carbon particles increases and this can be explained by using a percolation model [ 48],[49],[50].
In this model the coating can be seen as a grid of conducting and non-conducting sites. A carbon
particle resembles a conducting site and the fraction of conducting sites is related to the fraction rc of
the mass paint ratios. As the relative quantity of the resistive paint is increased, the chance of finding
a conducting path related to the number of carbon particles drops and thus the resistivity increases.
When the critical ratio is reached, there will be no conducting path left, accordingly the resistivity
increases rapidly. The described behaviour can be formulated by a power law [48] as in equation 4.2.
(
R0 (rc − r)−β
(r < rc )
Rtot =
(4.2)
Rc
(r > rc )
28
CHAPTER 4. DESIGN AND CONSTRUCTION OF A GRPC PROTOTYPE
The manufacturer gives datapoints of the resistivity Rtot for different ratios of rc at a coating thickness
of 25 µm. Some mixtures have been painted using the procedure described in the next section since
a different coating thickness is expected. These points were used to fit the power law in 4.2 which is
shown in figure 4.3. Strictly speaking this power law is only valid in the immediate vicinity of the
critical value rc . The resulting parameters from the fit are given below:
Parameter
R0
rc
β
Value
121 ± 288
0.12 ± 0.01
2.1 ± 0.6
Table 4.2: Fitted parameters of the power law described by equation 4.2.
The parameter R0 and rc differ from system to system but the value of β should be within the same
range of 1 to 2 [48]. More measurements are needed to provide a better fit. Next to that, it was not
possible with the available equipment to measure the surface resistivity of the resistive paint only. This
problem is related to the range of resistivity that could be measured.
Figure 4.3: Resistivity of the coating for different values of the mixing ratio rc . The red points represent the
experimental values and the green point represent the used ratio for the prototype. These points
are fitted with a power law.
The measurements of the surface resistivity of the painted samples together with the fitted power
law lead to the choice of a paint ratio rc around 0.16 to reach a resistivity between 100 kΩ/ and 1
MΩ/ indicated as the green point in figure 4.3.
Table 4.3 shows the mixtures of two used samples of paint for the gRPC prototype:
Sample number
1
2
mc (g)
48.03 ± 0.05
48.19 ± 0.05
mr (g)
252.20 ± 0.05
251.95 ± 0.05
rc (%)
16.00 ± 0.02
16.06 ± 0.02
Table 4.3: Ratio of the mass of the conductive to the total amount of paint
4.3. RESISTIVITY MEASUREMENT
4.2.2
29
Painting procedure
Before the glass plates were painted they were cleaned thoroughly with glass cleaning products (see
table 4.1). In order to keep the edge spacers and gas adapters out of the active region, a masking tape
was applied on the glass sheets. The masking tape has a thickness of 1.8 cm reducing the active region
of the detector to 26.4x26.4 cm2 . The two paints were mixed for 5 to 10 minutes to ensure that the
resistivity of the paint is uniform throughout the hole sample. The glass plates were painted by use of
a spray gun (Powerplus Powair 0105 [51]) being held under a pressure between 3 and 4 bar by an air
compressor. A dilution product 2-butoxy ethyl acetate was added to obtain a steady flow of the paint.
To ensure a uniform thickness the spray painter was moved as steady as possible with a constant speed
and a constant height. Each glass plate has got at least two layers of paint. The paint was dried by
putting the glass plates in an oven on a temperature between 95 °C and 100°C . The temperature has
to be below 100°C to ensure that the glue does not start melting. It was found to be that the glue
MV 760 gave the most reliable results in contrast to the glue B690-0. After about 30 to 60 minutes
the oven was shut down but the glass plates were kept inside the oven to avoid exposure to moisture
and to ensure that the temperature drop was steady. In some cases the paint started to show cracks
and came loose. The reason for this is not immediately clear but the following factors might play an
important role in it: the temperature during the drying process should be kept high enough, a thicker
coating might lead to easier cracking of the paint. Another possible source is related to the production
mechanism of the glass. During the manufacturing process, the molten glass floats on a pool of tin
hence the name float glass. One side of the glass contains traces of tin and tin oxide while the other
side had only contact with air. This side is smoother than the other side but reacts negatively with
paint and enamels. The tin side can be identified by radiating the glass with UV radiation. The tin
side will glow with a fluorescent milky white light. All used materials were cleaned using acetone. A
visual check of the coating was done by putting a light source beneath the painted plate. If no light
could pass through, no paint was further added.
Another way to apply a uniform coating to the plates, is to use silk screen painting. Silk screen
painting uses a fine mesh of wires that is put om top of the plates. Paint is deposited on the mesh,
pulled across and reaches the plate. As opposed to silk screen painting, the spray painter has the
advantage of being a lot faster and it does not require much cleaning work afterward. The spray painter
is however less accurate in depositing a uniform coating thickness since this requires a steady hand and
this is more tiresome when painting large surfaces.
4.3
Resistivity measurement
Measuring the resistivity of a surface cannot be done by simple measuring the resistance between two
points, because the electrons have multiple paths to go from one point to another point. A solution to
this problem is provided by a concentric ring probe [52]. This probe consists of two concentric rings
between which the resistance is measured and contains a weight to ensure proper contact with the
surface. The output of this probe was read by a digital multimeter [53] and is just the resistance itself.
To convert this resistance into units of Ω/ i.e. the surface resistivity, the result needs to be multiplied
by 10 −1 according to the manufacturer. The glass plates were divided into 9 measuring points equally
spread across the surface. Figure 4.4 shows a sketch of a glass plate and its subdivision. For each of
the nine positions, the surface resistivity has been measured at different times. Figure 4.5 shows the
measured surface resistivity for each of the nine different positions through time for one glass plate.
The fact that some lines lay far apart from other lines is due to the variations in the coating thickness.
The error bars only indicate the errors of the digital multimeter and do not take into account the highly
position dependent sensitive coating thickness. Where the layer is thick the surface resistivity is at its
lowest as opposed to the places were the layer is thin the surface resistivity is large and tends more to
the surface resistivity of glass as expected. Variations in the surface resistivity for a particular position
are again due to variations in coating thickness. Moving the concentric ring probe a few millimeters
can already lead to a variation of a few kΩ/. A possible way to improve this measuring method is to
design a grid in the shape sketched in figure 4.4. The grid could than be temporary attached to the
glass plate. The distribution of the surface resistivity measurements should than behave more like a
30
CHAPTER 4. DESIGN AND CONSTRUCTION OF A GRPC PROTOTYPE
Figure 4.4: A sketch of the four glued glass plates and its subdivision
gaussian distribution for each independent position. For the plate of figure 4.5 the mean value of the
Figure 4.5: Measured surface resistivity for one plate off all positions through time. The horizontal axis
denotes the elapsed time when the plates were put out of the oven.
surface resistivity is 437 kΩ/ which is well within the desired range. The following table gives the
mean resistivity of the four glass plates used in the prototype. The most important criteria for the
prototype is that the surface resistivity stays within the range between 100 kΩ/ and 1MΩ/ and
that it is as uniform as possible.
Plate number
1
2
3
4
Resistivity [kΩ/]
473
204
365
120
Table 4.4: Surface resistivity of the graphite coating for the four used plates
4.4. GAS GAP CONSTRUCTION
4.4
31
Gas gap construction
Once the resistive coating has been applied on the glass plates, the next step is to construct the gas
gap. Uniformity of the gap width is ensured by adding spacers in between the plates. This is an
important property since a change in gap width can mean a change in electric field and thus a change
in gas multiplication. Nine ball spacers have been put in between the plates equally positioned. The
ball spacers have a diameter of 1 mm and are made of ceramic. They have the advantage that the
surface touching the electrodes is at a minimum and ceramic has a good radiation resistance. Taking
into account the glue, the effective gap width is 1.2 mm. For the edge spacers, PET [54] was chosen
as material. PET is relatively cheap and has a good radiation resistance. These spacers were also
used as a part of a glass flow system. This was done by performing several simulations with different
constructions using GERRIS[55] in order to find a good gas distribution. A schematic view of the gas
flow is given in figure 4.6.
Figure 4.6: Schematic view of the gas flow designed by A. Fagot. The dotted line represents the gas flowing
in through the gas inlet adapter.
Our first gap uses gas inlet adapters which were designed last year [56] and consists out of two parts as
shown in figure 4.7. The first part consist of the spacer while the second part represents a tube called
the inlet. The reason to split the gas inlet adapters is that if an inlet breaks off, it can be easily replaced
by drilling out the remains of the inlet in the adapter. The inlet was glued into the adapter with glue
that cures quickly [57]. The adapter contains a smaller spacer for placing one of the glass sheets in
between. A small groove ensures that the second glass sheet can be glued at the very end when all the
spacers are glued. The complete gas adapter was produced by 3D printing by Shapeways[58].
(a) Inlet
(b) Adapter
(c) Improved Adapter with inlet
Figure 4.7: Gas inlet
The design of the adapter was further improved to incorporate the edge spacers more easily. In
figure 4.7(c) a small groove can be seen used for the edge spacer.
All spacers were glued using the same glue used for gluing the glass plates [45] since it has the
advantage that it is easy to clean. Just like in the case of the gluing of the glass plates, the drying
process is relatively fast. Each curing process only takes about 5 minutes and one side of the plate can
be cured at once. To be able to easily glue the ball spacers a sheet of plastic with holes equally distant
from each other has been designed. The edge and ball spacers together with the gas inlet adapters
32
CHAPTER 4. DESIGN AND CONSTRUCTION OF A GRPC PROTOTYPE
are shown in figure 4.8. The sheet made proper placement of the ball spacers possible. Once all the
spacers were glued, the second glass sheet was put on top. During the curing of the glue, some weight
(i.e. metal bars) was added on top of the plate and pressure was applied upon the plate. This process
is especially important for the spacers that regulate the gas flow. Any gas leak could disturb the gas
flow from being uniform. Finally, to ensure gas tightness in the outer edge spacers silicone glue has
been used [59] which is ideally suited for this problem. This curing process takes however 24 hours.
Table 4.5 sums up the materials used for the spacers and the gas inlet adapter.
Figure 4.8: Construction of the gas gap: The white dots are the ball spacers while on the edges the edge
spacers can be seen. In the top left corner one of the gas inlet adapters can be seen.
Item
gas inlet adapter
edge spacer
gas flow spacer
ball spacer
material
PEEK[60]
PET[54]
PET[54]
ceramic [61]
Table 4.5: Materials used for the spacers and the gas inlet adapter
4.5
High voltage connection
The high voltage cables are connected to the resistive coating where the high voltage is distributed
across the plates. First a small sheet of copper is glued on the resistive coating. The used glue [62]
is silver-filled epoxy used for making electrical connections. The glue can be left to cure at room
temperature for 24 hours. Faster curing is possible at higher temperatures. The high voltage cables are
soldered on top of the copper sheet. Insulation of this connection is provided by silicone glue [63] and
by using dielectric tape on top of it. The copper sheet of the ground cable is bend to the edge of the
gap since the construction is a double gap RPC and thus as much space as possible needs to be saved
between the two gaps. Care has also been taken by placing the ground connection and high voltage
connection not to close to each other to avoid discharges between the connections. The other end of the
cables are soldered into a Jupiter connector for easy connection as depicted in figure 4.9. The cables are
insulated by a metallic sleeve providing protection against electrical noise and by a plastic sleeve that is
electrically insulating. The inactive part of the glass sheets are completely isolated with dielectric tape.
4.6. READOUT STRIPS
33
(a) High voltage connection of the gaps
(b) Jupiter connector
Figure 4.9: High voltage connection of the gaps. The white cable represent the high voltage cable while the
black one is the ground cable as can be seen in the left picture. The right picture shows the other
end of the cables where they form a jupiter connector. Also shown is the yellow dielectric tape
used as insulation.
4.6
Readout strips
The gRPC has a double gap structure with the readout strips in between the two gaps. The strips of
last year were improved [56]. These consist of 10 readout strips with a width of 25.4 mm and a spacing
of 2 mm made out of copper tape that are applied on a Mylar sheet that acts as an insulation foil.
Compared to the previous readout strips, the newly used strips have a strip width of 15 mm with a strip
spacing of 1.5mm making a total of 16 strips. These strips have a length of 297mm and thus are slightly
longer than the active area of the detector to allow easy connection with the readout wires. They are
made out of copper with a thin layer of tin on top of it and were produced by Eurocircuits [ 64]. The
gaps together with the readout strips are wrapped into a copper foil to minimize the external noise that
can be picked up by the strips, it acts as a Faraday cage. The readout wires are both soldered on the
readout copper strips and on the copper foil for grounding. There are however no connections between
the strips and the grounding. The readout wires are connected to an adapter board for connection
with a CMS front-end board.
Figure 4.10: The readout strips with readout wires
34
4.7
CHAPTER 4. DESIGN AND CONSTRUCTION OF A GRPC PROTOTYPE
Detector casing
The two gas gaps are put into an aluminum casing as shown in figure 4.11. The casing basically consists
of two square plates that are screwed onto four bars that enclose the gap between the plates. Grooves
in the plates are present to prevent damaging the gas inlets and the high voltage connection. The holes
are used to be able to connect the high voltage connection, the gas inlets and the readout strips. The
inside of the metal casing is electrically insulated with Mylar foil and dielectric tape. To ensure that
the gaps do not move inside the casing, they were taped to the casing. One side of the gas inlets of
the gaps are connected to a gas tube for later connection with the gas flow system. The other two gas
inlets of the two different gaps are connected to each other with another gas tube. The completed
detector is shown in figure 4.12.
(a) Top
(b) Bottom
Figure 4.11: Design of the prototype casing.
Figure 4.12: The double gap gRPC in its metal casing. Visible are the readout strips, high voltage cables and
gas connections.
4.8
Multi-gap gRPC
Apart from the double gap structure, a multi-gap gRPC was designed and constructed. The design is
inspired by Tsinghua University [ 18]. The mRPC follows a double gap structure which is subdivided
into five gaps. A cross-sectional view of the multigap structure is given in figure 4.13.
4.8. MULTI-GAP GRPC
35
Figure 4.13: Cross-sectional view of the mRPC, design by A. Fagot
The outer plates are coated with a graphite coating to apply the high voltage. The edge spacers
are made of PET material. Instead of using ball spacers, a PET wire was used as seen in figure 4.14.
The wire is glued outside the active region and wound up like a screw with the plates in between the
different levels. Instead of turning the wire only around the four cylindrical spacers, the two corner
pieces were used. Although this configuration is not perfectly stable, it ensures that the gas can flow
more easily in the gaps. Only the beginning and end of the wire need to be glued, the only constraint
is that the tension of the wire is high enough. The gluing of all the spacers was done with the same
glue[45] as for the double gap gRPC. The inner glass plates have a thickness of 0.7 mm and a surface
area of 15x15 cm2 while the outer plates have a thickness of 1.1 mm with a surface area of 20x20 cm2 .
The lateral dimensions of the inner plates are within the area of the resistive coating. Each gap has a
width of 0.2 mm. Gas tightness was again assured by using silicone glue [59].
Figure 4.14: Top view of the multigap gRPC (gas adapters and edge spacers are not shown)
The design of the gas inlet adapter was slightly changed to fit in the multigap gRPC structure. The
most important change is the width of the adapter since the total gap width is now 3.8 mm. As opposed
to the design for the double gap gRPC, an additional second small spacer is added for extra rigidity.
This was later removed from the design to suppress costs and due to the fact that the edge spacer that
is slid in makes this extra spacer not necessary. The gas inlets themselves remain unchanged. The
36
CHAPTER 4. DESIGN AND CONSTRUCTION OF A GRPC PROTOTYPE
corner spacer has one large groove for putting in some glue. This makes the gluing of the spacer easier
given the tiny dimensions of the spacer O(cm). The cylindrical spacer is slightly thicker on the top and
bottom surface so that the PET wire can be kept easier at the same place.
(a) Gas Adapter
(b) Cylindrical spacer
(c) Corner spacer
Figure 4.15: Adapter and spacers used for the gmRPC
4.9
Possible Improvements
This section describes some improvements that can be made for the production process of gRPCs.
ˆ To obtain more reliable results in the coating thickness and thus the uniformity of the resistivity
of the coating, one can switch over to silk screen painting which is however more time consuming
as opposed to spray painting. The spray painting could be improved by using a robotic arm that
moves steadily over the surface to be painted.
ˆ A varnish could be added on top of the paint to protect it against moisture and to prevent any
damage.
ˆ The gas distribution was checked by means of a simulation, this could also be tested by constructing
a gap without the coating and by adding a coloured gas to see the gas distribution inside the gap
and confirm the results from the simulations.
ˆ One of the most occurring problems was the presence of dust. Every time the glass sheets and
spacers were glued or painted, the plates needed to be cleaned thoroughly from all the dust that
gathered on top of it and this takes a non negligible time. Therefore future production of gRPCs
can be made easier and less time consuming by working in a clean room.
ˆ Configurations of the spacers of the mRPC have not yet been simulated. The current configuration
tries to obtain a good gas flow while at the same time obtaining a uniform gap width. The gap
width might vary slightly in the corners of the quadrants where there is no wire present due to
the weight of the glass plates.
ˆ The active area of the mRPC is larger than the inner glass plates. Since the glass plates have
sharp edges, this might increase the noise rate and thus the active surface should be kept smaller
than the area of the inner plates.
4.9. POSSIBLE IMPROVEMENTS
37
ˆ As noted in section 3.5, variations in the gap width can lead to a nonuniform efficiency. Therefore,
the glued glass plates must be well aligned. Up to now, only a metal plate was used as explained
in section 4.1. This can be further improved by using a set of magnets to tightly push the glass
plates on the metal plate as illustrated in figure 4.16. Of course, much depends on how smooth
and uniform the thickness the metal plate is.
Figure 4.16: Illustrated method to improve the glass plate alignment
38
CHAPTER 4. DESIGN AND CONSTRUCTION OF A GRPC PROTOTYPE
Chapter 5
Characteristics of the prototype
Once the prototype was finished, some test were performed to characterize the prototype. These
include a dark current and a detection efficiency test. All the test for characterizing the prototype were
performed at the Ghent RPC lab.
5.1
Experimental setup
The Ghent RPC laboratory contains a well air conditioned hut in order to keep humidity and temperature
steady since the properties of RPCs are dependent on both. Five test benches, sometimes referred as
stations, form a cosmic stand where cosmic ray tests are performed. On each bench an RPC can be
placed. Two scintillator planes are present right below and above the test bench which are used as
triggers. The scintillation light is recorded by a series of photomultiplier tubes. Another separate stand
is used for performing dark current tests. Next to the benches is a gas flow system and a high voltage
operator provided by CAEN A1526N modules. These modules are installed in a CAEN SY1527LC crate
together with CAEN A1513B modules which provide the low voltage supply for the FEBs. The high
voltage system is connected to a pc where voltage and current of the RPC gaps can be monitored and
be set by a Cosmic DAQ program. The Cosmic DAQ program makes it possible to set the high voltage
at regular time intervals. This makes it possible to run a test during night or during the weekend.
The high voltage is corrected for the ambient temperature and pressure according to the formula 3.5.
Both the inlet and outlet of the (g)RPCs can be coupled to the system where a ”bubbler” is used
to check whether there is a gas flow through the detectors. The gas system mixes the three gasses
used for RPCs at CMS and this mixture can be controlled by a pc. Both a dark current test and a
detection efficiency test (cosmic ray test) were done as explained in the following sections. Gas leak
test were avoided since the glass is very fragile and an overpressure might lead to breaking the glass
plates. As explained before, silicon glue was used to prevent any gas leaks. For all the tests the gas
mixture of CMS was used. Referring to the masterthesis of S. Vanheule last year[56] who also made a
gRPC prototype, a name scheme was used to make a distinction between the gaps and the chamber
itself. For example GHENT-GRPC-002-001 refers to the first gap of the second prototype constructed
and GHENT-GRPC-002-C02 refers to the complete chamber. In this chapter a comparative study will
follow with the prototype of last year.
5.2
Dark current test
The dark current of a gap is the current which is drawn when the high voltage is powered but with
the source powered off. The current arises from incoming cosmic rays, electrons coming from the
cathode and even the spacers separating the plates. This dark current is important concerning power
consumption and detection efficiency and should be kept as low as possible. The maximum power
consumption allowed at CMS is 3 W/m 2 . A too high dark current leads to a voltage drop in the gas
gap and thus a reduction in efficiency. The usual CMS gas mixture was used for the test whereas the
39
40
CHAPTER 5. CHARACTERISTICS OF THE PROTOTYPE
high voltage was increased step by step. Before running the test, the gap was flushed for several hours
to ensure that there was no air left inside the gap. After each step the high voltage was maintained for
15 min in order to obtain a steady-state current and an average value of this current corresponds to
one data point in figure 5.1. Before the actual dark current test, the high voltage was increased step by
step to burn away any dust inside the gap since the gap was not constructed in a clean room. The dark
current first slowly rises up to about 6000 V. Up to this point there is no gas multiplication and the
gas gap acts as a resistor and the current is determined by the resistance of the spacers. Afterward
there is a very rapid increase due to gas breakdown by ohmic conduction.
14
I2 Gap 1
I1 Gap 1
GHENT-GRPC-002-001
I2 Gap 2
I1 Gap 2
GHENT-GRPC-002-002
12
Ι (μA/m2)
10
8
6
4
2
0
0
2000
4000
6000
8000
HVeff (V)
Figure 5.1: Dark current of the gaps
A linear fit with equation 5.1 was done for the rapidly rising part.
I(V ) =
V − gEc
R
(5.1)
Where V is the applied voltage, g is the gap width, R the resistance of the gap and Ec is a critical
value which corresponds to the value of the electric field that where the extrapolated curve crosses the
HV axis. This critical value of the electric field corresponds to the breakdown of the gas. From [65],
this critical value is found to be E c =5.3 kV/mm. The current I is normalized to the active surface of
the gap which corresponds to 26.4x26.4 cm2 . The slow rising and fast rising part were both fitted with
a linear equation denoted as I1 and I2 for both gaps. The fast rising part was fitted with equation 5.1
from which the gap width can be found. It is found to be 1.19 mm for the first gap and 1.17 mm for
the second gap which is slightly smaller than the desired value of 1.2 mm. Table 5.1 sums up the fitted
parameters.
5.3. DETECTION EFFICIENCY TEST
Parameter
a
b
Parameter
g (mm)
R (GΩ)
41
I1 (V) (µ A/m2 )=a*V+b
Value Gap 1
Value Gap 2
0.000163542 ± 0.00004503 0.00194574 ± 0.00003148
-0.308384 ± 0.2163
-0.284396 ± 0.149
I2 (V) (µ A/m2 )= 5.1
Value Gap 1
Value Gap 2
1.18625 ± 0.006863
1.16906 ± 0.002391
7.9622 ± 0.3451
6.57716 ± 0.1003
Table 5.1: Fitted parameters of the dark current
Since the second gap has a smaller gap width compared to the first gap, its dark current is higher.
Compared to the gRPC of last year, the dark current of both gaps seems to be higher. Furthermore the
transition point between the two different regimes of the current is shifted towards lower high voltage.
The reason for this behaviour is that the gap width is smaller compared to last year and thus the
electric field is higher.
5.3
Detection efficiency test
Once the complete detector was finished, the detection efficiency by means of a cosmic ray test was
determined. During the test, a time to digital converter (TDC) is used to record the arrival times of
the cosmic rays in the scintillators and in the readout strips of the RPCs which are placed in the cosmic
stand for different operating voltages of the gRPC. Two RE/4/3 RPCs were used as additional triggers
and were placed on the bench right below the top scintillator and right above the bottom scintillator.
This improves the tracking of the cosmic rays and allows a more efficient triggering. A C++ script was
written that analyzes the data from the TDC and calculates the detection efficiency. The script selects
the events from the TDC that fulfill certain criteria:
ˆ Only when the hits multiplicity1 in both bottom and top scintillators is one and if there is both a
hit in the top and bottom RE/4/3 RPC, the signal passes through the first selection.
ˆ Only hits from the channels of the scintillators above and below the gRPC are considered since
only these signals will pass through the gRPC.
Events that surpass the above selection criteria are denoted as ”GoodScintillatorTriggers” Ns . Additional
criteria lead to another classification of hits:
ˆ Events that have both a hit in the top and bottom trigger RPCs with a cluster multiplicity2
equal to one are selected.
ˆ The angles of the reconstructed tracks on the detector planes of the scintillators and RE/4/3
RPCs are compared. The difference between these angles should be small if the signals are
generated from the same event.
ˆ Channels from the trigger RPCs above and below the gRPC are chosen.
Events that fulfill these additional criteria are referred to as ”GoodTriggers” Nt . A last set of criteria
determines the ”GoodRPCEvents” Ne :
ˆ Hits that are to early given by the TDC time are considered as electrical noise, while too late
hits are considered as ”muon noise” 3 . This is done by fitting a Gaussian to the time spectrum
and selecting events that are within an interval enclosed by twelve times the standard deviation.
1 The
total number of fired hits.
number of clusters per event.
3 Muon noise means that when a high energetic muon passes it may distort the electric field long enough to create fake
events.
2 The
42
CHAPTER 5. CHARACTERISTICS OF THE PROTOTYPE
ˆ From the position of the clusters in the two trigger RPCs the hit position in the gRPC is
reconstructed. If the difference between this reconstructed position and the position inferred from
the data was too big, this hit is considered as noise.
ˆ Only single cluster events in the gRPC are considered.
The detection efficiency m is calculated as the ratio of the number of ”GoodgRPCevents” to the number
of ”Goodtriggers”.
Ne
m =
(5.2)
Nt
Various calculated values such as noise rates, cluster size, detection efficiency and effective high voltage
3.5 are saved into a CSV file. The detection efficiency m determined by this script is not the actual
detection efficiency a and is corrected for geometrical reasons. The correct placement of the gRPC on
the cosmic bench relative to the scintillator and RPC strips is crucial in obtaining a high efficiency.
A Monte Carlo simulation has been written that simulates cosmic ray events that are distributed
according to cos2 θ with θ the zenith angle. The position of the gRPC relative to the scintillators and
trigger RPCs is varied step by step. For each step the geometrical acceptance α has been calculated
that gives the fraction of cosmic rays that generates a trigger and passes through the gRPC. It is
related to the actual detection efficiency according to equation 5.3.
a =
m
α
(5.3)
Figure 5.2(a) shows the result of the simulation. Initially the gRPC and trigger RPCs are placed with
their front edge on the front edge of the scintillator plane. The front edge of the gRPC and trigger
RPCs is defined as the edge where the readout cables are located. The variable ”setup shift” is the
relative position of the trigger RPC and gRPC to the scintillators while the variable ”gRPC shift” is the
relative position of the gRPC relative to the trigger RPCs and scintillators. The highest geometrical
acceptance has been calculated to be 98% ± 0.02% if the setup shift is -269 mm and the gRPC shift is
+265 mm as illustrated in figure 5.2(b). This is equivalent by letting the gRPC in the same position
and moving the trigger RPCs out of the cosmic benches.
(a) Results of the simulation
(b) Schematic view of the configuration. The
black lines on the RE/4/3 RPC indicate the
three different readout segments.
Figure 5.2: Geometrical acceptance for different configurations
5.3. DETECTION EFFICIENCY TEST
43
For each point of the cosmic ray test, 10000 triggers were used and the discrimination threshold
of the front-end electronics was set to 215 mV. The used front-end electronics were those from the
assembly of RE/4/3 RPCs at Ghent. These used front-end electronics gave however noisy signals. The
efficiency curve has been calculated by discarding strip 5 and 6 due to their high noise rates. The result
of the efficiency scan is depicted in figure 5.3. The error bars take into account both the statistical
error on the efficiency and the systematic error due to the geometrical acceptance. A sigmoid curve as
in equation 3.7 has been fitted to the data and the fit parameters are summarized in table 5.2.
max
a
0.62 ± 0.03
λ (V−1 )
0.007 ± 0.002
HV50 (V)
5411 ± 46
HVW P (V)
5976 ± 89
Table 5.2: Fit parameters of the efficiency curve for the gRPC prototype
Figure 5.4 shows the monitored currents during the cosmic ray test. These currents are slightly
higher than the dark current, nevertheless the same gas mixture and equipment have been used. The
top gap corresponds to the second build gap (GHENT-GRPC-002-002) while the bottom to the first one
(GHENT-GRPC-002-001). Compared to the prototype of last year[56], the efficiency starts to rise closer
to the breakdown point. Since the gap width of this prototype is smaller, the streamer probability
increases and this might explain why the efficiency is not close to 100 % as one would expect. The
smaller gap width also explains the small plateau width. All of this implies that the gas mixture is
probably not optimized for this detector. Therefore the gas mixture should be changed to further reduce
the streamer probability. Compared to a CMS RPC, the working point of the HV is approximately
3300 V lower. However as explained before, a smaller gap width means a higher electric field. The
power consumption of the prototype at its operating voltage is about 5 mW/m 2 which is still within
the requirements of CMS.
1
0.9
fit
data
0.8
Efficiency εa
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
4000
4500
5000
5500
6000
6500
HVeff (V)
Figure 5.3: Actual detection efficiency of prototype.
7000
7500
Current (µA/m2)
44
CHAPTER 5. CHARACTERISTICS OF THE PROTOTYPE
17
top gap
16
bottom
gap
15
14
13
12
11
10
9
8
7
6
5
4
3
5000
5500
6000
6500
7000
HVeff (V)
Figure 5.4: Measured currents of the top and bottom gap.
7500
Chapter 6
CMS RPC characterization
RPCs operated in avalanche mode need preamplifiers to amplify the small signals as noted in section
3.7. The aging effects of RPCs can be reduced by reducing the total amount of integrated charge.
This however requires more sensitive electronics. A lower charge per count also means that the rate
capability increases since the gap can recharge faster after an ionizing event. A high detection efficiency
requires good amplifiers that detect the smallest signals. To conclude: more sensitive electronics are
a good improvement towards the performance of a gRPC operated in a high radiation environment.
Before starting to test new front-end electronics, a good knowledge of the experimental setup and the
to be tested RPC is required. Any source of noise must be identified or removed if possible. Only than
a comparative study of the CMS front-end electronics with the new front-end electronics can be done
successfully. Accordingly, several preliminary tests were done on the to be tested CMS RPC.
6.1
Experimental setup
All tests were performed at building 904 at the CERN Prevessin site. The test room is ventilated
to keep temperature and humidity constant. An RE/2/2 CMS RPC was used for the measurements.
Only 8 out of 32 readout strips were used during the tests and are located on one edge of the RPC in
partition B (one of the three η segments)[66]. The power supply for the high voltage was delivered by
a CAEN SY1527 system and the usual gas mixture of CMS was used. From previous measurements
on this RPC, an unknown source of noise was detected through an oscilloscope. To investigate this
problem more deeply, the front-end electronics were disconnected, to eliminate any noise coming from
the electronics. The readout cables were connected with another set of cables to an oscilloscope. These
cables were checked for noise by using a dual timer module (2255B) that sends logic pulses at regular
time intervals. Any unusual shape was reported and only cables that gave good logic pulses were used.
Next, the signals coming from the RPC at an operating voltage of 9500 V were checked through the
oscilloscope. Later on, the signals coming from the RPC were send through a discriminator module
(LeCroy 612 CL). This module produces a logic pulse when the analog signal (NIM signals) coming
from the RPC crosses a preset threshold (in this case around 34 mV). Further ahead, measurements of
the strip rates were done for several values of the high voltage. The output of the discriminators was
connected to CAEN N145 scalers. These modules count the number of events during a time interval
of 300 s set by the dual timer. After each measurement, the number given by these scalers has to be
manually written down. Pressure and temperature were monitored by a digital weather station to
correct the HV to HVef f using equation 3.5 with a reference pressure of P0 =965 mb and temperature
of T0 =293K. Before the start of each measurement, 5 minutes were interleaved to stabilize the current
in the gaps. Coincidence measurements of strips were done by using coincidence modules (LeCroy
465). The experimental setup is depicted in figure 6.1 where the RE/2/2 RPC and two out of three
scintillators can be seen. These scintillators have been used as external triggers. The third scintillator
is located below the RPC.
45
46
CHAPTER 6. CMS RPC CHARACTERIZATION
Figure 6.1: Experimental setup for the CMS RPC together with the scintillators. The third scintillator is
located below the RPC.
6.2
Scintillator characterization
To determine the efficiency of the RPC, triggers are needed. The efficiency of the RPC is calculated
as the ratio of the number of events registered by the RPC to the number of triggers. As triggers,
scintillators were used that were read out by photomultiplier tubes. Three scintillators (labeled as S1,
S2 and S3) were tested to find their optimum operating voltage. The output of the PMTs was connected
to discriminators with a threshold that was initially set at 35.2 mV. The number of events registered
was read out by scalers during a time window of 100s. The rate dependence of the scintillators on the
HV shows first a rapid increase followed by a plateau and increases further on. This latter increase is
due to regeneration effects (discharges, afterpulsing) [67]. The scintillators are operated on this plateau
to ensure a minimum variation in the rates due to drift in the PMT gain. S2 shows a clear plateau
at around 1930 V. For S1, this behaviour was less clear. From the output of the oscilloscope, it was
observed that this scintillator gave noisy signals. Accordingly, the threshold of the discriminator was
first changed to 60 mV. Even then, fluctuations made the determination of a plateau difficult because
of the low rate. To obtain better statistics, the time window was changed to 300 s. The signals from
the third scintillator were even more noisy. The threshold of the discriminator has been set to 100 mV
with a time window of 100 s to avoid the same problems as before. The results of the measurements
are shown in figure 6.2 and are summarized in table 6.1. The large error bars for scintillator S1 are
mainly because of the low count rate.
Scintillator
S1
S2
S3
Operating voltage (V)
1820
1930
1950
Discriminator threshold (mV)
36
90
90
horizontal dimensions (cm2 )
4.2x76
9.4x32
9.4x32
Table 6.1: Characteristics of the scintillators
Once the operating voltage of the three scintillators was found, the optimum threshold of the
discriminators has been determined. The scintillators were set at their operating voltage according to
the previous measurements. The rate was measured for each of them during 100 s for several values
of the discriminator threshold. For S2 and S3 a slowly decreasing plateau was found around 90 mV.
The scintillator S1 required an efficiency scan to determine the appropriate threshold since the low
rate made reliable results difficult. Scintillators S2 and S3 were used as triggers and S1 was placed
perpendicular to S2 and S3. Figure A.3 shows the used configuration. The events recorded by S2
and S3 in coincidence are the triggers, referred to as Nt . Together with the coincidence of S1 gives
the number of events, denoted as Ne . Likewise as in chapter 5, the measured detection efficiency m
Normalized rate
6.3. RPC CHARACTERIZATION
15
14
13
12
11
10
9
8
7
6
5
4
3
2
1
0
47
S1
1780 1790 1800 1810 1820 1830 1840 1850 1860 1870
Operating voltage (V)
(a) Rate of scintillator S1
4.5
4
S2
S3
Normalized rate
3.5
3
2.5
2
1.5
1
0.5
0
1700 1750 1800 1850 1900 1950 2000 2050 2100 2150
Operating voltage (V)
(b) Rate of scintillator S2 and S3
Figure 6.2: Signal rates of the scintillators normalized to the working voltage as given in table 6.1.
has been determined as in equation 5.2. The geometrical acceptance factor α has been found to be
0.13 form the common area of the scintillators. From the geometrical acceptance factor α, the actual
detection efficiency a has been calculated as in equation 3.7. The actual detection efficiency of S1
is displayed in figure 6.4. Each point corresponds to a measuring time of 600 s. The curve shows at
low values of the discriminator threshold, large fluctuations. Furthermore, the efficiency attains values
above 100 %. This behaviour can be accounted to noise. Since at low values of the threshold, the
signals arising from noise are also converted to a logic pulse.
6.3
RPC characterization
The observed signals coming directly from the RPC without the front-end electronics showed multiple
subpeaks and a long relaxation time. The mean of 128 signals coming from strip 5 is shown in figure
6.5(a). All the signals indicate the same behaviour: first a rapid rise time, then a slowly relaxation
time with one or multiple subpeaks superimposed. These subpeaks have a delay of a few ns. This is
problematic since one event can trigger multiple signals. Moreover, these signals are quite large (in case
of figure 6.5(a) around 60 mV) compared to the signals arising from cosmic rays (only 3 mV) as shown
in 6.5(b). Some of these signals even reached beyond 500 mV. To further suppress sources of noise, the
48
CHAPTER 6. CMS RPC CHARACTERIZATION
3.5
S2
S3
3
Normalized rate
2.5
2
1.5
1
0.5
0
40
60
80 100 120 140 160 180 200 220 240 260 280
Discriminator threshold (mV)
Figure 6.3: Signal rates of the scintillators normalized to the operating discriminator threshold.
2.2
Actual detection efficiency εa
2
1.8
1.6
1.4
1.2
1
0.8
0.6
0.4
0.2
0
30
35
40
45
50
55
60
65
70
75
80
85
Discriminator threshold (mV)
Figure 6.4: Efficiency scan of scintillator S1
readout strips coming out of the RPC were shielded with Mylar foil and tape. Possible explanations
for this behaviour might be due to reflections in the readout strips since they are not terminated on
one end. Sources of noise could also lead to these additional peaks. Further investigation was done by
searching for sources of electrical noise. The power supplies were switched off and were grounded with
cables. However, no improvements were observed.
The rate of the discriminated signals from each of the eight channels were monitored for several
values of the HV. They all show a decreasing exponential behaviour for decreasing HV indicating that
the signals arise from gas multiplication. Investigation of cluster sizes and individual strip rates was
done by adding the coincidence modules. Strip 2,3 and 4 were connected in coincidence. From these
measurements, the single strip rates (i.e. the rate of one strip without any hits in adjacent strips),
double and triple strip rates were calculated. These measurements were done with both gaps powered
on and for each single gap. The observed rate in the top gap is about a factor of two larger compared
to the bottom gap. To make a better estimate of the single rates of strips 2 and 4, we included strips 1
and 5. A first set of measurements allowed us to directly read the single cluster rates of strip 2, strip 4
and strip 2&3&4 with the used scheme illustrated in figure A.1. The first figure gives the rate of strip 2
only. The signals from the adjacent strips (in this case 1 and 3) are used as anticoincidence. An extra
coincidence module was used to extent the width of the signal of strip 3 since the ”real” signal from
strip 3 was also needed. The same scheme was used to determine the single strip rate for channel 4. A
6.3. RPC CHARACTERIZATION
(a) Mean of 128 signals of strip 3
49
(b) Mean of 128 signals of strip 5 using scintillators S2
and S3 in coincidence
Figure 6.5: Example of an analog signal of a RPC at an operating voltage of 9500 V.
last configuration was used for the single rate determination of strip 2, 3 and 4 together. In that case,
the signals from strip 1 and 5 were used as anticoincidence. To match the signal from strip 2,3 and
4 within the time window of the anticoincidence signal (150 ns wide), longer cables were used (16 ns
long) to delay them. Another set of measurements provided the single rates of strip 3, 2&3 and 3&4.
The used schematic is shown in figure A.2.
The results from the measurements showed that the rate of strip 3 is quite high in comparison with
the other strips. The rates of the strips to the total number of independent event rates are shown in
figure 6.6. To check the rate dependence on the stability of the currents, the rate was monitored during
Figure 6.6: Single,double and triple strip rate percentages of a dark current measurement.
time intervals of 100 s right after the high voltage reached its preset value of 9.5 kV. No significant effect
was found. From the measurements of the strip rates, a rough estimate of the cluster size was calculated
by using the occurrence of single,double and triple strip rates. The cluster size at the operating voltage
around 9500 V is 1.65 ± 1.22 and is shown in figure 6.9(a). It is however important to note that no
electronics were used or any external triggers. The signals are coming from the dark current only since
the muon signals are too weak to be converted by the discriminators.
A second estimate of the cluster size was done by adding the three scintillators as external triggers.
50
CHAPTER 6. CMS RPC CHARACTERIZATION
S2 was put below the RPC, while S1 on top. The third scintillator was put on top of S1. The signals
from strips 4 to 7 were connected through the oscilloscope. During the measurements, the HV of the
RPC has been kept constant at an operating voltage of 9500 V. Each time a signal was triggered, it
was visually checked whether the signals from each individual strip crossed the threshold of 1 or 2 mV.
From the intensity of these signals, it was estimated whether the neighbouring strips 3 and 8 could
also be fired. The mean cluster size over 25 events was found to be 2.52 ± 0.76 and 2.08 ± 0.80 for a
threshold of 1 mV respectively 2 mV. In contrary to the previous estimate, these signals arise from
muons only.
Finally the front-end electronics were connected and an efficiency scan test was conducted with a
threshold of the front-end electronics set to 220 mV. The rate of strip 3 to 8 together with the logic OR
of the 6 strips with triggers have been monitored. For each point of the HV, 1000 triggers have been
used. The efficiency is calculated as any triggered hit in one of the six strips to the total number of
triggers. A third estimate of the cluster size has been calculated by taking the ratio of the sum of the
individual strip counts to the total number of independent events. The results of the efficiency and the
cluster size are depicted in figure 6.7 and figure 6.8. The sigmoid curve of equation 3.7 has been fitted
to the efficiency and is clearly visible. The resulting fit parameters are given below in table 6.2. The
decrease of the efficiency at higher values of the HV can be accounted by the presence of streamers.
Compared to previous tests on this RPC[56], the results given by table 6.2 do not differ that much
which means that the RPC still gives reliable results.
max
0.975 ± 0.005
λ (V−1 )
0.0124 ± 0.0003
HV50 (V)
9194 ± 3
HVW P (V)
9582 ± 3
Table 6.2: Fit parameters of the efficiency curve for the RE/2/2 RPC
The estimation of the cluster size in figure 6.8 shows an increase starting from around 9200 V. This
behaviour can be accounted to the increased charge that fires multiple strips. Around the working point
of the RPC, the cluster size is approximately equal to 2 which is slightly too high for CMS requirements
[41]. The second cluster size estimation is consistent with this value. For HV < 9000 V where the
efficiency is very low, the cluster size is quite large and this might be due to external noise. The large
error bars indicate the low rate. To further comprehend the noisy signals, the rate was also monitored
without the coincidence of the scintillators. These rates reached values higher than 250 Hz which is
quite high. The cluster size has been estimated from this dark current as shown in figure 6.9(b). The
behaviour is quite different since the cluster size decreases with increasing HV. From these estimations
of the cluster size, it can be concluded that the behaviour of the muons only is quite different compared
to the dark current.
1
0.9
data
fit
Detection efficiency ε
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
8000 8200 8400 8600 8800 9000 9200 9400 9600 9800 10000 10200
HVeff (V)
Figure 6.7: Calculated efficiency of the RE/2/2 RPC with front-end electronics connected.
Cluster size
6.3. RPC CHARACTERIZATION
51
6.5
6
5.5
5
4.5
4
3.5
3
2.5
2
1.5
1
0.5
0
8000 8200 8400 8600 8800 9000 9200 9400 9600 9800 10000 10200
HVeff (V)
Figure 6.8: Estimation of the cluster size using the front-end electronics. One point gave no counts and thus
is not visible on the plot.
3
Cluster size
2.5
2
1.5
1
0.5
0
8200
8400
8600
8800
9000
9200
9400
9600
9800 10000 10200
HVeff (V)
(a) Estimation without front-end electronics
3.5
Cluster size
3
2.5
2
1.5
8000 8200 8400 8600 8800 9000 9200 9400 9600 9800 10000 10200
HVeff (V)
(b) Estimation with front-end electronics but without triggers
Figure 6.9: Estimations of the cluster size from the dark current.
52
CHAPTER 6. CMS RPC CHARACTERIZATION
Chapter 7
Conclusions
For the second Phase of the upgrade of CMS, new detectors need to be installed in the high pseudorapidity
region that sustain the higher particle rates. Glass Resistive Plate Chambers with semiconductive glass
are a possible candidate. This type of glass shows limited aging effects in a high radiation environment.
Before constructing a full size RE/4/1 gRPC, a smaller prototype with float glass was constructed to
get familiar with the construction procedure. Its design follows as close as possible the one of CMS
RPCs to make a comparative study easier. Since semiconductive glass is only available in limited
dimensions, the glass plates have to be glued to make a full size detector. An important criteria to this,
is the alignment of the glass plates which should be uniform as possible to obtain a constant efficiency
over the whole active area. A method was used to obtain a good alignment but it is still not sufficient
enough. An improved gluing procedure was suggested for further study. The resistive coating on top of
the plates consists out of a mixture of two paints with different resistivity. The mixture was chosen
to reach a surface resistivity between 0.1 and 1 MΩ/. The resistive coating was applied by use of
a spray painter. Although spray painting is fast and easy to use, it still needs further improvements
towards a uniform coating. The design of the prototype compared to the prototype of last year, was
further improved by implementing spacers that regulate a uniform gas distribution of the gas gap.
Different gas adapters were used to incorporate the edge spacers and the segmentation of the readout
strips was refined. The choice of material was chosen for radiation hardness. Next to the construction
of the double gap gRPC, a multi-gap gRPC was constructed. This structure has an even better time
resolution and rate capability than a double gap structure. Although most steps in the construction
are the same, different spacers were used for this multi-gap structure. To characterize the constructed
gRPC, both a dark current and efficiency scan test were conducted at the UGent RPC laboratory. The
dark current test gave an estimated gap width slightly smaller than 1.2 mm. For the efficiency scan,
a program was written to calculate the optimum geometrical acceptance of the experimental setup.
Compared to the previous prototype, the gap width is smaller leading to a lower efficiency due to an
increased streamer probability. To obtain a higher efficiency, the CMS gas mixture should be adjusted
to further decrease the streamer probability. The next step in this study is to construct a RE/4/1 gRPC
that should be tested for its rate capability. The larger dimensions of this detector requires a large
oven to cure the paint. Considering the segmentation and weight of the glass plates, alignment will be
the most important issue. Apart from the gRPC construction and characterization, some preliminary
tests were done for new front end electronics. More sensitive electronics allow to detect smaller signals
resulting in a lower detection efficiency. The first tests withheld the characterization of the tested RPC
and corresponding trigger PMTs. An unknown source of noise has been discovered giving an unusual
behaviour of the cluster size. Nevertheless, the test with CMS front-end electronics showed a perfectly
normal efficiency curve together with a cluster size which is slightly too high.
53
54
CHAPTER 7. CONCLUSIONS
Chapter 8
Nederlandse Samenvatting
Voor de tweede fase van de upgrade van de CMS detector aan de Large Hadron Collider wordt verwacht
dat het aantal gecreëerde deeltjes tijdens de proton-proton botsingen zal toenemen. In de regio dicht bij
de deeltjesstraal zal het tempo van inkomende deeltjes zeer hoog zijn voor het muonsysteem. Daarnaast
zal de opgenomen lading van de detectoren zeer hoog zijn wat kan leiden tot een daling van de efficiëntie.
Een mogelijke detector die deze problemen kan beperken is een glass resistive plate chamber (gRPC).
Deze bevat halfgeleidend glas dat het hoge tempo en geaccumuleerde lading van de deeltjes kan
weerstaan. Vooraleer een volledige RE/4/1 gRPC te bouwen, werd een smaller prototype met vlakglas
gebouwd om de constructie beter onder handen te krijgen. Aangezien halfgeleidend glas slechts in
beperkte afmetingen kan geproduceerd worden, moeten deze glasplaten aan elkaar gelijmd worden om
een volledige RE/4/1 gRPC te construeren. Dit vormt een eerste aspect naar de constructie van een
gRPC. Aangezien de detectie efficiëntie afhangt van de breedte van de holte tussen de twee platen, is het
belangrijk dat de glasplaten tijdens het lijmen goed gealigneerd zijn. Een methode werd gesuggereerd
om deze alignering te verbeteren. Echter, verder onderzoek is nodig. Een tweede stap in de constructie,
is het aanbrengen van een resistieve coating. Deze bestaat uit een mengeling van een geleidende en
resistieve verf en werd aangebracht met behulp van een luchtdrukpistool. De verhouding van het
mengsel werd zo gekozen dat het elektrisch veld na een ioniserend evenement voldoende snel herstelt.
Terzelfdertijd dient de omvang van de ontlading beperkt te blijven om een goede positieresolutie te
behouden. Deze coating is nodig voor het aanbrengen van de hoogspanning van de resistieve platen
en dient zo uniform mogelijk te zijn voor het bereiken van een uniforme detectie-efficiëntie. Alhoewel
het luchtdrukpistool niet de beste uniformiteit biedt, kan deze verbeterd worden met behulp van
mechanische toestellen. De verdere constructie hield het aanbrengen van gasingangen en een gassysteem
in voor het uniform verspreiden van het inkomende gasmengsel. De gebruikte materialen werden zo
gekozen om het hoge stralingsniveau te kunnen weerstaan. Naast de constructie van deze gRPC werd
ook nog een multi-gap gRPC gebouwd. Dit ontwerp heeft het voordeel van een betere tijdsresolutie
te hebben en nog beter te kunnen weerstaan aan het hoge tempo van inkomende deeltjes. Gezien de
universiteit van Gent in het verleden bijgedragen heeft aan de constructie van RPCs, werd de constructie
en karakterisering van het prototype uitgevoerd binnen het UGent RPC-labo. De resultaten van de
efficiëntietest en donkerstroomtest tonen aan dat de breedte van de holte tussen de platen kleiner is dan
verwacht. Dit heeft als gevolg dat het elektrisch veld hoger is wat leidt tot sterkere ontladingen. Om dit
te vermijden moet het gasmengsel verder aangepast worden om deze grote ontladingen (de streamers)
te vermijden en zo een hogere effciëntie te bekomen. De volgende stap in deze studie is het construeren
van een volledige RE/4/1 gRPC met halfgeleidend glas. Deze dient vervolgens getest te worden om het
hoge aantal inkomende deeltjes te kunnen weerstaan. Gezien de omvang en segmentatie van dit model,
zal de alignering van de glasplaten één van de belangrijkste aandachtspunten zijn in de constructie.
Naast de constructie en karakterizering van een gRPC, werden een aantal voorbereidende tests gedaan
voor het testen van nieuwe elektronica. Het gebruik van gevoeligere elektronica zal toelaten om de
efficiëntie van RPCs te verbeteren. Gezien deze elektronica zal toestaan om kleinere ladingen waar
te nemen, zal dit ook leiden tot een vermindering van verouderingseffecten van RPCs. Tijdens deze
metingen werd een karakterisering gedaan van de experimentele opstelling ten einde een vergelijkende
55
56
CHAPTER 8. NEDERLANDSE SAMENVATTING
studie van de oude en nieuwe elektronica mogelijk te maken. Een onbekende bron van ruis werd
ontdekt die ongewone karakteristieken vertoont aangaande de clustergrootte. Desondanks leverde de
efficiëntie-meting met de CMS electronica een betrouwbaar resultaat op. Verder leverde deze meting
een clustergrootte op die te hoog is voor de vereisten van CMS.
Appendix A
CMS RPC characterization
The following figures depict the used schemes in oder to determine the single strip rates of the tested
RPC.
(a) Coincidence of strip 2,3̄,1̄
(b) Coincidence of strip 4,3̄,5̄
(c) Coincidence of strip 2,3,4,1̄,5̄
Figure A.1: Schematic view of the processing of the discriminated signals.
57
58
APPENDIX A. CMS RPC CHARACTERIZATION
(a) Coincidence of strip 2,3,4̄ and 1̄
(b) Coincidence of strip 3,2̄ and 4̄
(c) Coincidence of strip 3,4,2̄ and 5̄
Figure A.2: Schematic view of the processing of the discriminated signals.
Figure A.3: Used configuration for determination of the efficiency of scintillator S1.
List of Figures
2.1
2.2
2.3
2.4
Energy loss of muons in Cu normalised to the mass density of the target
A slice of the CMS detector . . . . . . . . . . . . . . . . . . . . . . . . .
Layout of the CMS detector . . . . . . . . . . . . . . . . . . . . . . . . .
Simulated RPC trigger efficiency for three and four RPC stations[11] . .
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3.1
3.2
3.3
3.4
3.5
3.6
3.7
3.8
3.9
3.10
3.11
Schematic view . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Resistivity vs. transferred charge . . . . . . . . . . . . . . . . . . .
Behaviour of the density current through time[16] . . . . . . . . . .
Effect of adding SF6 to the efficiency and streamer probability[24]
Charge spectra for a narrow and wide gap[14] . . . . . . . . . . . .
Different gap structures . . . . . . . . . . . . . . . . . . . . . . . .
Diagram of one channel of the front-end circuit . . . . . . . . . . .
Illustration of the zero crossing technique:[36] . . . . . . . . . . . .
Design of the CMS gaps[42] . . . . . . . . . . . . . . . . . . . . . .
Double gap structure of the Forward RPCs[42] . . . . . . . . . . .
View of the CMS detector for the first upgrade . . . . . . . . . . .
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13
16
16
18
18
20
21
22
22
23
23
4.1
4.2
4.3
4.4
4.5
4.6
4.7
4.8
4.9
4.10
4.11
4.12
4.13
4.14
4.15
4.16
Cross-sectional view of the double gap CMS-like gRPC . . . . . . . . . .
Glass resistive plate chamber construction . . . . . . . . . . . . . . . . .
Resistivity measurements of the coating . . . . . . . . . . . . . . . . . .
A sketch of the four glued glass plates and its subdivision . . . . . . . .
Measured surface resistivity for one plate off all positions through time.
Schematic view of the gas flow . . . . . . . . . . . . . . . . . . . . . . .
Gas inlet . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Construction of the gas gap . . . . . . . . . . . . . . . . . . . . . . . . .
High voltage connection . . . . . . . . . . . . . . . . . . . . . . . . . . .
The readout strips with readout wires . . . . . . . . . . . . . . . . . . .
Design of the prototype casing . . . . . . . . . . . . . . . . . . . . . . .
The double gap gRPC in its metal casing . . . . . . . . . . . . . . . . .
Cross-sectional view of the mRPC . . . . . . . . . . . . . . . . . . . . .
Top view of the multigap gRPC . . . . . . . . . . . . . . . . . . . . . . .
Adapter and spacers used for the gmRPC . . . . . . . . . . . . . . . . .
Illustrated method to improve the glass plate alignment . . . . . . . . .
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25
26
28
30
30
31
31
32
33
33
34
34
35
35
36
37
5.1
5.2
5.3
5.4
Dark current of the gaps . . . . . . . . . . . . . . .
Geometrical acceptance for different configurations
Actual detection efficiency of prototype. . . . . . .
Measured currents of the top and bottom gap. . .
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40
42
43
44
6.1
6.2
6.3
6.4
Experimental setup for the CMS RPC . . . . . . . . . . . . . . . . . . . . . . . . . . .
Signal rates of the scintillators normalized to the working voltage as given in table 6.1.
Signal rates of the scintillators normalized to the operating discriminator threshold. .
Efficiency scan of scintillator S1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
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47
48
48
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60
LIST OF FIGURES
6.5
6.6
6.7
6.8
6.9
RPC signal . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Strip rate percentages of the RE/2/2 dark current . . . . . . . . .
Calculated efficiency of the RE/2/2 RPC with front-end electronics
Estimation of the cluster size using the front-end electronics. . . .
Estimations of the cluster size from the dark current. . . . . . . . .
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49
49
50
51
51
A.1 Schematic view of the processing of the discriminated signals. . . . . . . . . . . . . . . . 57
A.2 Schematic view of the processing of the discriminated signals. . . . . . . . . . . . . . . . 58
A.3 Used configuration for determination of the efficiency of scintillator S1. . . . . . . . . . . 58
List of Tables
2.1
2.2
Fermions in the Standard Model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Bosons in the Standard Model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.1
Bulk resistivity of plate materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
4.1
4.2
4.3
4.4
4.5
Used items for cutting and gluing the glass sheets. . . . . . . . .
Fitted parameters of the power law described by equation 4.2. . .
Ratio of the mass of the conductive to the total amount of paint
Surface resistivity of the graphite coating for the four used plates
Materials used for the spacers and the gas inlet adapter . . . . .
5.1
5.2
Fitted parameters of the dark current . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41
Fit parameters of the efficiency curve for the gRPC prototype . . . . . . . . . . . . . . . 43
6.1
6.2
Characteristics of the scintillators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46
Fit parameters of the efficiency curve for the RE/2/2 RPC . . . . . . . . . . . . . . . . 50
61
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4
4
27
28
28
30
32
62
LIST OF TABLES
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[61] Ceramic zirconia oxide: http://www.sceram.com/en/68-ZIRCONIA_OXIDE.html;
[62] EPO-TEK EE129-4 http://www.rikei.co.jp/epo-tek/pdf/EE129-4.PDF;
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