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Triple-GEM detectors for the
innermost region of the LHCb muon
apparatus
M. Poli-Lener
LNF-INFN
on behalf of the LHCb-GEM group
 Introduction
 GEM: principle of operation
 Detector Performances
 Detector construction
 Conclusion
M. Poli Lener LNF
Catania 31 March 2004
LHCb: GEM detector in M1R1
(*)
Muon detector (5 stations):
L0 high pT trigger + offline muon ID
M1-Station
M2-M5
Station
RICH
Muon station: front view
Vertex
Tracker
E+H Cal
(*) the first GEM detector at LHC
M. Poli Lener LNF
All equipped with MWPC but for M1R1
station  triple-GEM detector
12 Chambers, area ~ 1 m2, but ~20%
of triggered muons !!!
challenging for:
aging, st, cluster size & rate
Catania 31 March 2004
Gas Electron Multiplier
A Gas Electron Multiplier (*) is made by 50 m thick kapton
foil, copper clad on each side and perforated by an high
surface-density of bi-conical holes.
70 µm
M. Poli Lener LNF
140 µm
50 µm
(*) F.Sauli, NIM A386 1997 531
Catania 31 March 2004
GEM foil construction
Copper 5 m
Kapton 50 m
Commercially available
(Dupont)
U.V.
Photo-resist
Mask
Double-mask
photolitographic process
Copper etching by
chemical solution
50 m
70  m
M. Poli Lener LNF
Kapton etching using
the copper as a mask
Catania 31 March 2004
Gas Electron Multiplier
Filed lines
By applying a voltage
between the two copper
sides an electric field as
high as 100 kV/cm is
produced in the holes
acting as multiplication
channels.
Voltage ranging between
400 - 500 V
M. Poli Lener LNF
Cathode
conversion
and drift
amplification
induction
Anode Readout
Catania 31 March 2004
GEM operation: electrons
Time
Time =
~3
~6
ns
ns
~1
ns
Time
== 0
ns
electron cluster
ions
some clusters are collected
on the
side
the
first
cluster
starts
one bottom
cluster
couldof
bethe
GEM
or multiplication
drifted
toward to
the
collected
by copper
the next electrode
M. Poli Lener LNF
Catania 31 March 2004
GEM operation: ions
Time == ~10
~1 s
Time
ns
electron cluster
ions
most ions are collected
the
ions
versus
on the
upper
side
of
the
now
onlydrift
ions remain
upperside
GEMgem
or drifted
toward the
inside
the holes
cathode
The ions are completely removed from the hole after 1 s
high rate capability
M. Poli Lener LNF
Catania 31 March 2004
Single vs triple GEM
Measurements with alfa particle
With a single
GEM
is possible reach
gains ~103
but with “high”
discharge
probability
M. Poli Lener LNF
Ar/C02=70/30
Catania 31 March 2004
Triple-GEM detector
Detector peculiarities:
 The regions of conversion,
multiplication and signal induction are
physically distinct
 freedom in readout design choice;
 Signal is purely due to the motion of
electrons in the induction region
 no ion tail, fast signal;
 Ions are quickly removed from
multiplication region
 high rate capability
 Multiplication is divided in 3 steps
 robustness to discharges;
Small angle
 Light detector ( 3 ‰ X0 /GEM foil)
Cartesian
M. Poli Lener LNF
Pads
Catania 31 March 2004
Detector Requirements in LCHb





Rate Capability
Station Efficiency
PAD Cluster Size
Radiation Hardness
Chamber active area
up to 0.5 MHz/cm2
> 96% in a 20 ns time window (*)
< 1.2 for a 10x25 mm2 pad size
~ 1.6 C/cm2 in 10 years (**)
20x24 cm2
Main tasks:
 improve time resolution
 obtain good gain uniformity and
safe operation mode
gas mixture,
detector geometry
(*) A station is made of two detectors “in OR”, pad by pad.
This improves time resolution and provides some redundancy.
(**) Estimated with 50 e-/particle at 184 kHz/cm2 with a gain of ~ 6000
M. Poli Lener LNF
Catania 31 March 2004
Rate capability
Normalized gain (a.u.)
LHCb M1-R1 maximum rate
500 kHz /cm2
Beam Flux (Hz/cm2)
M. Poli Lener LNF
A very good gain stability was
found up to a photon flux of
about 50 MHz/cm2
M. Poli Lener, “Studio e sviluppo di un rivelatore a GEM per la zona
centrale delle camere a muoni di LHCb”, Degree Thesis 2002
Catania 31 March 2004
Gas mixture choice
The intrinsic time spread (for full efficiency
on single electron):
s(t) = 1/nvdrift
n number of primary clusters per unit length
vdrift drift velocity in the ionization gap
fast & high yield gas mixture
M. Poli Lener LNF
Ar/CO2
(70/30)
Ar/CO2/CF4
(60/20/20)
Ar/CO2/CF4
(45/15/40)
Ar/CF4/isoC4H10
(65/28/7)
Drift
Velocity
7
(@3 kV/cm)
9
(@ 3 kV/cm)
10.5
(@ 3.5 kV/cm)
11.5
(@ 2 kV/cm)
clusters
/mm
3.3
5
5.5
5.7
Catania 31 March 2004
Single chamber time resolution
9.7ns
4.5ns
5.3ns
4.5ns
Considerable improvement,
respect to the Ar/CO2=70/30
gas mixture (10 ns rms),
is obtained with the new CF4
based gas mixtures, which
allow to reach time resolutions
better than 5 ns rms
Our Choice:
Ar/CO2/CF4 45/15/40
Fast & Non-flammable
M. Poli Lener LNF
G.Bencivenni et al., NIM A 518 (2004) 106
Catania 31 March 2004
OR chamber:
time resolution & efficiency
Required efficiency in 20 ns
time window is achieved with
2 chambers in “OR”
PAD Cluster Size limit
2.9ns
 80 V
Required OR efficiency
A wide working region of  80 V is obtained
M. Poli Lener LNF
Catania 31 March 2004
Aging Studies (*)
Local Aging: performed with a high intensity X-ray tube;
irradiated area of about 1 cm2 (~ 5000 GEM holes).
Integrated charge 4 C/cm2  25 LHCb years.
Large Area Aging: performed with positive hadrons at the
PSI M1 beam line, with an intensity up to 300 MHz;
irradiated area of about 15 cm2.
Integrated charge 0.5 C/cm2  3 LHCb years.
Global Aging: performed at ENEA Casaccia with a 25 kCi
60Co source. Detectors were irradiated at 0.5  16 Gray/h.
Integrated charge up to 2 C/cm2  12.5 LHCb years.
(*)M.
M. Poli Lener LNF
Alfonsi et al., “Studies of etching effects on triple-GEM
detectors operated with CF4 based gas mixtures”, presented at
Catania 31 March 2004
ROME 2004 IEEE conference and submitted to TNS
Chamber construction & tools
All construction operations are performed in clean room (class 1000)
1
Construction steps:

Frames & components preparation

GEM framing with foil stretching

Chamber assembly

Coupling of the two chambers
3
2
6
4
5
GEM foil stretching
M. Poli Lener LNF
Catania 31 March 2004
GEM FRAMING: stretching
GEMs that pass the HV test (see quality
control) are stretched with a specific
tool.
The foil is clamped with jaws equipped
with plastic O-ring.
18 cm
M. Poli Lener LNF
Mechanical tension (18kg/jaw
 20 MPa) applied to the
edge of the foil is monitored
with gauge meters.
Kapton creep is negligible for
this mechanical tension (see
http://www.dupont.com)
Catania 31 March 2004
Chamber quality control
Before the construction several checks are
performed on:
 Panels (cathode and pad PCB)
 GEM foil
Tests on assembled detector concern:
 Gas chamber leakage
 Gain uniformity with X-ray
M. Poli Lener LNF
Catania 31 March 2004
Checks on panel
3-D machine
All panels will be checked for planarity with
a 3-D machine at LNF. The measure will be
performed on a grid of 35 points.
The planarity requirement is ≤ 50 µm
Preliminary measurements on panel preproduction  better than 30 µm
GEM foil HV test
GEM foils will be tested before frame
gluing. The test, sector by sector, is
performed in a gas tight box, flushed for
about 1 hour with nitrogen to reduce the
humidity level @ < 5%, before the GEM
test
M. Poli Lener LNF
Catania 31 March 2004
Gas leak test
The gas leak rate measurement of a chamber is referred to that one of a
reference chamber (same volume, “no leak”) to take into account for atmospheric
pressure and temperature variations.
Both test and reference chambers are inflated in
parallel, up to an overpressure of  5 mbar.
N2
Ref chamber
Patm
Chamber to test
S1
M. Poli Lener LNF
T, P
S2
The difference between
P(S1) e P(S2) measures
the gas leak rate of the
test chamber
foam box for thermal insulation
< 1 mbar/day
 RH 50 ppmV with
a 80 cc/min gas flow.
Ambient parameters (T and P) are
monitored with additional sensors
Catania 31 March 2004
X-ray test
Gain uniformity, pad by pad, is measured with a high intensity 5.9 keV X-ray tube
X-Y plane moved with
step-motors
Gas humidity and temperature are
monitored with a probe mounted on the
gas line outlet.
Also atmospheric pressure is recorded
collimator diameter  5 mm
A gain uniformity  10% , including edge
effects (6% without)
M. Poli Lener LNF
Catania 31 March 2004
FEE board: CARIOCA & Cardiac GEM
The CARDIAC FEE boards consist of:
-2 CARIOCA chips for amplification and discrimination of the signals
of each chamber;
- 1 DIALOG chip for logic OR of the coupled channels and digitization
of the output.
The features of the CARDIAC are:
- each channel threshold is adjustable
I2C
LV
- remote control via caenet interface
Specific modifications respect to the standard muon
station FEE are developed for the GEM detectors.
top
LVDS
Signal is purely due to the motion of electrons:
 NO ion tail cancellation circuit
M. Poli Lener LNF
bottom
Catania 31 March 2004
Conclusions
 GEM detectors are suitable to operate in the harsh
environment around the beam pipe of LHCb;
 The mechanical design of the detector does not
present critical points;
 Tools, construction procedures and quality checks are
well defined;
 The chamber construction will start in may 2005.
M. Poli Lener LNF
Catania 31 March 2004
M. Poli Lener LNF
Catania 31 March 2004
GEM Chamber Layout
GEM Chamber top view
Cathode Panel
gas fitting
gas inlet
PAD Panel
7 mm
Cathode Panel
Honeycomb
G3
M. Poli Lener LNF
G2
G1
GEM Chamber side view
Catania 31 March 2004
Single GEM detector
Cathode
Electrons:
I in
Drift
Field
Diffusion
Losses
Ions
Ion trap
~40%
~60%

I out
M. Poli Lener LNF

I drift
I-out = I-in . G . T
(gain x transparency)
Induction
Field
Anode
Ion Feedback =
I+drift / I-out
Catania 31 March 2004
Working with Fields
electron
ecollection
Collection efficiency
decrease at high drift field
values due to defocusing of field
lines above the GEM
Extraction efficiency
decrease at low transfer fields values
due to a worst electron extraction
capability from the lower side of the
GEM
M. Poli Lener LNF
Catania 31 March 2004
The gas gain
The GEM detector gain was measured by using X-rays
for the different gas mixtures;
The detector gain is an exponential
function of the sum of the 3 GEM
supply voltages :
G = A ea(Vgem1+Vgem2+Vgem3)
A and a depend on the gas mixture.
M. Poli Lener LNF
Catania 31 March 2004
Aging test
By irradiating the detector with an X-rays flux of 50 MHz/cm2 :
~ 20 C/cm2 was integrated with 60-20-20 @ Gain 2x104 (15 LHCb Y)
~ 4.5 C/cm2 was integrated with 45-15-40 @ Gain 6x103 (10 LHCb Y)
~ 11 C/cm2 was integrated with 65-28-7 @ Gain 1x104 (17 LHCb Y)
G/G < 5%
60-20-20
G=2x104
45-15-40 G=6x103
G/G < 10%
65-28-7 G=1x104
10 years of
LHCb R1-M1
P and T variations are corrected by using a low irradiated 3-GEM chamber
M. Poli Lener LNF
Catania 31 March 2004
GEM Types
DOUBLE-CONICAL (STANDARD)
50 µm kapton, 70 µm holes at 140 µm
50 µm kapton, 140 µm holes at 280 µm
25 µm kapton, 70 µm holes at 140 µm
CONICAL
50 µm kapton, 60/120 µm holes
M. Poli Lener LNF
CYLINDRICAL
50 µm kapton, 70 µm holes
Catania 31 March 2004