Download Time of Fight Detectors

Examples of Time of Fight
Detectors
• Variety of methods for measuring time it takes for a
particle to travel through a medium
• A time of flight (TOF) detector is a particle detector
which can discriminate between lighter and heavier
elementary particles of same momentum using their
time of flight between two scintillators
• One Method- The first of the scintillators activates a
clock upon being hit while the other stops the clock
upon being hit. If the two masses are denoted by
m1and m2 and have velocities v1 and v2 then the time
of flight difference is measured where L is the distance
between the scintillators. The approximation is in the
relativistic limit at momentum p and c denotes the
speed of light in vacuum. Usually, δt is the apparatus'
time resolution
ALICE Experiment
• Measures the particles produced in collisions so the evolution
of the system in space and time can be reconstructed.
• Subdetectors- Ensemble of cylindrical detectors embedded in
the magnetic field ( bending the trajectories) measures at
many points the passage of particles carrying electric charge.
• Particle identification- TOF measures with a precision better
than a tenth of a billionth of a second, the time that each
particle takes to travel from the vertex to reach it, so that one
can measure its speed
• Includes a Muon spectrometer, HMPID measures the faint
light patterns generated by fast particles and the TRD
measures the special radiation very fast particles emit when
crossing different materials, thus allowing to identify
electrons.
• Charged particles in the intermediate
momentum range are identified in ALICE by
the Time Of Flight (TOF) detector
• The time measurement with the TOF, in
conjunction with the momentum and track
length measured by the tracking detectors is
used to calculate the particle mass
The goal of ALICE is to study
• The QCD thermodynamics via the
measurement of π, K and p transverse
momentum distributions and particle ratios
on an Event-by- Event basis.
• Signatures of QGP (Quark Gluon Plasma)
formation via open charm and φ meson
production.
ALICE’s TOF Detector
• The TOF detector has a cylindrical shape,
covering polar angles between 45 degrees and
135 degrees over the full azimuth.
• It has a modular structure with 18 sectors;
each of these sectors is divided into 5 modules
along the beam direction.
• The modules contain a total of 1638 detector
elements (MRPC strips), covering an area of
160 m2 with 157248 readout channels (pads).
Detector Description
• The detector chosen for the ALICE TOF is the
Multigap Resistive Plate Chamber (MRPC),
developed within the CERN LAA project.
• The detector element is a long MRPC strip with
an active area of 7.4 × 120 cm2.
• It has 96 readout pads of 2.5 × 3.5 cm2 arranged
in two rows.
• It consists of 2 stacks of glass, each with 5 gas
gaps of 250 μm; spacers made of nylon fishing
line keep the distance between the glass plates
fixed.
Operations
• The MRPC –( Multigap Resistive Plate Chambers) is a stack of
resistive glass plates. A high voltage is applied to the external
surfaces of the stack. Further out there are pickup electrodes.
• A charged particle ionizes the gas and the high electric field
amplifies this ionization by an electron avalanche.
• The resistive plates stop the avalanche development in each gap;
they are however transparent to the fast signal induced on the
pickup electrodes by the movement of the electrons.
• So the total signal is the sum of the signals from all gaps (the reason
for many gaps is to achieve high efficiency), whereas the time jitter
of the signal depends on the individual gap width (the reason for
narrow gaps is to achieve good time resolution).
• The pattern of hit strips gives a quick measure, which is then used
by the trigger to make immediate decisions about whether the data
are worth keeping. RPCs combine a good spatial resolution with a
time resolution
Electronics
• The signal from the MRPC must be amplified
and discriminated and the time measured
with an accuracy matching the intrinsic time
resolution of the detector.
For the readout the HPTDC (High Performance Time to Digital Converter), a
chip developed by the CERN EP/MIC group, with bin width of 25 ps. Time-todigital converters register the arrival of a single ion at discrete time "bins";
thresholding discriminates between noise and ion arrival events.
For the Front End the NINO ASIC, developed recently by
the CERN LAA project, which combines a fast amplifier,
discriminator and stretcher.
Detector Performance
The time resolution of the
TOF MRPC is in the 50 ps
range, as can be seen in
the figure showing the
time distribution after
slewing correction. A
typical efficiency and time
resolution plateau as a
function of the high
voltage is more than 2 kV
long before the onset of
streamers, with efficiency
reaching 99.9 %.
π, K and p mass peaks
clearly show up in the
momentum range up to
3 GeV/c, in a simulation of
250 Pb-Pb events generated
with HIJING for magnetic
field B = 0.4 T
CDF II Collider at FermiLab
CDF System
Time of Fight at CDF
• CDF II experiment incorporated TOF into the detector to
provide charge kaon identification to improve neutral B
meson flavor determination.
• Expected TOF of 100 ps, the system will provide 2 standard
deviation separation between K± and π± for momenta of
p<1.6 GeV/c
• TOF was added to improve particle identification capabilities.
• TOF is to identify the b (particle/antiparticle) flavor of B
hadrons produced in the collisions
• An enhancement of the b flavor identification is crucial to
improve the statistical precision in CP violation
measurements and oscillations.
• Determining b flavor (content of b or b’) of B mesons
• Quantify the power of a b flavor tag is the total tag
effectiveness εD^2, where ε is the tag efficiency and D is the
tag dilution which is related to the probability P that the tag
dilution is correct; D=2P-1
• CDF developed several methods of “tagging” that will be
incorporated into its second run
• Same Side Tags- exploit charge correlation of particles
produced in association with hadronization (formation of
hadrons from Quarks and Gluons) of a b quark into a B meson
• Opposite Side Tags-identifies flavor of the second B hadron of
interest
• Identification of charged Kaon can be used as an opposite tag
b/c B’ (anti) hadrons produce more K- than K+ through a
decay sequence BeautyCharmStrange
• Kaon identification also enhances effectiveness of same side
tagging. B (B’) mesons produce in association with K+ (K-)
Significant fraction of kaons from the B hadron are in the
identification range of TOF. Opposite side kaon was not
established in the first CDF run.
Combining it with the same side tagging ( Run I),
TOF is expected to double the flavor tag
effectiveness of B/ B- mesons
• Particle identification ( kaon Identification)
with TOF is performed by measuring the
difference ( time of flight) in the time of arrival
of the particle at the scintillator with the
collision time t(o). The mass m can be
determined from the momentum p, the path
length L, and the time of flight t
TOF System
• 216 Bars of Bicron-408 Scintillators chosen for
a long attenuation length
• Bars are 279 cm X4 cm X 4 cm installed at a
radius of 138 cm from the beam in 4.7 cm of
radial space between the main drift chamber
and cryostat of the superconducting solenoid
• 432 Photomultiplier tubes (PMT) operating in
the 1.4 T magnetic field are attached to each
end of the scintillator bars.
• Signal form from anode and dynode stage of
PMT. PMT is fed into a preamplifier which
drives the signal to the electronics
• From the preamplifier the signal follows 2
paths, one for timing and the other for charge
measurements.
• First path the signal is fed into a discriminator
and the output serves as the start signal for
the time to amplitude conversion circuit.
• Second path used to correct the time
measurement for pulse height dependence
TOF Results
• TOF system with 20 130 cm bars was installed inside the CDF
solenoid at the end of the Run I to learn how to achieve the
best timing resolution (100 ps) from a full system.
• Flight time resolution average over all the
tubes was 220-250 ps from the fit of the TOF
mass, which is a low resolution. It was
rectified in the full system