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Testing Minimum Bias Trigger Scintillators with Cosmic Rays
Eric Feng
Supervisor: Professor James Pilcher
Department of Physics, University of Chicago, Chicago, IL 60637
Introduction
Results
This project used cosmic rays to test a prototype Minimum Bias Trigger
Scintillator (MBTS) that will be used in the ATLAS experiment at CERN
(Fig. 1). The efficiency of the MBTS counter at detecting minimum
ionizing particles was characterized. Two separate sections in the MBTS
prototype were tested.
The efficiency of the cosmic telescope was measured as a function
of the photomultiplier HV for one trigger counter (Fig. 4a). The
PMT high voltage for the other trigger counter was fixed to 1600V
for full efficiency.
The efficiency of the cosmic telescope was also measured as a
function of the relative delay imposed between the signals from the
two trigger scintillators (Fig. 4b).
The number of photoelectrons produced per minimum ionizing
particle (MIP) was measured to be:
• 7.4 for the outer section of the MBTS counter
• 10.3 for the inner section.
43cm
14cm
Fig. 4(a,b). Cosmic telescope efficiency.
3.6m
3.6m
Fig. 1. Planned deployment of the MBTS counters in the ATLAS
detector.
The 3-in-1 electronics card in the photomultiplier tube reading out
the MBTS counter was modified to derive the trigger output from
the high gain output instead of the low gain, increasing the signal
by a factor of 64.
The probability of fewer
than three charged particles
is 2.25%.
A Monte Carlo simulation was performed to simulate the
experiment. The predicted energy deposited in the counter is shown
in Fig. 6. The predicted number of photoelectrons per MIP is shown
in Fig. 7(a,b). The predicted signal shape agrees well with the
shape of the measured signal in Fig. 5(b,d).
The energy deposition follows a
Landau distribution. The mean
energy deposited is calculated
from the Bethe-Bloch formula.
88cm
The probability that no
charged particles pass
through any of the
MBTS counters in one
bunch crossing is 0.3%.
The number of photoelectrons
reflects the composition of the
Landau distribution for energy
deposition with a Poisson
distribution for photon statistics.
Fig. 6. Monte Carlo prediction of the energy deposited in
the MBTS counter.
Fig. 9. Charged particle multiplicity expected in the entire
region spanned by the MBTS counters.
Conclusions
The efficiency of a prototype Minimum Bias Trigger
Scintillator was measured using cosmic rays.
The default (low gain) trigger output of the 3-in-1 electronics
card did not provide sufficient signal-to-noise ratio to trigger
with full efficiency. Rerouting the trigger output to derive
from the high gain output boosted the gain by a factor of 64.
The number of photoelectrons produced per MIP and the
signal-to-noise ratios for the MBTS counter will be
incorporated in the GEANT full simulation in the future. These
will be used along with the expected charged multiplicity to set
requirements for triggering ATLAS on MBTS signals when the
LHC turns on.
The integrated signal charge from the trigger cable is shown in
Fig. 5(a-d).
Materials and Methods
A simple apparatus was built to test the MBTS counter. The MBTS
scintillator was laid horizontally on a table, supported by wooden blocks.
Two trigger counters sandwiched the MBTS counter and were used as a
cosmic telescope to select cosmic rays passing through it (Fig. 2).
The MBTS counter was read out with Tile Calorimeter electronics. The
signal out of the trigger cable was measured with additional electronics,
including a pulse height analyzer (Fig. 3).
Literature cited
Fig. 7(a,b). Monte Carlo prediction of the number of
photoelectrons produced per MIP.
Cosmic Ray
Trigger Scintillator
ATLFast, the fast simulation program for ATLAS, was used
to determine the charged particle multiplicity in a single LHC
bunch crossing. The charged multiplicity for a section of the
MBTS counter is shown in Fig. 8(a-b). The charged
multiplicity for the entire region spanned by the MBTS
counters is shown in Fig. 9.
Lead bricks
MBTS
Trigger Scintillator
M. Nessi. Minimum Bias Trigger Scintillator Counters (MBTS) For
Early ATLAS Running. Technical report, CERN, 2004.
S. Eidelman, et al. Review of Particle Physics. Physics Letters B,
592:1+, 2004.
D.E. Groom, N.V. Mokhov, and S.I. Striganov. Muon Stopping
Power and Range Tables: 10 MeV-100TeV. Atomic Data and
Nuclear Data Tables, 78:183–356, 2001.
A. Artikov, D. Chokheli, J. Huston, B. Miller, and M. Nessi.
Minimum Bias Scintillator Counter Geometry. Technical Report
AT-GE-ES-0001, CERN, 2004.
Fig. 2. Elevation view of the cosmic telescope.
MBTS
Counter
8X Linear Amplifier
5X Attenuator
High Pass Filter
(τ ~ 5ms)
3m Optical
Fiber
PMT
(3-in-1 Card)
Low Gain
Differential- to SingleEnded Converter
80m Trigger
Cable
High Gain
Signal
Pulse Height
Analyzer
Trigger
Fig. 3. Flow chart of the MBTS readout chain.
Acknowledgments
Trigger
Summing Card
Cosmic
Telescope
Fig. 5(a-d). Each plot has three spectra:
• Scintillator signal (red): This is the signal charge with the
photomultiplier HV set to 900V for maximum gain.
• Total noise (blue): This is the noise associated with the
system, including the TileCal electronics. This noise was
measured by turning off the HV to the photomultiplier.
• Measurement noise (green): This is the noise introduced by
the electronics used to measure the signal out of the trigger
cable; it excludes the noise of any Tile Calorimeter electronics.
I am grateful to Professor Jim Pilcher for supervising this project. I
also thank Kelby Anderson and the rest of the ATLAS group at the
University of Chicago for their help.
Fig. 8(a,b). The charged multiplicity expected in the outer section
and the inner section of a counter in a single LHC bunch crossing.
For further information
Please contact [email protected]. The poster and the final report
for this project can be found at http://home.uchicago.edu/~ejfeng/ .