Transcript F. Cormier - CERN Indico

First phase of McGill Thin Gap Chamber Testing Facility
F. Cormier, B. Lefebvre, S. Robertson, B. Vachon
Department of Physics, McGill University
TGC Construction
Motivation
TGC’s are thin gap chambers,
gaseous muon detectors first
used in a large scale in the
OPAL experiments. A TGC
operates
with
these
characteristics:
The ATLAS detector is a collaboration of
institutions from 37 different nations, part of the
Large Hadron Collider (LHC) operated by
CERN.
The goal of this detector is to test the predictions
of the Standard Model, as shown when the
discovery of the Higgs Boson was announced in
March 2013, and also to search for new physics
beyond the Standard Model.
•Wires at 3 kV create a very
strong electric field in the gap
•The resistive graphite layer,
about 100 kΩ/cm2 creates a
uniform electric field with the
wires
•Strips are capacitively coupled
with the graphite plane such that
any discharge on the plane is
registered in the strip
•A CO2-pentane (55:45 ratio) gas mixture
is used. Pentane is used as a quenching
gas to contain the amount of ionized
electrons and emitted photons, reducing
sparking and detection time
TGC Theory
The McGill TGC group is tasked with the quality control of
the sTGC’s produced in Canada. This includes testing the
spatial resolution and the efficiency of the detectors. If the
detectors pass these tests, they will be sent to the ATLAS
detector, where they will be used for approximately 10
years.
My job is to write a data acquisition program using VME
hardware in order to analyse data from a prototype sTGC
at McGill. Software and techniques developed at this time
are critical to the evolution of the testing facility at McGill.
Results & Analysis
The main advantage of gaseous detectors is having a multitude
of wires, strips and pads, which allows you to use an amplitdue
(using a QDC) or time-over-threshold (using TDC) distribution to
calculate a one-dimensional coordinate of the interaction point.
By having wires and strips perpendicular to each other, the
ultimate goal is to have a precise x and y position on each muon,
allowing us to track these particles across the detector.
Using our hodoscope and VME data acquisition software, a large
amount of data can be obtained. Using offline analysis, we can
remove pedestal and noise values, and fit a Gaussian to every
event of interest. This allows us to reconstruct every event, as
seen in the picture below, to find:
• Amplitude of the event (proportional to muon energy)
•Mean of the event (the strip position where the muon passed)
• Standard deviation of the event (the uncertainty in the position
measurement).
The New Small Wheels, upgrades to the small
wheels seen on both sides of the detector
(right), contains detectors which act as triggers.
The first trigger, called Level 1, is programmed
to with precisely determined parameters to
reject hits deemed background noise.
The detection process is as follows:
•A muon enters the gas gap
•Depending on momentum and angle, it
will interact with a number of gas
particles in the gap.
VME
•The gas particle will separate into an
ionized, negatively charged electron,
and a positively charged ion.
•Due to the high electric field, the
electron will be accelerated towards the
positive wire
•An avalanche of about
electrons converges on the wire;
this motion will induce a charge on
the wire
•This high acceleration causes the
primary electron to ionize other
particles, creating an electron & photon
avalanche to the wire.
•The ions converge on the graphite,
creating a signal on the capacitvely
couple strips
•Meanwhile, the ion is (compared to the
electrons) slowly moving towards the
graphite layer
To do this, the LHC must continuously upgrade
its luminosity (rate of collisions) as well as its
collision energy. Luminosity will be increased up
to 5 × 1034 cm-2 s-1 and Beam Energy up to 7
TeV.
Because of these upgrades, the ATLAS detector
will see a much greater level of background
noise. This background noise is produced by
slow particles emanating from collisions, such
as protons; this noise account for about 90% of
detections.
The sTGC, which will be built in Canada and tested at
McGill, is one of these detectors. Due to its fast rise time,
a Thin Gap Chamber is ideal as a Level 1 Trigger. This
trigger is very important as it must limit the amount of data
saved, as saving every event detected would take up too
much space.
106
•The signals are sent to readout
electronics, which lets us obtain
data on events in the sTGC
Data Flow & Hodoscope
A hodoscope is an apparatus designed to
track microscopic particles. The current
McGill hodoscope for the sTGC project
consists of an sTGC between two
scintillators, as shown on the right.
Custom-built
ASD
(amplifier-shaperdiscriminator) readout electronics allow us to
read signals from the sTGC strips and wires.
These electronics shape the raw signal which has many jitters and can be divided
between primary and secondary ionizations
– into a stable pulse, proportional to the
energy deposited onto the wire or strip.
VERSAModule Eurocard is a
standardized computer architecture
used in science and industry
throughout the world.
It is used here as data acquisition software. VMEbus is an
asynchronous architecture, leading to fast and reliable data taking.
VME crate operation is as follows:
•Master (left in picture) – a module which is connected to a PC,
and takes input commands from computer software, and outputs
data.
With large amounts of data, we are able to reconstruct a position
distribution. In theory, this distribution should be completely even
for all positions, however, edge effects will lower the numbers for
the first and last strips, and inefficiencies in the apparatus can
cause further distortions. A position reconstruction can be seen in
the figure below.
•Slave (right in picture) – modules which take data, They normally
contain multiplexers and fast ADC’s for rapid data taking, as well
as a buffer not to lose data.
•Lab modules – Optical bridge (Master), QDC (Charge to
amplitude converter) and TDC (Time to amplitude converter).
•My project was to design and write custom software to read out
these cards in order to meet our testing needs. All analysis
presented in this poster are a direct result of this software.
The scintillators are used as gate triggers for the VME system. Like the TGC’s
are used in the ATLAS detector as a trigger to reject noise and background
events, so are the scintillators used here to determine real cosmic muon
events. Thus, only when the two PMT’s give signals within about 100ns is a
gate generated, vetting real muons and outputting VME data, allowing us to
determine the position of the muon crossing using the sTGC.
The hodscope structure for the McGill testing facility, planned to be finshed in
2014, will be an expansion of the current hodscope – larger scintillators and
sTGC’s (over 1 × 1m), and precision chambers above and below the sTGC’s in
order to accurately track the position of the muon as it enters and exits the
hodoscope.
Using scintillators to trigger a data acquisition system, like the one developed
this summer, and precision chambers to track the muons, chamber efficiencies
can be calculated, allowing us to detect any defects, and decide whether or not
to allow the sTGC to be used at the ATLAS detector.
Conclusions
Hodoscope design and data acquisition software are both
shown to be extremely useful in studying the new sTGC
technology. The experience and software used in this poster will
be extended in order to test the sTGC’s built for the ATLAS
detector in 2015.
Acknowledgements
I would like to thank NSERC for funding this experiment, as well
as Professors Steven Robertson and Brigitte Vachon for giving
me the opportunity to work on this project. Finally, I would also
like to thank students Benoit Lefebvre and Kyle Johnson for their
contributions to this project.