First phase of McGill Thin Gap Chamber Testing FacilityF. Cormier, B. Lefebvre, S. Robertson, B. Vachon

Department of Physics, McGill University

MotivationTGC Construction

TGC Theory

Data Flow & Hodoscope

VME

Results & AnalysisThe 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.

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.

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.

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.

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 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.

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:

• 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

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.

• 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

• This high acceleration causes the primary electron to ionize other particles, creating an electron & photon avalanche to the wire.

• Meanwhile, the ion is (compared to the electrons) slowly moving towards the graphite layer

• An avalanche of about 106 electrons converges on the wire; this motion will induce a charge on the wire

• The ions converge on the graphite, creating a signal on the capacitvely couple strips

• The signals are sent to readout electronics, which lets us obtain data on events in the sTGC

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-shaper-discriminator) 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.

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.

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.

• 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 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).

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.

Conclusions

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.

Acknowledgements

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.