TPC
A Time Projection Chamber (TPC) for particle tracking is being installed at BNL by the Laser Electron Gamma Source (LEGS) group and the Instrumentation Division for use at beamline X5 at the NSLS. It will be part of the Spin ASYmmetry Detector Array (SASY) for LEGS experiments.
The TPC will be used to discriminate between π+ and π- particles by measuring their track curvatures in a magnetic field of 1.8 Tesla. It is composed of a cylindrical can 90 cm long and 36 cm in diameter with a high-voltage electrode in the middle, a dual-stage Gas Electron Multiplier (GEM) with associated anode pad plane and dedicated front-end electronics at one end, and a ground electrode with HV and filling gas connections at the other end.
Charged particles traversing the gas create tracks of electrons and ions along their paths. The multiplied electrons are collected by the anode plane, which is pixellated into about eight thousand pads to provide a digitized representation of the projection of the tracks. The time of arrival of the drifting electrons (relative to a trigger scintillator surrounding the TPC) at each pad is measured and digitized by the dedicated front-end electronics.
The TPC can be operated with mixtures of up to three of the following gasses: argon, methane, ethane, carbon dioxide, and carbon tetraflouride. These mixtures are prepared by a gas handling system which can also re-circulate and purify the gas. The TPC (a volume of 0.085 cubic meter) operates at the local atmospheric pressure and the gas system stores about 5 times the TPC volume at a pressure of three atmospheres. A current limited 25 kV high voltage supply with an external RC filter is connected to the cathode at the center of the TPC by a coaxial HV cable and a custom connector. The GEM foils are biased by 1 kV current limited supplies, and low voltage (2 to 5 volts at up to 50 amp) supplies operate the circuitry for the electronic readout. The TPC and its readout electronics will be contained inside a 1.8 Tesla superconducting solenoidal magnet. The cryostat for this magnet is 100 cm long with an inside bore of 50 cm. The magnet is cooled with liquid helium.
The TPC is presently undergoing bench-testing with cosmic tracks using P10 counter gas. The re-circulating and purifying gas system has been tested and will be integrated with the TPC upon its installation at LEGS. The steel return yoke for the large bore superconducting magnet has been assembled around the SASY detector at LEGS and the solenoid is now in the process of installation. The TPC is expected to be moved to LEGS in October, 2005.
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