ALICE TPC Upgrade Activities for LHC Run 3 and Beyond: "SAMPA ASIC Tests with GEM Detector Prototype"
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The Time Projection Chamber (TPC) signal readout of the ALICE detector is being upgraded to accommodate the higher collision rates and -energies during LHC Run 3 in 2018. Due to the increased collision rates, the TPC drift time of about 100 μs will be 5 times longer than the average time between interactions, rendering the presently employed gating of the TPC wire-chambers insufficient. Therefore, a Gas Electron Multiplier (GEM) based system will be replacing the wire-chambers. In addition, the front-end electronics need to be replaced to match the new readout chamber technology and increased data rates. This will be done by the new SAMPA chip which combines the functionality of the previous PASA (PreAmplifier ShAper) and ALTRO (ALICE TPC ReadOut) chips currently used as front-end electronics. The focus of this work has been twofold: (1) Characterizing a SAMPA Chip 1 analog prototype for use in the upgraded ALICE TPC signal readout. (2) Construct and characterize a GEM detector prototype for use with the SAMPA Chip 1. A fully differential buffer has been designed and mounted on the Chip 1 carrier board for ADC readout capabilities. The buffer performed well and the ADC readout was successful. The power consumption of the SAMPA Chip 1 failed to reach the requirement of 6mW per channel, achieving no better than ~9mW per channel at the nominal supply voltage. Results from the gain and pulse shape stability showed linear gain and a stable pulse shape, with some deviations for low and high input charges as a result of poor signal-to-noise ratio and the amplifier saturating, respectively. The crosstalk measurement showed scattered results, but none of the carrier boards achieved the requirement of less than 0.2 %. The best crosstalk results averaged ~0.3 %, while the worst exceeded 1 %. Simulated noise for the SAMPA Chip 1 doesn’t fulfill the requirement of 385 ENC at 12 pF capacitance. When corrected for stray capacitances on the carrier boards, the measured noise coincided to some extent with the simulated values. The high noise levels seem to originate from the inside the SAMPA ASIC. A GEM detector has been designed and produced in close collaboration with the group stationed at the Wigner Research Institute in Budapest, Hungary. Testing of the GEM detector has been done at the Wigner Institute using both Fe-55 and Sr-90 radioactive sources, measuring the energy resolution of the detector and calibrating its gain. The gain was set to ~2000 and the energy resolution was measured to be ~8 %, which is below the 12 % requirement. Further testing of the prototype detector has been conducted in Bergen, establishing its gain for different gas mixtures and measuring the signal-to-noise ratio of the SAMPA Chip 1 for MIPs. This work has been done in close collaboration with Ganesh Jagannath Tambave, whose main focus has been on the signal readout and data acquisition of the SAMPA Chip 1. The signal-to-noise ratio measured to be 27:1, the requirement being 38:1. Increasing the drift gap of the detector will increase the signal-to-noise ratio.
PublisherThe University of Bergen
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