Upgrade of ASACUSA's Antihydrogen Detector

The ASACUSA (Atomic Spectroscopy And Collisions Using Slow Antiprotons) experiment located at the Antiproton Decelerator at CERN aims to compare the ground state hyperfine splitting of antihydrogen to hydrogen, providing a direct test of the CPT symmetry. The ASACUSA hodoscope detects the secondary particles produced by annihilation of the antiproton of antihydrogen, which are mostly pions. It is a barrel-type octagonal detector consisting of an outer and an inner layer of 32 plastic scintillator bars each and two layers of scintillating fibres. The bars are individually read out by two silicon photomultipliers (SiPMs) on each end of the bar and additionally every fibre (100 in the outer layer, 64 in the inner) is read out by a SiPM. The SiPMs are connected to IFES boards (Intelligent Frontend Electronics for SiPMs), previously developed at the Stefan-Meyer-Institute, which are pre-amplifiers and provide a digital time-over-threshold (ToT) signal via leading edge discrimination. Previously, the possible detection rate of the hodoscope was at about 50 Hz. In order to provide detection at a higher rate, several changes had to be made in the data acquisition (DAQ) system. After performing changes in the data transfer of the time-to-digital converters (TDCs) as well as reprogramming the DAQ software, events at a rate of over 1000 Hz could be detected without loss. The ToT signals of the hodoscope bars are fed into FPGA logic units to produce a trigger from coincidences but are also used for further time measurements with TDC units. Since the ToT signals are produced by leading edge discrimination with a fixed threshold, the position of the leading edge varies with the height of the corresponding analogue signal produced. To find a way to prevent this time-walk effect, several test measurements were performed.