Master Thesis Defense by Alexander Buhl

Title: Development and Testing of ASIC Based Readout and Control System for Large Silicon Photomultiplier Arrays

Abstract:

In a letter of intent from 2020, the ALICE Collaboration outlines the plan for the addition to the ALICE detector, in the form of a Forward Calorimeter (FoCal). This detector is comprised of the two sub-detectors, the Hadronic part (FoCal-H) and the electromagnetic (FoCal-E). Prototyping of the FoCal-H began in the Summer of 2020, the first prototype was tested soon after. The following year, a second prototype was constructed and is still used for testing. 

Reading out the FoCal-H has always been a challenge, and to this day, 4 different systems have been into consideration for this task. For the earlier testing we used two different systems from CAEN; the A1702 and the DT5202, both based on the CITIROC-1A ASIC. Recently, we have tested the VMM+SRS system, based on the VMM ASIC, and finally, an upcoming system is based on the H2GCROC ASIC. This thesis will focus on a test beam run of the FoCal-H in the Autumn of 2022, where the DT5202 was thoroughly tested, and some preliminary testing of the VMM+SRS was done. Extensive physics analysis has been performed on the data from these runs, in order to both evaluate the detectors performance, as well as the readout systems performance. This includes examining the linearity of the energy response of the detector, the detectors energy resolution, how well the detector contains the developing particle showers, and using the variance of the particle showers to identify what particle started them. A key part of the analysis is the successful synchronization of the FoCal-H and the FoCal-E. Due to the construction of a trigger logic, also described in this thesis, it was possible to synchronize the triggers delivered to the individual detectors, and thus it was possible to synchronize the data after the fact. 

Another central part of this thesis, is the use of Silicon Photomultipliers (SiPMs) in the detector. Specifically, what criteria these would impose on the readout system, and how each of the systems (to the extent that they have been tested) lives up to those criteria. Central to this is a study of how the systems maintain high trigger rates, and handles a large range of signal sizes. The latter is particularly in focus, as it relates to channel saturation, and how this will affect the results of the analysis. While it is shown that the channel saturation is not as prevalent as feared, it is also shown how it does artificially lower the energy resolution.

In addition the stability of the entire detector and readout system is examined. These will be done with the DT5202 in focus, examining how it performs, and finally evaluate why this systems is not a good solution for a readout system for the final FoCal-H detector. On top of that, the preliminary testing of the VMM+SRS is discussed, detailing how this system successfully works for calorimeter (for which it was not designed). Finally, the current status of the final readout system is discussed, and the near future of testing is laid out, as it relates to further testing of the VMM+SRS and the first field tests of the H2GCROC system. The latter is supposed to be tested in September 2023.