Design and development of the MAGIX Trigger veto system

Abstract

At the new high-intensity, low-energy electron accelerator MESA, the MAGIX setup will be used for high-precision scattering experiments including dark sector searches, the study of hadron structure and few-body systems, as well as investigations of reactions relevant to nuclear astrophysics. The MAGIX experiment features a window-less scattering chamber housing an internal gas jet target that can be operated with a variety of different gases, two high-precision magnetic spectrometers, as well as sophisticated detector systems positioned at the spectrometers’ focal planes. This setup, combining a gas jet target with MESA’s high-intensity electron beam, with a fully window-less design, allows for an exceptionally clean experimental environment, in which background effects like multiple scattering are drastically reduced. The focal plane detectors include a tracking detector realized by a time projection chamber and the MAGIX trigger veto system, which forms the central focus of this thesis.

The trigger veto system will provide a fast and reliable trigger decision for the data acquisition, and will furthermore allow for coincidence time measurements and particle identification information. Moreover, it will provide the basic hit and position information for the triggered readout of the time projection chamber. It comprises a segmented trigger layer made of 22 plastic scintillation detectors read out by photomultiplier tubes, complemented by a flexible veto system consisting of three layers of additional scintillation detectors read out by silicon photomultipliers and passive lead absorber layers, mounted below the trigger layer. Its data readout uses the ultrafast preamplifier-discriminator NINO chip, which encodes the signal amplitudes using the time-over-threshold method, followed by FPGAs programmed as TDCs.

During the course of this thesis, a finalized and optimized detector design for the MAGIX trigger veto system was successfully developed, which is now ready to be installed at the MAGIX experiment at MESA. Additionally, important detector properties, such as efficiency, coincidence time resolution, and spatial resolution, were characterized for the trigger layer using data collected during a dedicated beam time at MAMI. A detection efficiency of about 99.93 % was achieved, the coincidence time resolution reached values of around (180± 10) ps (FWHM), and the spatial resolution along a trigger scintillator was determined to be (25.07± 1.05) mm. The readout system’s trigger rate tolerance was also evaluated, along with an assessment of its long-term stability. The entire readout system successfully withstood readout rates exceeding 4 MHz, with the individual channels reliably operating at rates above 700 kHz.