The MainzTPC experiment is a research and development project dedicated to the study of scintillation and ionization processes in liquid xenon, which are induced by low-energy electronic and nuclear recoils. Furthermore, the chamber is designed with a future measurement of the hypothesised Migdal Effect in xenon mind. The setup consists of an dual-phase xenon time projection chamber (TPC), the MainzTPC, along with its surrounding cryogenic system, and a gas storage and purification system. It also features a collimator and a rotating table for a germanium detector, as well as dedicated software and electronics. The software includes a slow control system to monitor the thermodynamics of the setup and a data acquisition system to record measurement data.
The MainzTPC was optimized for the use as a primary target in Compton and neutron scattering experiments to measure recoil energies in liquid xenon down to 1 keV. To minimize multiple scattering of incident gamma rays and neutrons, the time projection chamber was designed to be small. Additionally, the amount of passive material (e.g. field cage) surrounding the active volume (liquid xenon) was minimized to reduce energy losses.
These considerations led to the design shown in the Figure above, which differs from large-scale dual-phase time projection chambers as e.g. XENONnT. (A short introduction to XENONnT and dual-phase time projection chambers can be found here.) Instead of an array of multiple photomultiplier tubes at the top and bottom, the MainzTPC employs a single monolithic photomultiplier tube with 2-inch diameter on each side of the active volume. This approach maximizes light collection by avoiding non-photosensitive areas; however, it does not provide information for xy-position reconstruction. To compensate, eight additional photosensors, so-called avalanche photodiodes, were placed along the edges of an octagonal holding structure surrounding the upper PMT.
Due to inconsistencies in the avalanche photo diodes’ gain, a reliable xy-position reconstruction proved difficult. Therefore, the MainzTPC is currently being redesigned to replace the top assembly of photomultiplier tube and avalanche photodiodes with an array of silicon photomultipliers (SiPMs) in order to improve position resolution in x and y. Achieving precise position reconstruction, which is crucial for a measurement of the Migdal effect, requires optimizing the detector geometry, particularly the silicon photomultipliers array and the PTFE holder.
For this purpose, a Geant4-based simulation is being developed to model the interactions of low-energy neutrons with liquid xenon, calculate the quantum yield of the S1 and S2 signals, and simulate the optical propagation of these photons up to their final readout.
To address known instabilities in the liquid level of the MainzTPC, the liquid-gas interface was observed using commercially available cameras. Based on these observations, improvements to the level meters and level control system are planned.
