Simulation Study of Photon-to-Digital Converter (PDC) Timing Specifications for LoLX Experiment

The Light-only Liquid Xenon (LoLX) experiment is a prototype detector aimed at studying liquid xenon (LXe) light properties and various photodetection technologies. LoLX is also aimed to quantify LXe’s time resolution as a potential scintillator for 10-ps time-of-flight positron emission tomography (TOF-PET). Another key goal of LoLX is to perform a time-based separation of Cerenkov and scintillation photons for new background rejection methods in LXe experiments. To achieve this separation, LoLX is set to be equipped with photon-to-digital converters (PDCs), a photosensor type that can provide a timestamp for each observed photon. To guide the PDC design, we explore the requirements and potential outcomes for time-based Cerenkov separation. We use a PDC simulator, whose input is the light information from the Geant4-based LoLX simulation model, and evaluate the separation quality against time-to-digital converter (TDC) parameters of the PDCs. Compared with the current filter-based approach, the simulations predict a few different configurations that offer a Cerenkov separation level increase from 50% to 66% when using PDCs and time-based separation. A separation of 65% is also achievable with just 16 TDCs for 14 400 micro-cells per PDC, or one TDC per 2.25 mm2. These simulation results will lead to a specification guide for the upcoming PDC design as well as expected results to compare against future PDC-based experimental measurements. In the longer term, the overall LoLX results will assist large LXe-based experiments and motivate the assembly of an LXe-based TOF-PET demonstrator system.