Picosecond laser test unit for photosensor characterization at ambient and low temperatures

TL;DR

This paper introduces a compact setup for precise photosensor characterization at low temperatures using picosecond laser pulses, providing detailed measurements of PMT properties relevant for neutrino and dark matter experiments.

Contribution

It presents a novel, reproducible method for single photoelectron characterization of photosensors under controlled conditions, including a new data-driven approach to analyze pulse charge spectra.

Findings

Cooling increases PMT gain by ~0.1%/°C

TTS decreases with voltage and shows no clear temperature dependence

Cable length influences apparent gain and TTS

Abstract

Accurate single photoelectron (SPE) characterization of photosensors is essential for controlling systematic uncertainties in low-light neutrino and dark matter detectors. We present a compact laboratory setup for the characterization of photosensors under controlled, low-light conditions. Specifically, we demonstrate its use with photomultiplier tubes (PMTs) operated at the SPE-level, using picosecond laser pulses and waveform digitization to determine key PMT properties. Measurements as a function of supply voltage and temperature ($-50^\circ$C to $+20^\circ$C) are performed on ET Enterprises 9821(Q)B tubes and a Hamamatsu R9980 assembly, which show exponential gain-voltage behavior and device-to-device variation. Cooling increases the gain by $\sim 0.1\,\%/^\circ$C, while the transit time spread (TTS) and peak-to-valley ratio (P/V) exhibit no clear temperature dependence. TTS decreases with voltage. Late pulses remain at the percent level and prepulses at the sub-percent level. Cable length affects both apparent gain and TTS. A model-independent, data-driven self-convolution method is introduced to quantify double photoelectron contributions from pulse charge spectra. The procedures provide a reproducible, practice-oriented reference for SPE-level PMT characterization and can be extended to other photosensor types.