First test of a cryogenic scintillation module with a CaWO4 scintillator and a low-temperature photomultiplier down to 6 K

TL;DR

This study demonstrates that a commercially available low-temperature photomultiplier can effectively detect scintillation light from CaWO4 crystals at 6 K, maintaining performance and energy resolution suitable for cryogenic rare event experiments.

Contribution

First experimental validation of a low-temperature PMT for scintillation detection at cryogenic temperatures, showing its suitability for rare event search experiments.

Findings

PMT retains single photon counting ability at 6 K

Responsivity decreases but remains high at cryogenic temperatures

Energy resolution remains comparable to room temperature measurements

Abstract

Future cryogenic experiments searching for rare events require reliable, efficient and robust techniques for the detection of photons at temperatures well below that to which low-temperature photomultipliers (PMT) were characterised. Motivated by this we investigated the feasibility of a low-temperature PMT for the detection of scintillation from crystalline scintillators at T = 6 K. The scintillation module was composed of a CaWO4 scintillator and a low-temperature PMT D745B from ET Enterprises. The PMT responsivity was studied at T=290, 77 and 6 K using gamma-quanta from 241Am (60 keV) and 57Co (122 and 136 keV) sources. We have shown that the low-temperature PMT retains its single photon counting ability even at cryogenic temperatures. At T = 6 K, the response of the PMT decreases to 51 +- 13 % and 27 +- 6 % when assessed in photon counting and pulse height mode, respectively. Due to the light yield increase of the CaWO4 scintillating crystal the overall responsivity of the scintillation modules CaWO4+PMT is 94 +- 15 % (photon counting) and 48 +- 8 % (pulse height) when cooling to T = 6 K. The dark count rate was found to be 20 s-1. The energy resolution of the module remains similar to that measured at room temperature using either detection mode. It is concluded that commercially available low-temperature PMT are well suited for detection of scintillation light at cryogenic temperatures.