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

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 γ-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 of the detection modes. It is concluded that commercially available low-temperature PMT are well suited for detection of scintillation light at cryogenic temperatures.