CsI:Tl scintillation pulse shapes measured with a SiPM photodetector in a liquid nitrogen cryostat

A custom designed cryostat was constructed to measure the response of a CsI:Tl scintillator at temperatures close to the boiling point of liquid nitrogen (LN2). The scintillation light was collected by an HUV-HD SiPM from FBK with 6×6 mm2 area and 25×25 μm2 cell pitch. The crystal size was 5×6×7 mm3. All surfaces except the one facing the SiPM were covered with Teflon tape to enhance light collection by the photodetector. The performance of the experimental setup was verified at room temperature using analog electronics for signal processing. The crystal was mounted on a copper frame placed inside the LN2 cryostat. Since our goal was to measure the scintillation decay profiles, and the SiPM response at low temperatures becomes substantially slower than that observed at room temperature, the SiPM was mounted on a separate copper frame connected with the outer housing to keep it close to room temperature. The separation between the crystal surface and the SiPM was about 1.5 mm at room temperature, and it became smaller once the setup was cooled down to LN2 temperature, but even so the crystal and the photodetector were still separated. This approach allowed us to analyze scintillation pulse shapes of CsI:Tl at LN2 temperatures. An energy spectrum of 662 keV γ-rays from a 137Cs source was also recorded. The light yield of the CsI:Tl sample at LN2 temperature stands at about 6 % ÷ 8 % of the value observed at room temperature.