Quantum efficiency of the R5509-73 IR PMT at low photocathode bias during cryogenic operation

Ascertain the quantum efficiency of the Hamamatsu R5509-73 InP/InGaAs photocathode PMT when operated at a low photocathode bias (approximately −1 V) under the cryogenic conditions of the HeXe TPC, to enable reliable quantification of infrared light yield.

Background

The R5509-73 IR PMT used in the HeXe TPC requires a specific negative bias to optimize quantum efficiency and dark count rate. During operation inside the TPC, the photocathode bias was often lower than intended due to suspected issues with a Zener diode at low temperature, which likely reduced the PMT’s quantum efficiency.

Because the PMT’s quantum efficiency at this reduced bias is unknown, the study cannot report an absolute IR light yield. Determining the QE in these conditions is necessary to calibrate IR measurements and evaluate potential detector applications of IR scintillation.

References

The measurements presented here cannot be used to quantify the IR light yield, primarily because the PMT’s quantum efficiency at the applied low bias voltage is unknown.

— Operation of a dual-phase xenon detector with wavelength sensitivity from ultraviolet to infrared (2505.24682 - Hammann et al., 30 May 2025) in Section 6, Discussion and outlook