Cryogenic phonon-scintillation detectors with PMT readout for rare event search experiments and characterisation of a PMT at milli-Kelvin temperatures

Reliable, robust, efficient, and scalable light detectors operated at milli-Kelvin temperature that can resolve individual photons in a scintillation event are required by the cryogenic phonon-scintillation detectors (CPSD) for next generation rare-event search experiments. This thesis demonstrates a cryogenic detector with a scintillating crystal as the absorber, equipped with a Neutron Transmutation Doped Germanium (NTD-Ge) phonon sensor and a Hamamatsu R8520-06 photomultiplier tube (PMT) with platinum underlay as the light detector. The PMT is powered by a Cockcroft-Walton generator. In this experiment, two CPSD modules are successfully tested, one with CaWO4 and the other with CaMoO4 as the scintillating absorber. The detector modules were cooled down to ~ 18 milli- Kelvin inside a dilution refrigerator. The CPSD modules are demonstrated to be able to measure signals from the phonon channel and light channel simultaneously, which is critical for particle identification. The full-width-at-half-maximum (FWHM) energy resolutions at 122.1 keV for the phonon channel are 2.17 keV (1.8%) for CaWO4 and 0.97 keV (0.79%) for CaMoO4. The FWHM resolutions at 122.1 keV γ excitation for the scintillation/photon channel are 19.9% for CaWO4 and 29.7% for CaMoO4. At milli-Kelvin temperatures, the dark count rate of the PMT is determined to be less than 5 Hz while the gain of the PMT is found to be 3.3 × 10E6. These characteristics of the CPSD in this work compare favourably to conventional CPSDs with cryogenic calorimeter-based light detectors currently deployed in cryogenic rare-event search experiments, especially in the energy region of interest for dark matter direct detection. The PMT is demonstrated to have low dark count rate and high gain at milli-Kelvin temperatures, thus it remains an effective single photon detector. From the radioactivity of a similar model of PMT as input to a Geant4 simulation, the radioactivity introduced by the PMT contributes only minorly (<10%) to the overall radioactive background of a hypothetical CPSD module for WIMP searches. The CPSD module with PMT readout benefits a reliable, well-tested and commercially available apparatus, while the PMT as a photon detector at milli-Kelvin temperatures offers a much better time resolution than cryogenic light detectors, allowing to resolve individual photons. The technology in this study can potentially revolutionise the science reach of rare event search experiments using CPSD (e.g. the CRESST experiment), by offering an alternative light detector solution which is much more scalable and more consistent in performance than conventional CPSDs with cryogenic calorimeter-based light detectors, addressing two major issues that the present techniques face.