Development of a two phase xenon detector for use in direct dark matter searches

Abstract

This thesis presents research into the development of a two phase xenon detector for direct dark matter searches. The research was undertaken at Imperial College London, as part of the United Kingdom Dark Matter Collaboration during the years 1997 to 2003. Dark matter particles should interact with ordinary matter, depositing energy in a detector resulting in scintillation photons and ionisation. Xenon is a favourable target as it has a high light yield (78600 photons/MeV), has naturally occuring isotopes sensitive to spin dependent interactions, is dense when liquefied and can be obtained with low levels of intrinsic radioactivity. In this study, experiments with a scintillation test cell indicated the possibility for discrimination between nuclear recoil and electron recoil events using the pulse shape of the scintillation light. Measurements gave a decay time constant of 23±3ns for a-particle recoils and 43±8ns for electron recoils. Neutron recoil measurements at the Neutron Beam Facility at Sheffield University measured the quenching factor of liquid xenon to be 0.17±0.01. To reduce light losses the possibility of operating photomultipliers at liquid xenon temperatures was investigated. Results from these experiments indicated that photomultipliers remain linear at temperatures of -104.5°C, however, there is a decrease in response to pulsed light at frequencies of 2kHz. A two phase prototype detector was used to measure the scintillation photons and ionisation from interactions of a-particles and gamma-rays with xenon. These tests gave a discrimination factor of 90% between the two distributions of secondary to primary signal ratios. Monte Carlo analysis of the performance of ZEPLIN III addressed the light collection, charge drift and extraction, electroluminescence output and position sensitivity of the detector. A gamma-ray transportation code was also used to assess the background radioactivity due to the immersion of photomultipliers in the liquid xenon volume. Results from these programs indicate that ZEPLIN III will have a discrimination factor of 99.81% and be sensitive to spin independent cross sections of 7 x 10_44cm2.