An Xe detector for WIMP search

For the ever-challenging dark matter direct detection, we have been developing an optimized detector for more than a decade. The ZEPLIN II detector with liquid Xe as a target material is now under construction at the University of California, Los Angeles, which will be installed in the 1100-m deep U.K. Boulby salt mine in early 2002. A two-phase (liquid and gas) Xe scheme has been shown to be very promising for superior dark matter particle identification. We present Monte Carlo simulation studies of light collection for the ZEPLIN II central detector. The simulation code includes absorption, reflection, and refraction at the interface between liquid and gas. The response of photopeaks to each energy level from 5 to 100 keV with the 5-keV bin is linear without any significant statistical fluctuations. The system scintillation detection efficiency (f_sys=recorded photoelectrons/incident photons) is uniform throughout the entire range of gamma energies in this paper. We also discuss the need for signal amplification and a large-scale detector in order to cover the comprehensive SUSY-WIMP region with the lowest possible energy threshold.