A Xe detector for WIMP search

For the ever-challenging dark matter direct detection, we have developed an optimized detector for over a decade. The ZEPLIN II detector with liquid Xe as a target material is now under construction at UCLA, which will be installed in the 1100-m deep UK Boulby salt mine in early 2002. Two-phase (liquid and gas) Xe scheme has been shown to be very promising for a 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-keV to 100-keV with the 5-keV bin is linear without any significant statistical fluctuations. The modified light collection efficiency (f_mod = recorded photoelectrons/incident photons) is uniform throughout the entire range of gamma energies in this study. 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.