Preliminary results on position reconstruction for ZEPLIN III

ZEPLIN III is a two-phase xenon detector for direct dark matter search that will look for rare nuclear recoils from elastic scattering of Weakly Interacting Massive Particles off xenon atoms. ZEPLIN III measures both the scintillation and ionisation signals produced in the liquid by the interacting particle. The prompt scintillation is detected by an array of 31 photomultipliers (PM) immersed in the liquid, and a strong electric field extracts the ionisation electrons to the vapour phase. Electroluminescence photons are produced in the gas and detected by the same PM array. In this paper we describe position reconstruction methods that use both these signals to achieve a position accuracy of a few millimetres in the horizontal plane and sub-millimetre in the vertical coordinate