G. Salinas

Measurement of single electron emission in two-phase xenon

We present the first measurements of the electroluminescence response to the emission of single electrons in a two-phase noble gas detector. Single ionization electrons generated in liquid xenon are detected in a thin gas layer during the 31-day background run of the ZEPLIN-II experiment, a two-phase xenon detector for WIMP dark matter searches. Both the pressure dependence and magnitude of the single electron response are in agreement with previous measurements of electroluminescence yield in xenon. We discuss different photoionization processes as possible cause for the sample of single electrons studied in this work. This observation may have implications for the design and operation of future large-scale two-phase systems.

Limits on spin-dependent WIMP-nucleon cross-sections from the first ZEPLIN-II data

The first underground data run of the ZEPLIN-II experiment has set a limit on the nuclear recoil rate in the two-phase xenon detector for direct dark matter searches. In this Letter the results from this run are converted into the limits on spin-dependent WIMP-proton and WIMP-neutron cross-sections. The minimum of the curve for WIMP-neutron cross-section corresponds to 7 × 10−2 pb at a WIMP mass of around 65 GeV.

SIGN, a WIMP Detector Based on High Pressure Gaseous Neon

A newWIMP detector concept based on the measurement of Scintillation and Ionization in Gaseous Neon (SIGN) is presented. The detector employs room temperature gaseous neon at a pressure of ≥100 bars as the WIMP target. The ionization is readout using either charge gain or electrofluorescence or both in a modified cylindrical proportional chamber geometry. The primary scintillation is detected by placing a CsI photocathode on the inside wall of the cylindrical chamber. The neon is doped with xenon (≤0.5%) for signal enhancement. Theoretical considerations suggest that the measurement of both scintillation and ionization will provide discrimination between nuclear and electron recoils in this gas mixture.

A High Pressure Noble Gas Approach for WIMP Detection

Initial measurements of the charge and light yield in pure xenon gas at 20 bar are discussed. Preliminary findings are that the yields are not greatly different from those reported in liquid xenon. Also, they are similar to yields observed in 50 bar Ar(Xe) and 100 bar Ne(Xe) in the same apparatus. In addition, good nuclear recoil discrimination is observed at low recoil energies relevant to WIMP interactions. The findings suggest that a room‐temperature, high pressure gas approach may be an attractive alternative to liquid phase detectors for future large scale WIMP measurement experiments.

R & D for Future ZEPLIN

We propose a new concept for a very low background Dark Matter experiment using multi-ton liquid xenon. The detector consists of two concentric spheres and a charge readout device in the centre. Xenon between the two spheres forms a self-shield and veto device. The inner surface of the centre sphere is coated with CsI to form an internal photocathode with minimum of 2π coverage for any event in the active volume. Photoelectrons from the CsI photocathode drift toward the charge readout micro-structure in the centre of the detector. Both scintillation and ionisation is measured simultaneously for background rejection and 3-D event mapping. In addition to external shielding, the low background is achieved by eliminating PMTs and by using low radioactivity pure materials throughout the detector. We present detailed calculations of the charge readout system and design details. The detector is expected to probe the full SUSY parameter space.

Status of ZEPLIN II and ZEPLIN III

We describe the ZEPLIN II (30-kg) and ZEPLIN III (7-kg) discriminating dark matter detector using two-phase xenon designed for direct detection of cold dark matter in the form of Weakly Interacting Massive Particles. These two detectors are currently being commissioned. Both detector will begin operation in the Boulby Mine, UK in 2005. ZEPLIN II & III are capable of discriminating between nuclear recoils and background events and have a design reach up to two orders of magnitude beyond current limits. These two detectors will also serve as a step in the development program for a next-generation ton-scale detector.