J.E. McMillan

The ZEPLIN-III dark matter detector: Instrument design, manufacture and commissioning

We present details of the technical design, manufacture and testing of the ZEPLIN-III dark matter experiment. ZEPLIN-III is a two-phase xenon detector which measures both the scintillation light and the ionisation charge generated in the liquid by interacting particles and radiation. The instrument design is driven by both the physics requirements and by the technology requirements surrounding the use of liquid xenon. These include considerations of key performance parameters, such as the efficiency of scintillation light collection, restrictions placed on the use of materials to control the inherent radioactivity levels, attainment of high vacuum levels and chemical contamination control. The successful solution has involved a number of novel design and manufacturing features which will be of specific use to future generations of direct dark matter search experiments as they struggle with similar and progressively more demanding requirements.

Limits on WIMP cross-sections from the NAIAD experiment at the Boulby Underground Laboratory

The NAIAD experiment (NaI Advanced Detector) for WIMP dark matter searches at the Boulby Underground Laboratory (North Yorkshire, UK) ran from 2000 until 2003. A total of 44.9 kg x years of data collected with 2 encapsulated and 4 unencapsulated NaI(Tl) crystals with high light yield were included in the analysis. We present final results of this analysis carried out using pulse shape discrimination. No signal associated with nuclear recoils from WIMP interactions was observed in any run with any crystal. This allowed us to set upper limits on the WIMP-nucleon spin-independent and WIMP-proton spin-dependent cross-sections. The NAIAD experiment has so far imposed the most stringent constraints on the spin-dependent WIMP-proton cross-section.

Neutron background in large-scale xenon detectors for dark matter searches

Abstract Simulations of the neutron background for future large-scale particle dark matter detectors are presented. Neutrons were generated in rock and detector elements via spontaneous fission and (α,n) reactions, and by cosmic-ray muons. The simulation techniques and results are discussed in the context of the expected sensitivity of a generic liquid xenon dark matter detector. Methods of neutron background suppression are investigated. A sensitivity of 10 −9 –10 −10 pb to WIMP-nucleon interactions can be achieved by a tonne-scale detector.

Measurement of scintillation efficiencies and pulse-shapes for nuclear recoils in NaI(Tl) and CaF2(Eu) at low energies for dark matter experiments

Abstract Measurements have been performed with a 2.85 MeV mono-energetic neutron beam of relative scintillation efficiency and pulse-shape for nuclear and electron recoils in NaI(Tl) and CaF2(Eu). Scintillation efficiencies in NaI(Tl) relative to 60 keV gamma events were found to be 27.5±1.8% for Na recoils (recoil energy Erec>4 keV) and 8.6±0.7% for I recoils (Erec>10 keV). Relative scintillation efficiencies in CaF2(Eu) for Ca and F recoils show some evidence for a fall with energy (17% to 8% for F) for 10 keV 〈 t i 〉 of 263±15 ns for Na events (visible energy Evis in the range 2–8 keV and 272±10 ns for I events (2 keV 〈 t i 〉 of 4 keV 〈 t i 〉 for 2 keV

Simulations of muon induced neutron flux at large depths underground

The production of neutrons by cosmic-ray muons at large depths underground is discussed. The most recent versions of the muon propagation code MUSIC, and particle transport code FLUKA are used to evaluate muon and neutron fluxes. The results of simulations are compared with experimental data.

NaI dark matter limits and the NAIAD array – a detector with improved sensitivity to WIMPs using unencapsulated NaI

Re-analysis of published data from the UKDMC NaI Tl dark matter experiment is presented using latest spin factors and comparison is made with the sensitivity predicted for NAIAD, a 100 kg NaI detector concept based on unencapsulated . NaI Tl . We present experimental results and Monte Carlo simulations for NAIAD and show that a factor of 1.5-2 improvement in energy threshold is achievable over conventional NaI dark matter detectors with consequent ; 50% improvement in nuclear recoil discrimination at 10 keV. An overall improvement in sensitivity to spin dependent WIMP interactions of factor 50, based on 100 kg = yrs of data, is predicted relative to previous UKDMC limits. q 2000 Published by Elsevier Science B.V. All rights reserved.

Measurements of muon flux at 1070 m vertical depth in the Boulby underground laboratory

Abstract Measurements of cosmic-ray muon rates and energy deposition spectra in a 1 t liquid scintillator detector at 1070 m vertical depth in the Boulby underground laboratory are discussed. In addition, the simulations used to model the detector are described. The results of the simulations are compared to the experimental data and conclusions given. The muon flux in the laboratory is found to be (4.09±0.15)×10 −8 cm −2 s −1 .

CsI(Tl) for WIMP dark matter searches

Abstract We report a study of CsI(Tl) scintillator to assess its applicability in experiments to search for dark matter particles. Measurements of the mean scintillation pulse shapes due to nuclear and electron recoils have been performed. We find that, as with NaI(Tl), pulse shape analysis can be used to discriminate between electron and nuclear recoils down to 4 keV. However, the discrimination factor is typically 10–15% better than in NaI(Tl) above 4 keV. The quenching factor for caesium and iodine recoils was measured and found to increase from 11% to ∼17% with decreasing recoil energy from 60 to 12 keV. Based on these results, the potential sensitivity of CsI(Tl) to dark matter particles in the form of neutralinos was calculated. We find an improvement over NaI(Tl) for the spin-independent WIMP–nucleon interactions up to a factor of 5 assuming comparable electron background levels in the two scintillators.

The ZEPLIN-III dark matter detector: Performance study using an end-to-end simulation tool

We present results from a GEANT4-based Monte Carlo tool for end-to-end simulations of the ZEPLIN-III dark matter experiment. ZEPLIN-III is a two-phase detector which measures both the scintillation light and the ionisation charge generated in liquid xenon by interacting particles and radiation. The software models the instrument response to radioactive backgrounds and calibration sources, including the generation, ray-tracing and detection of the primary and secondary scintillations in liquid and gaseous xenon, and subsequent processing by data acquisition electronics. A flexible user interface allows easy modification of detector parameters at run time. Realistic datasets can be produced to help with data analysis, an example of which is the position reconstruction algorithm developed from simulated data. We present a range of simulation results confirming the original design sensitivity of a few times 10−8 pb to the WIMP-nucleon cross-section.

Characteristics of alpha, gamma and nuclear recoil pulses from NaI(Tl) at 10–100 keV relevant to dark matter searches

Abstract Measurements of the shapes of scintillation pulses produced by nuclear recoils, alpha particles and photons in NaI(Tl) crystals at visible energies of 10–100 keV have been performed in order to investigate possible sources of background in NaI(Tl) dark matter experiments and, in particular, the possible origin of the anomalous fast time constant events observed in the UK Dark Matter Collaboration experiments at Boulby mine [P.F. Smith et al., Phys. Rep. 307 (1998) 275]. Pulses initiated by X-rays (via photoelectric effect close to the surface of the crystal) were found not to differ from those produced by high-energy photons (via Compton electrons inside the crystal) within experimental errors. However, pulses induced by alpha particles (degraded from an external MeV source) were found to be ∼10% faster than those of nuclear recoils, but insufficiently fast to account for the anomalous events.