Radioactivity Backgrounds in ZEPLIN-III

Abstract

We examine electron and nuclear recoil backgrounds from radioactivity in the ZEPLIN-III dark matter experiment at Boulby. The rate of low-energy electron recoils in the liquid xenon WIMP target is 0.75$\pm$0.05 events/kg/day/keV, which represents a 20-fold improvement over the rate observed during the first science run. Energy and spatial distributions agree with those predicted by component-level Monte Carlo simulations propagating the effects of the radiological contamination measured for materials employed in the experiment. Neutron elastic scattering is predicted to yield 3.05$\pm$0.5 nuclear recoils with energy 5-50 keV per year, which translates to an expectation of 0.4 events in a 1-year dataset in anti-coincidence with the veto detector for realistic signal acceptance. Less obvious background sources are discussed, especially in the context of future experiments. These include contamination of scintillation pulses with Cherenkov light from Compton electrons and from $\beta$ activity internal to photomultipliers, which can increase the size and lower the apparent time constant of the scintillation response. Another challenge is posed by multiple-scatter $\gamma$-rays with one or more vertices in regions that yield no ionisation. If the discrimination power achieved in the first run can be replicated, ZEPLIN-III should reach a sensitivity of $\sim 1\times 10^{-8}$ pb$\cdot$year to the scalar WIMP-nucleon elastic cross-section, as originally conceived.

12 pages, 5 figures

References in corpus (13)

• First Dark Matter Results from the XENON100 Experiment
• Results from the First Science Run of the ZEPLIN-III Dark Matter Search Experiment
• First limits on WIMP nuclear recoil signals in ZEPLIN-II: a two phase xenon detector for dark matter detection
• The ZEPLIN-III dark matter detector: instrument design, manufacture and commissioning
• Limits on the spin-dependent WIMP-nucleon cross-sections from the first science run of the ZEPLIN-III experiment
• Nuclear recoil scintillation and ionisation yields in liquid xenon from ZEPLIN-III data
• Measurement of single electron emission in two-phase xenon
• Calculation of neutron background for underground experiments
• Measurements of neutrons produced by high-energy muons at the Boulby Underground Laboratory
• First measurement of low intensity fast neutron background from rock at the Boulby Underground Laboratory
• The ZEPLIN-III dark matter detector: performance study using an end-to-end simulation tool
• The ZEPLIN-III Anti-Coincidence Veto Detector
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• WIMP-nucleon cross-section results from the second science run of ZEPLIN-III
• The LUX-ZEPLIN (LZ) radioactivity and cleanliness control programs
• Single electron emission in two-phase xenon with application to the detection of coherent neutrino-nucleus scattering
• Nuclear recoil scintillation and ionisation yields in liquid xenon from ZEPLIN-III data
• A Review of Basic Energy Reconstruction Techniques in Liquid Xenon and Argon Detectors for Dark Matter and Neutrino Physics Using NEST
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• Performance data from the ZEPLIN-III second science run
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