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WIMP candidate events produce lower ionisation/scintillation ratios than the more prevalent background interactions. Critically for WIMP searches, the ratio between the two response channels (scintillation and ionisation) allows the rejection of the predominant backgrounds for WIMP searches: gamma and beta radiation from trace radioactivity in detector materials and the immediate surroundings. The horizontal coordinates can be reconstructed from the hit pattern in the photomultiplier array(s). This configuration is that of a time projection chamber (TPC) it allows three-dimensional reconstruction of the interaction site, since the depth coordinate (z) can be measured very accurately from the time separation between the two light pulses. These systems are also known as xenon 'emission detectors'. Once in the gas, they generate a second, larger pulse of light ( electroluminescence or proportional scintillation), which is detected by the same array of photomultipliers. In two-phase xenon – so called since it involves liquid and gas phases in equilibrium – the scintillation light produced by an interaction in the liquid is detected directly with photomultiplier tubes the ionisation electrons released at the interaction site are drifted up to the liquid surface under an external electric field, and subsequently emitted into a thin layer of xenon vapour. They can produce two signatures for each particle interaction: a fast flash of light ( scintillation) and the local release of charge ( ionisation). Their nature is not yet known, but no sensible candidates remain within the Standard Model of particle physics to explain the dark matter problem.Ĭondensed noble gases, most notably liquid xenon and liquid argon, are excellent radiation detection media. These hypothetical elementary particles could be Weakly Interacting Massive Particles, or WIMPs, weighing as little as a few protons or as much as several heavy nuclei. It ruled out cross-sections for elastic scattering of WIMPs off nucleons above 3.9 × 10 −8 pb (3.9 × 10 −44 cm 2) from the two science runs conducted at Boulby (83 days in 2008 and 319 days in 2010/11).ĭirect dark matter search experiments look for extremely rare and very weak collisions expected to occur between the cold dark matter particles that are believed to permeate our galaxy and the nuclei of atoms in the active medium of a radiation detector. The ZEPLIN-III project was led by Imperial College London and also included the Rutherford Appleton Laboratory and the University of Edinburgh in the UK, as well as LIP-Coimbra in Portugal and ITEP-Moscow in Russia.
ZEPLIN III SERIES
This was the last in a series of xenon-based experiments in the ZEPLIN programme pursued originally by the UK Dark Matter Collaboration (UKDMC). It operated at the Boulby Underground Laboratory (North-East England, UK) in the period 2006–2011. The ZEPLIN-III dark matter experiment attempted to detect galactic WIMPs using a 12 kg liquid xenon target. The shielding was completed by a 20-cm thick lead castle. The gamma-rays from neutron capture were detected by 52 modules of plastic scintillator placed around the moderator. The detector was surrounded by Gd-loaded polypropylene to moderate and capture neutrons, a potential source of background. The lower chamber contained liquid nitrogen to provide cooling. 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.ZEPLIN-III experiment: the WIMP detector, built mainly out of copper, included two chambers within a cryostat vessel: the upper one contained 12 kg of active liquid xenon an array of 31 photomultipliers operated immersed in the liquid to detect prompt scintillation as well as delayed electroluminescence from a thin gas layer above the liquid. 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 instrument design is driven by both the physics requirements and by the technology requirements surrounding the use of liquid xenon. 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. We present details of the technical design, manufacture and testing of the ZEPLIN-III dark matter experiment.