ZEPLIN-III

The ZEPLIN-III dark matter experiment attempted to detect galactic WIMPs using a 12 kg liquid xenon target.

This was the last in a series of xenon-based experiments in the ZEPLIN programme pursued originally by the UK Dark Matter Collaboration (UKDMC).

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.

In the late 1980s the UKDMC had explored the potential of different materials and techniques, including cryogenic LiF, CaF2, silicon and germanium, from which a programme emerged at Boulby based on room-temperature NaI(Tl) scintillators.

This self-shielding effect (alluded to by the 'zoned' term in the contrived ZEPLIN acronym) explains the faster gain in sensitivity of these targets compared to technologies based on a modular approach adopted with crystal detectors, where each module brings its own background.

ZEPLIN-III was proposed in the late 1990s,[6] based partly on a similar concept developed at ITEP,[7] and built by Prof. Tim Sumner and his team at Imperial College.

The photomultiplier array contained 31 photon detectors viewing the WIMP target from below, immersed in the cold liquid xenon.

[8] ZEPLIN–II and –III were purposely designed in different ways, so that the technologies employed in each sub-system could be appraised and selected for the final experiment proposed by the UKDMC: a tonne-scale xenon target (ZEPLIN-MAX) capable of probing most of the parameter space favored by theory at that point (1 × 10−10 pb), although this latter system was never built in the UK for lack of funding.

[11][12] It also ruled out an inelastic WIMP scattering model which attempted to reconcile a positive claim from DAMA with the absence of signal in other experiments.

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. The lower chamber contained liquid nitrogen to provide cooling. The detector was surrounded by Gd-loaded polypropylene to moderate and capture neutrons, a potential source of background. The gamma-rays from neutron capture were detected by 52 modules of plastic scintillator placed around the moderator. The shielding was completed by a 20-cm thick lead castle.
Signal from ZEPLIN-III two-phase xenon detector. The fast scintillation pulse (S1) is generated promptly by scintillation in the liquid; a larger, delayed pulse (S2) is obtained once the ionisation drifted from the interaction site is emitted into the thin gas phase above the liquid. The insets below the signal traces show Monte Carlo simulation of the optical signals.