ZEPLIN-III
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The ZEPLIN-III dark matter experiment attempted to detect galactic WIMPs using a 12 kg liquid xenon target. It operated from 2006 to 2011 at the
Direct
Detection technology
Condensed noble gases, most notably liquid xenon and liquid argon, are excellent radiation detection media. They can produce two signatures for each particle interaction: a fast flash of light (scintillation) and the local release of charge (ionisation). 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. 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. These systems are also known as xenon 'emission detectors'.[1]
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. The horizontal coordinates can be reconstructed from the hit pattern in the photomultiplier array(s). 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. WIMP candidate events produce lower ionisation/scintillation ratios than the more prevalent background interactions.
The ZEPLIN programme pioneered the use of two-phase technology for WIMP searches. The technique itself, however, was first developed for radiation detection using argon in the early 1970s. is developing similar systems that have set improved limits.
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History
The ZEPLIN (ZonEd Proportional scintillation in LIquid Noble gases) series of experiments was a progressive programme pursued by the UK Dark Matter Collaboration using liquid xenon. It evolved alongside the
ZEPLIN-I, a 3 kg liquid xenon target, operated at Boulby from the late 1990s.
ZEPLIN-II was the first two-phase system deployed to search for dark matter in the world;[2] it consisted of a 30 kg liquid xenon target topped by a 3 mm layer of gas in a so-called three-electrode configuration: separate electric fields were applied to the bulk of the liquid (WIMP target) and to the gas region above it by using an extra electrode underneath the liquid surface (in addition to an anode grid, located above the gas, and a cathode, at the bottom of the chamber). In ZEPLIN-II an array of 7 photomultipliers viewed the chamber from above in the gas phase.
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. It was deployed underground at Boulby in late 2006, where it operated until 2011. It was a two-electrode chamber, where electron emission into the gas was achieved by a strong (4 kV/cm) field in the liquid bulk rather than by an additional electrode. 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.
Results
Although the ZEPLIN-III liquid xenon target was built on the same scale as that of its ZEPLIN predecessors, it achieved significant improvements in WIMP sensitivity due to the higher discrimination factor achieved and to a lower overall background. In 2011 it published exclusion limits on the spin-independent WIMP-nucleon elastic scattering cross-section above 3.9 × 10−8 pb for a 50 GeV WIMP mass.
References
- ^ a b B. A. Dolgoshein, V. N. Lebedenko & B. I. Rodionov, "New method of registration of ionizing-particle tracks in condensed matter", JETP Lett. 11(11): 351 (1970)
- ^ S2CID 1044263.
- ^ See full UKDMC reference list in http://hepwww.rl.ac.uk/ukdmc/pub/fulpub.html
- ISSN 0370-2693.
- ISSN 0927-6505.
- ^ T. J. Sumner et al., "ZEPLIN-III: a two-phase xenon dark matter detector, in: Proc. 3rd Int. Workshop. Id. Dark Matter, Spooner & Kudryavtsev (Eds): World Scientific, pp. 452–456 (2001)
- ^ D. Yu. Akimov et al., "Scintillation two-phase xenon detector with gamma and electron-background rejection for dark matter search", in: Sources and Detection of Dark Matter in the Universe: North Holland, pp. 461–464 (1998)
- S2CID 11911700.
- S2CID 14136134.
- S2CID 9685630.
- S2CID 119237969.
- S2CID 1349055.
- S2CID 67836272.