Finally, tantalisingly, the Cryogenic Dark Matter Search (CDMS) released the results of its latest (and final) effort to search for the Dark Matter that seems to make up most of the matter in the Universe, but doesn’t seem to be the same stuff as the normal atoms that we’re made of. Under some theories, the dark matter would interact weakly with normal matter, and in such a way that it could possibly be distinguished from all the possible sources of background. These experiments are therefore done deep underground — to shield from cosmic rays which stream through us all the time — and with the cleanest and purest possible materials — to avoid contamination with both both naturally-occurring radioactivity and the man-made kind which has plagued us since the late 1940s.See:Doctors, Deep Fields and Dark Matter (Bold added for emphasis by me)
Observational studies of the rotation of galaxies and groups of galaxies strongly suggest the existence of a dominating amount of matter invisible at any electromagnetic wavelengths. One of the favoured forms of this "missing mass", both theoretically and observationally, is the WIMP (Weakly Interacting Massive Particle). These cold WIMPs are expected to be scattered by the nuclei of typical detector material at a rate of less than one per kg per day, yielding energy depositions in the 1-50 keV energy range.
ZEPLIN-III is a two-phase (liquid/gas) xenon detector looking for galactic WIMP dark matter at the Boulby Underground Laboratory, North Yorkshire, UK, at a depth of 1100 m. At this depth the cosmic-ray background is reduced by a factor of a million. The WIMP target consists of 12 kg of cold liquid xenon topped by a thin layer of xenon gas. These are viewed by an array of 31 photomultiplier tubes immersed in the liquid.
The detector operates at higher electric fields than other, similar systems, namely its predecessor ZEPLIN-II, and provides high-precision reconstruction of the interaction point in three dimensions. Together with the low-background construction (mainly high-purity copper), these features will give ZEPLIN-III higher sensitivity in direct WIMP searches.
The ZEPLIN-III Collaboration includes the University of Edinburgh, Rutherford Appleton Laboratory, Imperial College London, LIP-Coimbra (Portugal) and ITEP-Moscow (Russia).
ZEPLIN-III PICTURE GALLERY
Photomultiplier array covered by electrode grid