Sapphires that search for dark matter

12 August 1998

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The above photograph shows one of the four 262 g sapphire crystals which will be used by the CRESST (Cryogenic Rare Event Search with Superconducting Thermometers) Collaboration to search for WIMPs - weakly interacting, massive particles that may make up some, if not all, of the dark matter in the Universe. Each crystal has a small tungsten "superconducting phase transition thermometer" evaporated on to its surface. The four crystals are assembled together to form a 1 kg block.

WIMP events are expected to be rare, so all possible precautions are taken to minimise unwanted "background" signals. By installing CRESST deep under the Gran Sasso mountain in Italy in the Gran Sasso Underground Laboratory, the detector is protected from cosmic rays and other unwanted backgrounds. In addition, the effects of background radioactivity are minimised by surrounding the sapphire block with layers of only the highest purity copper and lead. In addition, all the copper components are electropolished to remove the last traces of surface contamination.

To increase the sensitivity of the detector to potential WIMP events, in which the particles might deposit tiny amounts of energy in the sapphire while scattering off its nuclei, the sapphire detector is cooled to 15 mK using a dilution refrigerator (pictured here). This lowers the heat capacity of the sapphire block, and so increases the temperature rise associated with any energy deposited in the block. Moreover, at 15 mK the tungsten thermometers are in the middle of the transition between their superconducting and normal conducting phases and so their resistance is very sensitive to small temperature changes.

Credit: Physics Photographic Unit, University of Oxford

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