‘Naked’ crystals go underground

30 June 2003

On 5 May, four “naked” high-purity germanium detectors were installed in liquid nitrogen in the GENIUS Test Facility (GENIUS-TF) at the Gran Sasso Underground Laboratory. This is the first time this novel technique for background reduction in searches for rare decays is being tested under realistic background conditions.

The goal of the GENIUS-TF, led by Hans Volker Klapdor-Kleingrothaus of Heidelberg, is to test detector techniques for the GENIUS project, which will search with extreme sensitivity for cold dark matter, double beta decay and low-energy solar neutrinos. In its dark-matter version, GENIUS will operate 40 naked germanium crystals (100 kg) in a 12 x 12 m tank of liquid nitrogen.


In the test facility the naked crystals sit on a plate made from a special type of Teflon, within a thin-walled copper box filled with 70 litres of purified liquid nitrogen. The copper is thermally shielded by 20 cm of special low-activity Styropor, surrounded by a 15 tonne shield of electrolytic copper (10 cm thick) and 35 tonnes of lead (20 cm thick). The complete set-up has a neutron shield of 10 cm boronpolyethylene. A digital data acquisition system allows the simultaneous measurement of energy, pulse shapes and other parameters of individual events.

On the day they were installed, the four detectors – a total of 10 kg of high-purity natural germanium – were tested with radioactive sources of 60Co and 228Th. They showed good energy resolution and it was found that microphonics in the liquid nitrogen is not a problem.

Besides testing construction parameters, one of the first goals of GENIUS-TF will be to investigate the signal of cold dark matter reported by the DAMA collaboration in 1999, which could originate from the modulation of the WIMP flux by the motion of the Earth relative to the Sun.

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