We are preparing the CRESST experiment to search for dark matter WIMPs using cryogenic detectors with superconducting phase transition thermometers. In the first stage we plan to use four 250 g sapphire detectors with thresholds of 0.5 keV and resolutions of 0.2 keV at 1 keV. This will provide sensitivity to WIMP masses below 10 GeV, and is thus complementary to other dark matter searches.

Iridium-gold proximity-effect bilayers with critical temperatures between 20 and 100 mK are made for use as superconducting phase transition thermometers for low temperature calorimeters. The reproducibility of the fabrication process of the iridium and gold films is discussed.

We are developing massive cryogenic detectors with low energy thresholds and high resolution for use in a direct Dark Matter search experiment. Our detector consists of a superconducting phase transition thermometer evaporated onto the surface of an absorber crystal. An energy resolution of 220 eV FWHM (for 6 keV X-rays) has been reached with a 31 g sapphire crystal using a proximity-effect thermometer. The latest results for sapphire detectors using tungsten superconducting phase transition thermometers, whose development is just beginning, are also presented. The planned further development and use of such detectors in a search for Dark Matter particles (WIMPs) is discussed, showing their advantages for low WIMP masses.

Taking advantage of the low critical temperatures of proximity-effect phase transition thermometers, superconductors with low Debye temperatures can be used as absorber materials for calorimetric detectors. We performed experiments with lead and tin absorbers of a mass of about 1 g. The energy resolutions for 6 keV X-rays are 1000 eV and 230 eV FWHM, respectively. A model which includes the effect of both thermal and non-thermal phonons explains the measured pulse shapes. The observed heat capacity of the absorbers agrees with that given by the Debye law. A comparison of heat pulses and of radiation induced pulses shows a thermalization efficiency close to 100%. No evidence for trapping of energy as quasiparticles is observed.

We have studied the proximity effect in bilayers of thin films of iridium covered by gold. These structures were evaporated onto sapphire single crystals for use as phase transition thermometers in cryogenic particle detectors. By varying the thicknesses of the iridium and gold layers, we achieved critical temperatures as low as 33 mK. The critical temperature of the bilayers is lower than predicted by the theory of de Gennes-Werthamer, but adding a free parameter to the theory allows good agreement. The transitions of the bilayers typically had widths of a few mK, with the narrowest reaching 0.2 mK, and were always sharper than those of pure iridium films evaporated simultaneously.

We plan a dark matter search using cryogenic calorimetric detectors with superconducting phase transition thermometers. We discuss such an experiment, compare its estimated sensitivity range with other dark matter searches, and discuss its planned realization.

We present results obtained with detectors consisting of dielectric absorber crystals and super-conducting phase transition thermometers made of Ir/Au bilayers. With a 31 g sapphire crystal and an Ir/Au thermometer operated at 45 mK we have obtained an energy resolution of 220 eV (FWHM) for collimated 5.9 keV X-rays. To explain the measured pulse shapes we have developed a model which includes the effect of non-thermal phonons. Results obtained previously with a 18 g Si absorber and an Ir/Au thermometer with broad transition allowed checking this model over a wide temperature range. A heater experiment performed with this calorimeter provides further support of our model.