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.

A fundamental question of astrophysics and cosmology is the nature of dark matter. Astrophysical observations show clearly the existence of some kind of dark matter, though they cannot yet reveal its nature. Dark matter can consist of baryonic particles, or of other (known or unknown) elementary particles. Baryonic dark matter probably exists in the form of dust, gas, or small stars. Other elementary particles constituting the dark matter can possibly be measured in terrestrial experiments. Possibilities for dark matter particles are neutrinos, axions, and weakly interacting massive particles (WIMPs). While a direct detection of relic neutrinos seems at the moment impossible, there are experiments looking for baryonic dark matter in the form of Massive Compact Halo Objects, and for particle dark matter in the form of axions and WIMPS.

A tungsten superconducting phase transition thermometer on a 32 g sapphire crystal has given an energy resolution of 100 eV (FWHM) for 1.5 keV X-rays, increasing to 440 eV at 14 keV. A possibility to obtain similar resolution in much larger crystals by using Al films as phonon collectors is presented.

We have studied the proximity effect in bilayers of thin films of iridium covered by gold. 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 [J. J. Hauser, H. C. Treuerer, and N. R. Werthamer, Phys. Rev. 136, A637 (1964)], 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 steeper than those of pure iridium films evaporated simultaneously. Such bilayers can be used as superconducting phase transition thermometers in cryogenic particle detectors.

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.

We use massive superconducting absorbers made of molybdenum and vanadium as low temperature calorimetric particle detectors. The high resolution of our thermometry system, consisting of a superconducting phase transition thermometer monitored with a dc-SQUID, enables us to detect α and γ particles with large single crystals. Heat pulses generated by passing current through a metal film on the surface of the crystal are used to study the response of the calorimeters. With a 35 g molybdenum single crystal we obtain an energy resolution of 10% FWHM on 5.8 MeV α particles at an operating temperature of 120 mK. The observed temperature rise of 1.2 μK is a factor of 8 less than expected from the calculated heat capacity. Using a 15 g vanadium single crystal, the energy resolution on 5.8 MeV α particles is 1.2% FWHM. In this case, the pulse height of 6.1 μK is a factor of 8 smaller than expected from the calculated heat capacity. A possible reason for the large deviations from the expected heat capacity is the presence of hydrogen dissolved in vanadium. Impurities and defects on the surface of the crystals probably contribute also to the heat capacity. The pulse shapes of all observed events are very similar, whether they are created by absorption of α or γ radiation or by an electrically created heat pulse, and consists of two exponential decaying parts. A comparison of the pulse heights of heater and radiation pulses leads us to the conclusion that all the energy deposited in the absorber crystal is converted into phonons.

We describe a thermometry system for low temperature calorimetric particle detectors, consisting of a phase transition thermometer combined with a SQUID readout. Studies were performed of a cadmium phase transition thermometer attached to a 1.1 g silicon absorber. Operated at 455 mK an energy resolution of ΔEE = 1% for irradiation with 5.8 MeV α-particles was achieved. This resolution was limited by the data acquisition system used. We also studied an iridium thermometer attached to a 280 g sapphire absorber operated at 135 mK. This calorimeter exhibited an energy resolution of ΔEE = 1.2% for irradiation with 5.8 MeV α-particles. The change in temperature caused by the absorption of one α-particles was measured to be 4.5 μK.