U. Nagel

Proximity effect in iridium‐gold bilayers

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.

Low-energy X-ray detection in cryogenic detectors with tungsten thermometers

Abstract In the course of our development of calorimetric particle detectors with superconducting phase transition thermometers, we have succeeded in depositing epitaxial α-tungsten films on sapphire which have critical temperatures T c near 15 mK. To our knowledge this is the first time that the T c of bulk tungsten has been observed in thin films. Such films used as thermometers are very sensitive and provide good energy resolution: with 4 g and 32 g sapphire crystals energy resolutions of better than 100 eV (FWHM) for 1.5 keV X-rays have been achieved.

A massive cryogenic particle detector with good energy resolution

Abstract Massive cryogenic particle detectors are being developed for use in a search for dark matter particles. Results with a 31 g sapphire crystal and a superconducting phase transition thermometer operated at 44 mK are presented. The observed signal includes a fast component which is significantly larger than the expected thermal pulse. The energy resolution is 210eV (FWHM) for 6keV X-rays.

Status and low background considerations for the CRESST dark matter search

Abstract 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.

Physics and Performance of Calorimetric Particle Detectors with Dielectric Absorbers and Superconducting Phase Transition Thermometers

We present results obtained with detectors consisting of dielectric absorber crystals and superconducting 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 ke V 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.

Munich dark matter search

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.

Progress on fabrication of iridium-gold proximity-effect thermometers

Abstract 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.

Superconducting Tungsten Films for Use as Phase Transition Thermometers for Calorimetric Detectors

To improve the sensitivity of calorimetric particle detectors we want to produce low-Tc superconducting thin films to be used as phase transition thermometers. We have succeeded in depositing epitaxial α-tungsten films on sapphire which have critical temperatures Tc near 15 mK. To our knowledge this is the first time that the Tc of bulk tungsten has been observed in thin films. Such a film has been produced on a 4 g sapphire crystal and operated as a calorimeter, giving an energy resolution of 75 eV (FWHM) for 1.5 keV X-rays.

Cryogenic particle detectors with superconducting phase transition thermometers

Abstract 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.

Use of Proximity Effect in Iridium-Gold Superconducting Phase Transition Thermometers

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.