S. Cooper

Model for cryogenic particle detectors with superconducting phase transition thermometers

We present data on a detector composed of an 18 g Si crystal and a superconducting phase transition thermometer which could be operated over a wide temperature range. An energy resolution of 1 ke V (FWHM) has been obtained for 60 keV photons. The signals consist of two components: a fast one and a slow one, with decay times of 1.5 ms and 30–60 ms, respectively. In this paper we present a simple model which takes thermal and non-thermal phonon processes into account and provides a description of the observed temperature dependence of the pulse shape. The fast component, which completely dominates the signal at low temperatures, is due to high-frequency non-thermal phonons being absorbed in the thermometer. Thermalization of these phonons then leads to a temperature rise of the absorber, which causes the slow thermal component. At the highest operating temperatures (T ∼ 80 mK) the amplitude of the slow component is roughly as expected from the heat capacity of the absorber. The strong suppression of the slow component at low temperatures is explained mostly as a consequence of the weak thermal coupling between electrons and phonons in the thermometer at low temperatures.

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.

Quasiparticle diffusion over several mm in cryogenic detectors

Abstract The use of quasiparticle diffusion in a superconducting film has the potential to allow an increase in the size of a cryogenic detector without proportional loss of energy resolution. The quasiparticle lifetime and the diffusion constant are critical parameters which have limited this development. Using W superconducting phase transition thermometers as the sensors and a W/Al bilayer as the diffusion film, we have measured quasiparticle diffusion over a distance of 2 mm and deduced a diffusion constant of D=2.5×10 −4 m 2 / s and a quasiparticle lifetime of τ=9.0 ms , which is, to our knowledge, by far the longest ever observed. With Ir/Au thermometers and an Ir/Au/Al diffusion film we found D=4.6×10 −3 m 2 / s and τ=0.43 ms with diffusion over 4 mm , the longest distance observed to date.

A calorimetric particle detector using an iridium superconducting phase transition thermometer

Abstract We report on a calorimetric particle detector consisting of an 18.3 g silicon crystal and an iridium superconducting phase transition thermometer. The cryogenic calorimeter and the associated apparatus are described in detail. The pulses from irradiation with an α-particle source have a large unexpected overshoot in addition to the component expected from a naive thermal model. The pulse height spectrum displays an energy resolution of 1% FWHM at 6 MeV and good linearity. The noise, electrothermal feedback, and position dependence are discussed.

Massive cryogenic particle detectors with low energy threshold

Abstract We have developed massive cryogenic particle detectors to be used in the CRESST dark matter search. Each detector is made of a sapphire crystal and a tungsten superconducting phase transition thermometer. In this paper, we report on the results obtained with four 262 g detectors, which show energy thresholds as low as 350 eV and good energy resolution at low energies. The shape of the experimental pulses, the linearity of the detector response and the energy dependence of the resolution are discussed.

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.