M. Bruckmayer

Limits on WIMP dark matter using sapphire cryogenic detectors

Abstract Data taken by CRESST with a cryogenic detector system based on 262 g sapphire crystals has been used to place limits on WIMP dark matter in the Galactic Halo. The experiment was especially sensitive for low-mass WIMPs with spin-dependent cross sections and improves on existing limits in this region.

The CRESST dark matter experiment: status and perspectives

Abstract The CRESST experiment in its first phase is using sapphire detectors with tungsten phase transition thermometers to search for dark matter WIMPs. At present four 262 g detectors are performing first measurements under low background conditions. Detector performance as well as preliminary results from the background runs are presented. A second phase of CRESST using CaWO 4 and simultaneous measurement of phonons and scintillation light is in preparation.

Active thermal feedback for massive cryogenic detectors

Abstract A method to stabilize cryogenic detectors with superconducting phase transition thermometers in their operating point is presented. Measurements of X-ray lines emitted by an 55 Fe X-ray fluorescence source showed an improvement in energy resolution from 230 to 133 eV on the 1.5 keV aluminium line with this technique. Furthermore the required set-up allows to simulate real events by injecting heat pulses into the thermometer and in this way to calibrate the detector and to monitor its long-term stability.

The CRESST dark matter search

We discuss the short-and long-term perspectives of the CRESST (Cryogenic Rare Event Search using Superconducting Thermometers) project and present the current status of the experiment and new results concerning detector development. In the search for elementary particle dark matter, CRESST is presently the most advanced deep underground, low-background, cryogenic facility. The basic technique involved is to search for WIMPs (Weakly Interacting Massive Particles) by the measurement of nonthermal phonons, as created by WIMP-induced nuclear recoils. Combined with our newly developed method for the simultaneous measurement of scintillation light, strong background discrimination is possible, resulting in a substantial increase in WIMP detection sensitivity. This will allow a test of the reported positive evidence for a WIMP signal by the DAMA Collaboration in the near future. In the long term, the present CRESST setup permits the installation of a detector mass up to 100 kg. In contrast to other projects, CRESST technology allows the employment of a large variety of detection materials. This offers a powerful tool in establishing a WIMP signal and in investigating WIMP properties in the event of a positive signal.We present the current status of CRESST(Cryogenic Rare Event Search using Superconducting Thermometers) project and new results concerning the development of new detectors based on the simultaneous measurement of phonons and scintillation light. A significant reduction in the background rate could recently be achieved. With our newly developed method for the simultaneous measurement of scintillation light, strong background discrimination is possible also in larger detectors, resulting in a substantial increase in WIMP detection sensitivity.We present the current status of CRESST(Cryogenic Rare Event Search using Superconducting Thermometers) project and new results concerning the development of new detectors based on the simultaneous measurement of phonons and scintillation light. A significant reduction in the background rate could recently be achieved. With our newly developed method for the simultaneous measurement of scintillation light, strong background discrimination is possible also in larger detectors, resulting in a substantial increase in WIMP detection sensitivity.

Imaging X-ray spectroscopy with superconducting phase transition thermometers

Abstract We report on the development of an imaging cryogenic X-ray spectrometer based upon X-ray absorption in sapphire and readout by two tungsten superconducting phase transition thermometers (SPTs). We achieve one-dimensional imaging over an absorber length of 10 mm with a position resolution of ≈200 μm .

Results of CRESST phase I

Results of the CRESST experiment at Gran Sasso using 262 g sapphire calorimeters with tungsten phase transition thermometers are presented. Calibration and analysis methods are described. Data taken in 2000 have been used to place limits on WIMP dark matter particles in the galactic halo. The sapphire detectors are especially sensitive for low-mass WIMPS with spin-dependent interaction and improve on existing limits in this region.

Superconducting phase transition thermometers for X-ray spectroscopy

Abstract We report on a first attempt on a cryogenic imaging spectrometer based on sapphire absorber and two superconducting phase transition thermometers. One-dimensional imaging was achieved over a length of 10 mm with a spatial resolution of ≈200 μm and an energy resolution of 216 eV for 5.89 keV X-rays.

Diffusion of long-lived quasiparticles over long distances

Abstract Diffusion of quasiparticles over distances up to 4 mm has been observed in various superconducting films. The quasiparticles were created by X-ray absorption in film strips with critical temperatures near 1 K and were detected in two superconducting phase transition thermometers at the ends of each strip. Position and energy of the absorbed X-rays as well as diffusion constants and lifetimes of the quasiparticles were determined. Very long lifetimes up to 9 ms allow the realization of large area phonon collector films on massive cryogenic particle detectors. Recently, with a first such detector a high efficiency of the phonon collection could be demonstrated.

Determination of quasidiffusive phonon propagation in BaF2 using pulse shape analysis and possible implications for particle detection

Abstract We have shown that for quasidiffusive phonon propagation in an isotropic crystal the diffusion coefficient D=KL8/9, where K is a material constant and L is the distance between the phonon source and point of detection. We have demonstrated this is the case for BaF2 bombarded by α-particles. This implies that pulse shape analysis may provide a mechanism to achieve position sensitivity in phonon mediated detectors.

Propagation of non-thermal phonons induced by α-particle bombardment in BaF2

We have measured the time dependence of the flux of non-thermal phonons generated by α particles in a BaF2 single crystal of mass 1 g using a Series Array of Superconducting Tunnel Junctions. Taking advantage of the almost perfect elastic symmetry of BaF2, we approximate the equations of quasidiffusive propagation to give a simple diffusion equation characterized by an effective diffusion coefficient D=KL8/9, where K is a material constant and L the distance between the phonon source and the point of detection. We use this model to perform pulse-shape analysis on our data and show that agreement is excellent, and highly preferred over a ballistic pulse shape. The distributions of values of K determined for different lengths L are found to overlap strongly, in support of the law D=KL8/9. We suggest that pulse-shape analysis may provide an alternative to time-difference measurements to achieve position sensitivity in a phonon-mediated detector based on a BaF2 absorber.