S. Uchaikin

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.

Discrimination between nuclear recoils and electron recoils by simultaneous detection of phonons and scintillation light

We have developed a detector, consisting of a cryogenic calorimeter with a scintillating crystal as an absorber, and a second calorimeter for the detection of scintillation light, both operated at 12 mK. Using a CaWO4 crystal with a mass of 6 g as the scintillating absorber, we have achieved a discrimination between nuclear recoils and electron recoils with a suppression factor of 99.7% at energies above 15 keV. This method will be applied for background rejection in the CRESST dark matter search (Cryogenic Rare Event Search with Superconducting Thermometers).

Fracture Processes Observed with A Cryogenic Detector

In the early stages of running of the CRESST dark matter search using sapphire detectors at very low temperature, an unexpectedly high rate of signal pulses appeared. Their origin was finally traced to fracture events in the sapphire due to the very tight clamping of the detectors. During extensive runs the energy and time of each event was recorded, providing large data sets for such phenomena. We believe this is the first time the energy release in fracture has been directly and accurately measured on a microscopic event-by-event basis. The energy threshold corresponds to the breaking of only a few hundred covalent bonds, a sensitivity some orders of magnitude greater than that of previous technique. We report some features of the data, including energy distributions, waiting time distributions, autocorrelations and the Hurst exponent. The energy distribution appear to follow a power law, dN/dE ∝ E −� , similar to the power law for earthquake magnitudes, and after appropriate translation, with a similar exponent. In the time domain, the waiting time w or gap distribution between events has a power law behavior at small w and an exponential fall-off at large w, and can be fit ∝ w −� e −w/w 0 . The autocorrelation function shows time correlations lasting for substantial parts of an hour. An asymmetry is found around large events, with higher count rates after, as opposed to before, the large event .

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.

High detection sensitivity achieved with cryogenic detectors in combination with matrix-assisted laser desorption/ionisation time-of-flight mass spectrometry.

Cryogenic detectors directly measure the impact energy of any impinging particle independent of its velocity. Thus, a very high, mass-independent, detection efficiency is expected from their application in time-of-flight mass spectrometry. The cryogenic detector applied here is based on a superconducting phase-transition thermometer and was implemented in a dual reflector time-of-flight mass spectrometer (N-geometry). A dilution series, using standard sample preparation procedures, shows that the detection limit for insulin (Mr: 5734) can be decreased by several orders of magnitude, down to 0.5 amol on the MALDI target. Detection limits for rhM-CSF β (Mr: 49,032) and for polyclonal IgG (Mr: ca 150,000) in the high femtomole and low picomole range, respectively, were established.

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.

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.

Fast cryodetector and SQUID read-out for mass spectrometry

We report a cryogenic detector with a superconducting phase transition thermometer and SQUID read-out for time of flight (TOF) mass spectrometry of biomolecules. Cryogenic detectors combine 100% detection efficiency independent of the mass of the molecule, an excellent energy resolution and a low threshold. Implementation of the cryogenic detectors into TOF mass spectrometers is expected to improve the efficiency of the detection of large biomolecules. To reach a high time resolution, we have developed two new read-outs based on a double stage SQUID and a cooled amplifier.