Publications (28)6.38 Total impact
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Article: Status of the CRESST Dark Matter Search
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ABSTRACT: The CRESST experiment aims for a detection of dark matter in the form of WIMPs. These particles are expected to scatter elastically off the nuclei of a target material, thereby depositing energy on the recoiling nucleus. CRESST uses scintillating CaWO4 crystals as such a target. The energy deposited by an interacting particle is primarily converted to phonons which are detected by transition edge sensors. In addition, a small fraction of the interaction energy is emitted from the crystals in the form of scintillation light which is measured in coincidence with the phonon signal by a separate cryogenic light detector for each target crystal. The ratio of light to phonon energy permits the discrimination between the nuclear recoils expected from WIMPs and events from radioactive backgrounds which primarily lead to electron recoils. CRESST has shown the success of this method in a commissioning run in 2007 and, since then, further investigated possibilities for an even better suppression of backgrounds. Here, we report on a new class of background events observed in the course of this work. The consequences of this observation are discussed and we present the current status of the experiment. Comment: Proceedings of the 13th International Workshop on Low Temperature Detectors, 4 pages, 3 figures12/2009; -
Article: Composite CaWO4 Detectors for the CRESST-II Experiment
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ABSTRACT: CRESST-II, standing for Cryogenic Rare Events Search with Superconducting Thermometers phase II, is an experiment searching for Dark Matter. In the LNGS facility in Gran Sasso, Italy, a cryogenic detector setup is operated in order to detect WIMPs by elastic scattering off nuclei, generating phononic lattice excitations and scintillation light. The thermometers used in the experiment consist of a tungsten thin-film structure evaporated onto the CaWO4 absorber crystal. The process of evaporation causes a decrease in the scintillation light output. This, together with the need of a big-scale detector production for the upcoming EURECA experiment lead to investigations for producing thermometers on smaller crystals which are glued onto the absorber crystal. In our Run 31 we tested composite detectors for the first time in the Gran Sasso setup. They seem to produce higher light yields as hoped and could provide an additional time based discrimination mechanism for low light yield clamp events. Comment: Proceedings of the Thirteenth International Workshop on Low Temperature Detectors 4 pages, 9 figures12/2009; -
Article: Scintillator Non-Proportionality and Gamma Quenching in CaWO4
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ABSTRACT: We measure and explain scintillator non-proportionality and gamma quenching of CaWO4 at low energies and low temperatures. Phonons that are created following an interaction in the scintillating crystal at temperatures of 15mK are used for a calorimetric measurement of the deposited energy, and the scintillation light is measured with a separate cryogenic light detector. Making use of radioactivity intrinsic to the scintillating crystal, the scintillator non-proportionality is mapped out to electron energies <5keV. The observed behavior is in agreement with a simple model based on Birks' law and the stopping power dE/dx for electrons. We find for Birks' constant $k_B=(18.5\pm0.7)$nm/keV in CaWO4. Gamma lines allow a measurement of the reduced light yield of photons with respect to electrons, as expected in the presence of scintillator non-proportionality. In particular, we show that gamma-induced events in CaWO4 give only about 90 percent of the light yield of electrons, at energies between 40keV and 80keV. Comment: 5 pages, 5 figures10/2009; -
Article: EURECA — the future of cryogenic dark matter detection in Europe
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ABSTRACT: EURECA (European Underground Rare Event Calorimeter Array) is an astro-particle physics facility aiming to directly detect galactic dark matter. The Laboratoire Souterrain de Modane has been selected as host laboratory. The EURECA collaboration unites CRESST, EDELWEISS and the Spanish-French experiment ROSEBUD, thus concentrating and focussing effort on cryogenic detector research in Europe into a single facility. EURECA will use a target mass of up to one ton, enough to explore WIMP – nucleon scalar scattering cross sections in the region of 1e(-9) – 1e(-10) picobarn. A major advantage of EURECA is the planned use of more than just one target material (multi target experiment for WIMP identification).EAS Publications Series 06/2009; 36:249 –255. -
Article: Electron and gamma background in CRESST detectors
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ABSTRACT: The CRESST experiment monitors 300 g CaWO4 crystals as targets for particle interactions in an ultra low background environment. In this paper, we analyze the background spectra that are recorded by three detectors over many weeks of data taking. Understanding these spectra is mandatory if one wants to further reduce the background level, and allows us to cross-check the calibration of the detectors. We identify a variety of sources, such as intrinsic contaminations due to primordial radioisotopes and cosmogenic activation of the target material. In particular, we detect a 3.6 keV X-ray line from the decay of 41Ca with an activity of , corresponding to a ratio 41Ca/40Ca=(2.2±0.3)×10-16.Astroparticle Physics. 05/2009; -
Article: Proceedings of the 1st International Workshop "Radiopure Scintillators for EURECA" (RPScint'2008)
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ABSTRACT: Workshop RPSCINT'2008 was organized in Kyiv (Ukraine) on 9th and 10th September 2008. The idea was to bring together physicists, chemists, crystal scintillator experts and manufacturers to discuss the requirements of low-count rate experiments, in particular the required radiopurity and scintillation properties; selection and screening of input materials; purification of materials; raw compound preparation; crystal growing, annealing and handling; test of crystals; search for and development of new scintillating materials. Some contributions to the RPSCINT 2008 workshop are presented in these proceedings.04/2009; -
Article: Discrimination of recoil backgrounds in scintillating calorimeters
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ABSTRACT: The alpha decay of 210Po is a dangerous background to rare event searches. Here, we describe observations related to this alpha decay in the Cryogenic Rare Event Search with Superconducting Thermometers (CRESST). We find that lead nuclei show a scintillation light yield in our CaWO4 crystals of 0.0142±0.0013 relative to electrons of the same energy. We describe a way to discriminate this source of nuclear recoil background by means of a scintillating foil, and demonstrate its effectiveness. This leads to an observable difference in the pulse shape of the light detector, which can be used to tag these events. Differences in pulse shape of the phonon detector between lead and electron recoils are also extracted, opening the window to future additional background suppression techniques based on pulse shape discrimination in such experiments.Astroparticle Physics. 03/2009; -
Chapter: OPTIMIZATION OF THE CZOCHRALSKI GROWTH PROCESS FOR CALCIUM TUNGSTATE DETECTOR CRYSTALS
01/2009: pages 269-270; -
Article: Commissioning run of the CRESST-II dark matter search
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ABSTRACT: The CRESST cryogenic direct dark matter search at Gran Sasso, searching for WIMPs via nuclear recoil, has been upgraded to CRESST-II by several changes and improvements. The upgrade includes a new detector support structure capable of accommodating 33 modules, the associated multichannel readout with 66 SQUID channels, a neutron shield, a calibration source lift, and the installation of a muon veto. We present the results of a commissioning run carried out in 2007.The basic element of CRESST-II is a detector module consisting of a large crystal and a very sensitive smaller light detector to detect the scintillation light from the CaWO4. The large crystal gives an accurate total energy measurement. The light detector permits a determination of the light yield for an event, allowing an effective separation of nuclear recoils from electron–photon backgrounds. Furthermore, information from light-quenching factor studies allows the definition of a region of the energy-light yield plane which corresponds to tungsten recoils. A neutron test is reported which supports the principle of using the light yield to identify the recoiling nucleus.Data obtained with two detector modules for a total exposure of 48 kg-days are presented. Judging by the rate of events in the “all nuclear recoils” acceptance region the apparatus shows a factor ∼10 improvement with respect to previous results, which we attribute principally to the presence of the neutron shield. In the “tungsten recoils” acceptance region three events are found, corresponding to a rate of 0.063 per kg-day. Standard assumptions on the dark matter flux, coherent or spin independent interactions, then yield a limit for WIMP-nucleon scattering of , at .Astroparticle Physics. 01/2009; -
Book: CRESST
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ABSTRACT: Export Date: 17 February 201001/2009: pages 231-236; -
Conference Proceeding: EURECA — The Future of Cryogenic Dark Matter Detection in Europe
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ABSTRACT: The European Underground Rare Event Calorimeter Array (EURECA) will be an astro-particle physics facility in the Laboratoire Souterrain de Modane, aiming to directly detect galactic dark matter. The EURECA collaboration unites CRESST, EDELWEISS and the Spanish-French experiment ROSEBUD, thus concentrating and focussing effort on cryogenic detector research in Europe into a single facility. The aim is to explore WIMP – nucleon scalar cross sections in the 1(-9) – 1(-10) picobarn region with a target mass of up to one ton. A major advantage of EURECA is the planned use of more than just one target material (multi target experiment for WIMP identification).Identification of Dark Matter 2008 (IDM2008), August 18-22, 2008, Stockholm, Sweden; 12/2008 -
Article: Cryogenic composite detectors for the dark matter experiments CRESST and EURECA
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ABSTRACT: Weakly interacting massive particles (WIMPs) are candidates for non-baryonic dark matter. WIMPs are supposed to interact with baryonic matter via scattering off nuclei producing a nuclear recoil with energies up to a few 10 keV with a very low interaction rate of ∼10−6 events per kg of target material and day in the energy region of interest. The dark matter experiment cryogenic rare event search with superconducting thermometers (CRESST) and the European underground rare event calorimeter array (EURECA) project are aimed at the direct detection of WIMPs with the help of very sensitive modularised cryogenic detectors that basically consist of a transition edge sensor (TES) in combination with a massive absorber crystal. In the CRESST experiment the search for coherent WIMP-nucleon scattering events is validated by the detection of two processes. In the scintillating absorber single crystal, CaWO4, heat (phonons) and scintillation light are produced and detected with two independent cryogenic detectors: a phonon channel and a separate light channel.The development of such cryogenic detectors and the potential ton-scale production are investigated in this paper. To decouple the TES production from the choice of the target material in order to avoid heating cycles of the absorber crystal and to allow pretesting of the TESs, a composite detector design (CDD) for the detector production has been developed and studied. An existing thermal detector model has been extended to the CDD, in order to investigate, understand, and optimize the performance of composite detectors. This extended model, which has been worked out in detail, can be expected to provide a considerable help when tailoring composite detectors to the requirements of various experiments.Optical Materials. 11/2008; -
Article: Thermal detector model for cryogenic composite detectors for the dark matter experiments CRESST and EURECA
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ABSTRACT: The CRESST (Cryogenic Rare Event Search with Superconducting Thermometers) and the EURECA (European Underground Rare Event Calorimeter Array) experiments are direct dark matter search experiments where cryogenic detectors are used to detect spin-independent, coherent WIMP (Weakly Interacting Massive Particle)-nucleon scattering events by means of the recoil energy. The cryogenic detectors use a massive single crystal as absorber which is equipped with a TES (transition edge sensor) for signal read-out. They are operated at mK-temperatures. In order to enable a mass production of these detectors, as needed for the EURECA experiment, a so-called composite detector design (CDD) that allows decoupling of the TES fabrication from the optimization procedure of the absorber single-crystal was developed and studied. To further investigate, understand and optimize the performance of composite detectors a detailed thermal detector model which takes into account the CDD has been developed.10/2008; -
Article: Characterization of the Response of CaWO4 on Recoiling Nuclei from Surface Alpha Decays
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ABSTRACT: A potentially harmful background for experiments attempting direct dark matter detection like the CRESST (= Cryogenic Rare Event Search with Superconducting Thermometers) experiment is caused by recoiling nuclei from 210Po alpha decays on surfaces close to the detector. In order to characterize this kind of background in CRESST, calibration measurements have been performed at the TU München. Afor this purpose an optimized version of the CRESST detector has been developed consisting of a 38g CaWO4 crystal and a separate cryogenic light detector, both equipped with Ir/Au transition edge sensors (TESs). The simultaneous measurement of the phonon signal and the scintillation light from the CaWO4 crystal allows to discriminate between electron and nuclear recoils using their different light outputs. The unexpected results of a first measurement with a 210Po source can be understood with the help of a Monte Carlo simulation performed for a similar system.Journal of Low Temperature Physics 04/2008; 151(3):824-829. · 1.19 Impact Factor -
Article: Properties of Tungsten Thin Films Produced with the RF-Sputtering Technique
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ABSTRACT: For the purpose of building very sensitive light and phonon detectors, as e.g. applied in the Dark Matter (DM) experiment CRESST (Cryogenic Rare Event Search with Superconducting Thermometers), transition edge sensors (TESs) in combination with a massive absorber crystal are used. To ensure high sensitivity of the detectors, low heat capacities, i.e. low working temperatures of about 10 mK are aimed at. Therefore, TESs made of tungsten thin films exhibiting the alpha-tungsten (α-W) phase with transition temperatures of T c =10–15mK are required. We have produced tungsten thin films with T c in the range of 25–55mK by rf-sputtering. To decouple the thermometer production from the choice of the target material and to avoid heating cycles of the absorber crystal, a composite design for detector production is applied. The composite design includes fabrication of the TES on a separate substrate and then attaching of this separate TES to a massive absorber crystal by gluing. For this purpose small sapphire substrates are used for the deposition of the TES. Properties of tungsten thin films grown with the rf-sputtering technique as well as first results of composite detectors built with these films acting as TESs will be presented.Journal of Low Temperature Physics 03/2008; 151(1):216-222. · 1.19 Impact Factor -
Article: Application of the Neganov-Luke Effect for Scintillation Light Detection
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ABSTRACT: For the phonon-light technique employed in the CRESST experiment very sensitive light detectors are needed as only a small fraction of the energy of incident particles is detected as light. Following Neganov and Luke, the sensitivity can be improved by drifting the generated charge carriers in a semiconductor absorber by an applied electric field. For an efficient charge collection substrates with low trap densities are required. For this purpose and for electrical decoupling the TES is glued onto the drift device. Results from measurements with Neganov-Luke amplification using glued TES will be presented.Journal of Low Temperature Physics 03/2008; 151(1):394-399. · 1.19 Impact Factor -
Article: Development of a Cryogenic Detector for Coherent Neutrino Nucleus Scattering
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ABSTRACT: In Coherent Neutrino Nucleus Scattering (CNNS) the neutrinos interact coherently with all nucleons leading to a cross section which is much larger than for all other neutrino interactions. Because of the small momentum transfer as well as the small recoil energy in CNNS, and the relatively low count rate, a low energy threshold and a large target mass (several hundred grams) are required to observe CNNS. Our aim is to build a cryodetector for that purpose. Such a cryodetector, installed in the vicinity of a nuclear power plant, could probe new physics like non-standard neutral current interactions or a neutrino magnetic moment. We describe the results of three detectors, with Ge absorbers of 0.8 and 3.2 g and with a CaWO4 absorber of 10 g. For Ge (0.8 g) an energy threshold of 0.43 keV and an energy resolution of 0.27 keV at ∼6 keV could be reached. We demonstrate that surface roughness effects deteriorate both threshold and resolution. For the 10 g CaWO4 absorber we obtained 0.27 keV and 0.35 keV for threshold and energy resolution at ∼6 keV respectively.Journal of Low Temperature Physics 01/2008; 151(3):629-634. · 1.19 Impact Factor -
Article: Results and status of the CRESST experiment
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ABSTRACT: CRESST (Cryogenic Rare Event Search with Superconducting Thermometers) employs cryogenic detectors for the direct search for weakly interacting massive dark matter particles (WIMPs). In the second phase of the experiment scintillating calcium tungstate crystals are used to discriminate background by means of different light yield for background and WIMP signals. After first results with this novel technique have been obtained, the experimental setup is being upgraded for further background reduction and larger target mass. The results and present status of the experiment will be presented.Journal of Physics Conference Series 06/2006; 39(1):75. -
Article: Dark-matter search with CRESST
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ABSTRACT: The CRESST experiment is looking for non-baryonic particle dark matter via nuclear scattering in CaWO4. The simultaneous measurement of the heat and the scintillation light generated by an event in a CaWO4 single crystal is used to discriminate between electron and nuclear recoils thanks to their different light output. This allows an efficient suppression of the electron recoil background. The set-up consists of modules with a 300 g CaWO4 crystal mounted in a reflective housing together with a light detector. The heat signal is read out using a superconducting transition-edge sensor (TES) made of tungsten evaporated directly on to the crystal that is operated at a few mK. Currently the second phase of the experiment is being set up at the Laboratori Nazionali del Gran Sasso in which it is planned to run 33 detector modules providing a total target mass of 10 kg. First test runs with prototype detectors have been successfully performed.Czechoslovak Journal of Physics 04/2006; 56(5):535-542. · 0.42 Impact Factor -
Article: Direct dark matter search with CRESST and EURECA
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ABSTRACT: The current status of the direct Dark Matter experiments CRESST (Cryogenic Rare Event Search with Superconducting Thermometers) and the planned EURECA (European Underground Rare Event Calorimeter Array) is presented. Both experiments are aimed at the direct detection of WIMPs (Weakly Interacting Massive Particles), potential candidates for the Dark Matter in the universe. New design developments of the cryogenic detectors operated at mK temperatures are investigated to optimize detector performance and to simplify mass production. Thus, CRESST is also providing a basis for the EURECA project, aimed at a ton of cryogenic detectors with a multi-material target.Progress in Particle and Nuclear Physics.
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Institutions
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2006–2008
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Technische Universität München
- Faculty of Physics
München, Bavaria, Germany
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