Search

We analyzed low-background data from the CRESST-II experiment with a total net exposure of 730 kg days to extract limits on double electron capture processes. We established new limits for $^{40}$Ca with $T_{1/2}^{2v2K}>9.9\times10^{21}$ y and $T_{1/2}^{0v2EC}>1.4\times10^{22}$ y and for $^{180}$W with T$_{1/2}^{2v2K}>3.1\times10^{19}$ y and $T_{1/2}^{0v2EC}>9.4\times10^{18}$ y at 90% CL. Depending on the process, these values improve the currently best limits by a factor of $\sim$1.4-30.

International audience; EURECA (European Underground Rare Event Calorimeter Array) is a new project, searching for dark matter, with largely the present groups of the CRESST and EDELWEISS experiments and already a few new groups. The aim is to explore scalar cross sections in the 10−9–10−10 picobarn region with a target mass of up to one tonne. A major advantage of EURECA is our planned use of more that just one target material (multi target experiment for WIMP identification). In preparation for this large-scale experiment, R&D for EURECA is provided through the current phases of CRESST and EDELWEISS.

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.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 (~ 300 g) CaWO_4 crystal and a very sensitive smaller (~ 2 g) light detector to detect the scintillation light from the CaWO_4.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 ~ten 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 4.8 \times 10^{-7}pb, at M{WIMP} ~50 GeV., Comment: 10 pages, 9 figs No essential changes in content. Minor errors corrected. Sentence added at end of introduction concerning increased sensitivity. Best limit on plots at M=50 GeV. Text labelling of candidate events corrected

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 Munchen. A for this purpose an optimized version of the CRESST detector has been developed consisting of a 38 g 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.

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.

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.

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 =10–15 mK are required. We have produced tungsten thin films with T c in the range of 25–55 mK 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.

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. © 2010 Elsevier B.V. All rights reserved.

CRESST is an experiment for the direct detection of dark matter particles via nuclear recoils. The CRESST detectors, based on CaWO 4 scintillating crystals, are able to discriminate γ and β background by simultaneously measuring the light and phonon signals produced by particle interactions. The discrimination of the background is possible because of the different light output (Quenching Factor, QF) for nuclear and electron recoils. In this article a measurement is shown, aimed at the determination of the QFs of the different nuclei (O, Ca, W) of the detector crystal at 40–60 mK using an 11 MeV neutron beam produced at the Maier-Leibnitz-Laboratorium in Garching (MLL).

To detect WIMP (Weakly Interacting Massive Particle)-nucleon scattering events and thus validate the dark matter hypothesis, the CRESST (Cryogenic Rare Event Search with Superconducting Thermometers) and the future EURECA (European Underground Rare Event Calorimeter Array) experiments use modularized cryogenic detectors operated at mK temperatures. We present an attempt to realize so-called composite detectors for this purpose. A detailed thermal detector model to investigate the composite detector design concerning signal evolution and detector performance is described.

The CaWO 4 detectors of CRESST are able to distinguish electron recoils as induced by ionizing radiation from nuclear recoils as expected from heavy dark matter particles (WIMPs) thanks to the coincident measurement of the phonon and scintillation light signals produced in an event. For a detailed understanding of the detector response to nuclear recoils, especially the light yield or quenching factor, measurements with different neutron sources have been performed. Standard neutron sources such as Am–Be yield some information but a better separation of the signals from the different nuclei can be obtained by a monoenergetic pulsed neutron source when additionally measuring the scattered neutrons. Such an experiment has been set up at the tandem accelerator at the Maier–Leibnitz Laboratory in Garching. Results from a first series of measurements at room temperature will be presented as well as the status of the experiment towards cryogenic measurements.

The CRESST dark matter experiment uses the simultaneous measurement of the scintillation light and the heat signal of a CaWO 4 crystal to discriminate between background electron recoil and nuclear recoil events. At the Technical University of Munich calibration measurements have been performed to characterize the detectors. These measurements include the determination of the light output and scintillation time constants of CaWO 4 at temperatures below 50 mK. The setup used in these measurements consist of a CaWO 4 crystal, which is mounted in a reflective housing together with a silicon light detector carrying an Ir/Au transition edge sensor (TES) evaporated directly onto it.

The combination of high-energy-resolution spectrometers with high-spatial-resolution scanning electron microscopes provides a powerful tool for material analysis. X-ray spectrometers based on superconducting transition edge sensors combine the advantages of commonly used energy- and wavelength-dispersive spectrometers, i.e. a fast spectrum acquisition and a high-energy-resolution over a wide energy range. While the energies of the characteristic lines present in a spectrum contain the qualitative composition of the analyzed material, the corresponding mass fractions can be obtained from their intensities. In this work first quantitative measurements performed with the POLARIS spectrometer system are presented.

For an active suppression of the gamma and electron background in the Cryogenic Rare Event Search with Superconducting Thermometers (CRESST) dark matter experiment both phonons and scintillation light generated in a CaWO 4 crystal are detected simultaneously. The phonon signal is read out by a transition edge sensor (TES) on the CaWO 4 crystal. For light detection a silicon absorber equipped with a TES is employed. An efficient background discrimination requires very sensitive light detectors. The threshold can be improved by applying an electric field to the silicon crystal leading to an amplification of the thermal signal due to the Neganov–Luke effect. Measurements showing the improved sensitivity of the light detectors as well as future steps for reducing the observed extra noise will be presented.

Direct Dark Matter detection with cryodetectors is briefly discussed, with particular mention of the possibility of the identification of the recoil nucleus. Preliminary results from the CREEST II Dark Matter search, with 730 kg-days of data, are presented. Major backgrounds and methods of identifying and dealing with them are indicated., Talk at DSU workshop, ITP Beijing, Oct. 2011. 9 figures, 2 tables

The CRESST-II experiment is searching for Dark Matter particles in the form of WIMPs via their elastic scattering off nuclei in a target material. The CRESST target consists of scintillating CaWO4 crystals which are operated as cryogenic calorimeters at millikelvin temperatures and read out by transition edge sensors. Each interaction in CaWO4 produces a phonon signal in the target crystal and also a light signal that is measured by a secondary cryogenic calorimeter, allowing a very efficient discrimination between electron recoils from radioactive e/γ background and nuclear recoils. Moreover, to some extent, the different types of recoiling nuclei (O, Ca, W) can be distinguished. In the paper we present the latest results of the experiment, obtained from a net exposure of 730kg days acquired with 8 detectors between July 2009 and March 2011. The data has shown a considerable number of events in our signal region. Since this large number of events is not consistent with the known sources of background in our experiment, we discuss the compatibility of this excess of events with a possible WIMP-signal.

The simultaneous measurement of phonons and scintillation light induced by incident particles in a scintillating crystal such as CaWO4 is a powerful technique for the active rejection of background induced by gamma's and beta's and even neutrons in direct Dark Matter searches. However, less than ~1% of the energy deposited in a CaWO4 crystal is detected as light. Thus, very sensitive light detectors are needed for an efficient event-by-event background discrimination. Due to the Neganov-Luke effect, the threshold of low-temperature light detectors based on semiconducting substrates can be improved significantly by drifting the photon-induced electron-hole pairs in an applied electric field. We present measurements with low-temperature light detectors based on this amplification mechanism. The Neganov-Luke effect makes it possible to improve the signal-to-noise ratio of our light detectors by a factor of ~9 corresponding to an energy threshold of ~21 eV. We also describe a method for an absolute energy calibration using a light-emitting diode., additional figure, other figures improved

In experiments for direct dark matter searches, neutrinos coherently scattering off nuclei can produce similar events as Weakly Interacting Massive Particles (WIMPs). The calculated count rate for solar neutrinos in such experiments is a few events per ton-year. This count rate strongly depends on the nuclear recoil energy threshold achieved in the experiments for the WIMP search. We show that solar neutrinos can be a serious background source for direct dark matter search experiments using Ge, Ar, Xe and CaWO_4 as target materials. To reach sensitivities better than approximatly 10^-10 pb for the elastic WIMP nucleon spin-independent cross section in the zero-background limit, energy thresholds for nuclear recoils should be approximatly >2.05 keV for CaWO_4, >4.91 keV for Ge, >2.89 keV for Xe, and >8.62 keV for Ar as target material. Next-generation experiments should not only strive for a reduction of the present energy thresholds but mainly focus on an increase of the target mass. Atmospheric neutrinos limit the achievable sensitivity for the background-free direct dark matter search to approximatly >10^-12 pb., accepted by Astroparticle Physics

The CRESST experiment monitors 300g CaWO_4 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 3.6keV X-ray line from the decay of 41-Ca with an activity of (26\pm4)\mu Bq, corresponding to a ratio 41-Ca/40-Ca=(2.2\pm0.3)\times10^{-16}., Comment: 7 pages, 12 figures

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., Proceedings of the Thirteenth International Workshop on Low Temperature Detectors 4 pages, 9 figures

The CRESST-II direct Dark Matter search is located in the Gran Sasso underground laboratories, Italy. CaWO4 crystals are used as scintillating targets for WIMP (weakly interacting massive particle) interactions. They are operated as cryogenic calorimeters in combination with a second cryogenic detector used to measure the scintillation light produced in the target crystal. For each particle interaction, the combination of phonon and light signals provides an event by event discrimination which allows to distinguish known particles (alphas, betas, gammas, neutrons) from the expected signal of WIMPs. A major upgrade of the setup comprises modifications of the shielding, installation of a muon-veto, and new read out electronics, as well as a new detector-support structure to accommodate up to 33 detector modules, i.e. 10 kg of target mass. The experiment was thereafter successfully commissioned in 2007. Data obtained during this commissioning phase from 2 detector modules are presented here. Combining the data collected with these two detector modules with data from one single module obtained during the CRESST-I phase, the experiment could already place a limit of ∼6 × 10-7 pb for the spin independent WIMP-nucleon scattering cross section at a WIMP mass of ∼60 GeV/c2. © BAS, BDP Sciences 2009.

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 figures

The alpha decay of $\n{{}^{210}Po}$ 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 $\n{CaWO_4}$ crystals of $0.0142\pm0.0013$ 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., Comment: 5 pages, 9 figures

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 CRESST (Cryogenic Rare Event Search with Superconducting Thermometers) and the EURECA project (European Underground Rare Event Calorimeter Array) 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, CaWO_4, 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., 7 pages, 8 figures; To appear in the proceedings of CryoScint08 (http://cryoscint08.in2p3.fr), Univ. Lyon 1, France, June 6th 2008, P. Di Stefano (IPN Lyon) and F. Petricca (MPP Munich) eds

CRESST (Cryogenic Rare Event Search with Superconducting Thermometers) is an experiment located at the Gran Sasso underground laboratory and aimed at the direct detection of dark matter in the form of WIMPs. The setup has just completed a one year commissioning run in 2007 and is presently starting a physics run with an increased target mass. Scintillating $\mathrm{CaWO_4}$ single crystals, operated at temperatures of a few millikelvin, are used as target to detect the tiny nuclear recoil induced by a WIMP. The powerful background identification and rejection of $\alpha$, e$^{-}$ and $\gamma$ events is realized via the simultaneous measurement of a phonon and a scintillation signal generated in the $\mathrm{CaWO_4}$ crystal. However, neutrons could still be misidentified as a WIMP signature. Therefore, a detailed understanding of the individual recoil behaviour in terms of phonon generation and scintillation light emission due to scattering on Ca, O or W nuclei, respectively, is mandatory. The only setup which allows to perform such measurements at the operating temperature of the CRESST detectors has been installed at the Maier-Leibnitz-Accelerator Laboratory in Garching and is presently being commissioned. The design of this neutron scattering facility is such that it can also be used for other target materials, e.g. $\mathrm{ZnWO_4}$, $\mathrm{PbWO_4}$ and others as foreseen in the framework of the future multitarget tonne-scale experiment EURECA (European Underground Rare Event Calorimeter Array)., Comment: 6 pages, 7 figures, To appear in the proceedings of CryoScint08 (http://cryoscint08.in2p3.fr), Univ. Lyon 1, France, June 6th 2008, P. Di Stefano (IPN Lyon) and F. Petricca (MPP Munich) eds

We present results on WIMP dark matter interaction, obtained using cryogenic phonon-scintillation detectors. The targets are CaWO4 scintillating crystals with phonon sensors, while scintillation is detected with an associated cryogenic light detector. The combination of phonon and light signals leads to a strong suppression of nonnuclear recoil backgrounds. Results obtained before the upgrade of CRESST are presented and a status report on the upgrade itself is given.

EURECA (European Underground Rare Event Calorimeter Array) is a new project, searching for dark matter. The collaboration is comprised largely of the present groups of the CRESST and EDELWEISS experiments and several new groups. The aim is to explore scalar cross sections in the - 10 -9-10-10picobarn region with a target mass of up to one tonne. A major advantage of EURECA is the planned use of more that just one target material (multi target experiment for WIMP identification). In preparation for this largescale experiment, RandD for EURECA is provided through the current phases of CRESST and EDELWEISS.

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.

The current status of CRESST (Cryogenic Rare Event Search using Superconducting Thermometers) and new results concerning the detector development are presented. The basic technique of CRESST is to search for particle Dark Matter (WIMPS, Weakly Interacting Massive particles) by the measurement of non-thermal phonons as created by WIMP-induced nuclear recoils. Combined with the newly developed method of simultaneous measurement of scintillation light, strong background discrimination is possible, resulting in a substantial increase in WIMP detection sensitivity. The short and long term perspectives of CRESST are discussed., Comment: 12 pages, 6 figures

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., Comment: To appear in Journal of Physics: Conference Series; Proceedings of Neutrino 2008, Christchurch, New Zealand

CRESST is a WIMP dark matter search using scintillating CaWO4 cryogenic detectors with active background suppression. First results obtained in the commissioning run are presented.

Transition edge sensors (TESs) have been developed to be used as high-resolution x-ray detectors. They show excellent energy resolution and can be used in many applications. TESs are a special kind of calorimeters that can determine small temperature changes after x-ray absorption. Such a temperature change causes a strong resistance change (superconducting to normal-conducting phase transition) that can be measured. The energy calibration of a TES based spectrometer is problematic due to the nonlinear behavior of the detector response. In this article, a method is introduced to calibrate the energy scale of TES spectra. This is accomplished by calculating the energy dependence of the response of the detector operated in electrothermal feedback mode. Using this method a calibration accuracy of a few eV for an x-ray energy of 6keV can be achieved. Examples of energy dispersive x-ray spectroscopy (EDS) measurements demonstrate the high quality of this method for everyday use of TES EDS detectors in material...

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 10-9 - 10-10 picobarn. A major advantage of EURECA is the planned use of more than just one target material (multi target experiment for WIMP identification). © EAS, EDP Sciences 2009.