Cryogenic Rare Event Search With Superconducting Thermometers Research Articles

Optimal Operation of Cryogenic Calorimeters Through Deep Reinforcement Learning

Cryogenic phonon detectors with transition-edge sensors achieve the best sensitivity to sub-GeV/c2\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$^2$$\\end{document} dark matter interactions with nuclei in current direct detection experiments. In such devices, the temperature of the thermometer and the bias current in its readout circuit need careful optimization to achieve optimal detector performance. This task is not trivial and is typically done manually by an expert. In our work, we automated the procedure with reinforcement learning in two settings. First, we trained on a simulation of the response of three Cryogenic Rare Event Search with Superconducting Thermometers (CRESST) detectors used as a virtual reinforcement learning environment. Second, we trained live on the same detectors operated in the CRESST underground setup. In both cases, we were able to optimize a standard detector as fast and with comparable results as human experts. Our method enables the tuning of large-scale cryogenic detector setups with minimal manual interventions.

Testing spin-dependent dark matter interactions with lithium aluminate targets in CRESST-III

In the past decades, numerous experiments have emerged to unveil the nature of dark matter, one of the most discussed open questions in modern particle physics. Among them, the Cryogenic Rare Event Search with Superconducting Thermometers (CRESST) experiment, located at the Laboratori Nazionali del Gran Sasso, operates scintillating crystals as cryogenic phonon detectors. In this work, we present first results from the operation of two detector modules which both have 10.46 g LiAlO2 targets in CRESST-III. The lithium contents in the crystal are Li6, with an odd number of protons and neutrons, and Li7, with an odd number of protons. By considering both isotopes of lithium and Al27, we set the currently strongest cross section upper limits on spin-dependent interaction of dark matter with protons and neutrons for the mass region between 0.25 and 1.5 GeV/c2.1 MoreReceived 25 July 2022Accepted 15 November 2022DOI:https://doi.org/10.1103/PhysRevD.106.092008© 2022 American Physical SocietyPhysics Subject Headings (PhySH)Research AreasParticle dark matterPhysical SystemsWeakly interacting massive particlesTechniquesCryogenicsDark matter detectorsGravitation, Cosmology & Astrophysics

Improving the Quality of CaWO4 Target Crystals for CRESST

The Cryogenic Rare Event Search with Superconducting Thermometers (CRESST) experiment aims at the direct detection of dark matter particles via their elastic scattering off nuclei in a scintillating CaWO_4 target crystal. The CaWO_4 crystal is operated together with a light detector at mK temperature and read out by a Transition Edge Sensor. For many years, CaWO_4 crystals have successfully been produced in-house at Technical University of Munich (TUM) with a focus on high radiopurity which is crucial to reduce background originating from radioactive contamination. In order to further improve the CaWO_4 crystals, an extensive chemical purification of the raw materials and the synthesised CaWO_4 powder has been performed. In addition, a temperature gradient simulation of the growth process and subsequently an optimisation of the growth furnace with the goal to reduce the intrinsic stress was carried out. We present results on the intrinsic stress in the CaWO_4 crystals and on the CaWO_4 powder radiopurity. A crystal grown from the purified material was installed in the current CRESST set-up. The detector is equipped with an instrumented holder which is used to measure the alpha decay rate of the crystal. We present a preliminary analysis showing a significantly reduced intrinsic background from natural decay chains.

Searches for Light Dark Matter with the CRESST-III Experiment

Cryogenic Rare Event Search with Superconducting Thermometers (CRESST) is a long-standing direct dark matter detection experiment with cryogenic detectors located at the underground facility Laboratori Nazionali del Gran Sasso in Italy. CRESST-III, the third generation of CRESST, was specifically designed to have a world-leading sensitivity for low-mass dark matter (DM) (less than 2 GeV/hbox {c}^{2}) to probe the spin-independent DM-nucleus cross section. At present, a large part of the parameter space for spin-independent scattering off nuclei remains untested for dark matter particles with masses below few GeV/hbox {c}^{2} although many motivated theoretical models having been proposed. The CRESST-III experiment employs scintillating hbox {CaWO}_{{4}} crystals of sim 25 g as target material for dark matter interactions operated as cryogenic scintillating calorimeters at sim 10 mK. CRESST-III first data taking was successfully completed in 2018, achieving an unprecedented energy threshold for nuclear recoils. This result extended the present sensitivity to DM particles as light as sim 160 MeV/hbox {c}^{2}. In this paper, an overview of the CRESST-III detectors and results will be presented.

Scheelite Crystals: (Crystal Research and Technology 2/2020)

Crystal Research and TechnologyVolume 55, Issue 2 2070012 Cover PictureFree Access Scheelite Crystals: (Crystal Research and Technology 2/2020) First published: 12 February 2020 https://doi.org/10.1002/crat.202070012AboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinkedInRedditWechat Graphical Abstract Single crystals of CaWO4, also known as Scheelite, are grown by the Czochralski method in an atmosphere with 1 % oxygen in argon using a pure rhodium crucible. Since they are used as detector crystals in the CRESST (Cryogenic Rare Event Search with Superconducting Thermometers) experiment, they need to have a high transparency and extremely low radioactive background. The two crystals which exhibit a somewhat gray color are as grown, the clear crystal in the middle underwent an annealing process in pure oxygen to eliminate the oxygen deficiency which is present for as grown crystals. The image is provided through the courtesy of Prof. Andreas Erb. Volume55, Issue2Special Issue: The 50th Anniversary of the German Association for Crystal Growth (Deutsche Gesellschaft für Kristallwachstum und Kristallzüchtung, DGKK)February 20202070012 RelatedInformation

Geant4-based electromagnetic background model for the CRESST dark matter experiment

The CRESST (Cryogenic Rare Event Search with Superconducting Thermometers) dark matter search experiment aims for the detection of dark matter particles via elastic scattering off nuclei in mathrm {CaWO_4} crystals. To understand the CRESST electromagnetic background due to the bulk contamination in the employed materials, a model based on Monte Carlo simulations was developed using the Geant4 simulation toolkit. The results of the simulation are applied to the TUM40 detector module of CRESST-II phase 2. We are able to explain up to (68 pm 16),mathrm {%} of the electromagnetic background in the energy range between 1 and 40,mathrm {keV}.

Nb on Al2O3 and doped-Si as cryogenic light detectors

For rare event searches, such as the direct dark matter search experiment CRESST (Cryogenic Rare Event Search with Superconducting Thermometers), highly sensitive cryogenic detectors are indispensable. A very low energy threshold (≪100eV) and excellent energy resolution are required to increase the experimental sensitivity, particularly for low mass dark matter particles (mDM<5 GeV/c2), and to differentiate between these rare events and natural radioactive backgrounds. For background suppression, CRESST benefits from using simultaneously two detection channels from scintillating calorimeters, in this case of heat and light. Measuring a heat signal and its corresponding scintillating light, allows event-by-event particle identification down to certain energies. Therefore, the development of cryogenic light detectors that can achieve single photon sensitivity, have high radio-purity and excellent energy resolution, is a key point to improve the experiment’s sensitivity at low energies. In this contribution we explain and report on novel ways to improve the light collection efficiency through a preliminary study on Nb films sputtered on Sapphire and through highly doped Si as cryogenic light detectors.

Energy-dependent light quenching in CaWO $$_4$$ 4 crystals at mK temperatures

Scintillating CaWO $$_4$$ single crystals are a promising multi-element target for rare-event searches and are currently used in the direct dark matter experiment CRESST (Cryogenic Rare Event Search with Superconducting Thermometers). The relative light output of different particle interactions in CaWO $$_4$$ is quantified by quenching factors (QFs). These are essential for an active background discrimination and the identification of a possible signal induced by weakly interacting massive particles (WIMPs). We present the first precise measurements of the QFs of O, Ca and W at mK temperatures by irradiating a cryogenic detector with a fast neutron beam. A clear energy dependence of the QF of O and, less pronounced, of Ca was observed for the first time. Furthermore, in CRESST neutron-calibration data a variation of the QFs among different CaWO $$_4$$ single crystals was found. For typical CRESST detectors the QFs in the region-of-interest (10–40 keV) are $$\hbox {QF}_\mathrm{O}^\mathrm{ROI}=(11.2\pm 0.5)$$ %, $$\hbox {QF}_\mathrm{Ca}^\mathrm{ROI}=(5.94\pm 0.49)$$ % and $$\hbox {QF}_\mathrm{W}^\mathrm{ROI}=(1.72\pm 0.21)$$ %. The latest CRESST data (run32) is reanalyzed using these fundamentally new results on light quenching in CaWO $$_4$$ having moderate influence on the WIMP analysis. Their relevance for future CRESST runs and for the clarification of previously published results of direct dark matter experiments is emphasised.

Alternative Scintillating Materials for the CRESST Dark Matter Search

The CRESST Dark Matter Search aims to detect Dark Matter particles via their elastic scattering off nuclei in scintillating target crystals. A simultaneous measurement of a phonon and light signal allows to efficiently discriminate the dominant beta and gamma background from expected nuclear recoil signals. For a given light detector and reflector, it is the scintillation efficiency of the crystal which determines the lowest energy for which the active background discrimination is still efficient. Crystals with a higher light output therefore can improve the overall sensitivity of the experiment. In this context, measurements of the light yield of CsI (undoped) and CdWO4 crystals at milli-Kelvin temperature are presented. Furthermore we report on first results of measurements of these crystals in conventional CRESST configuration.

The CRESST dark matter search

The aim of CRESST ( Cryogenic Rare Event Search with Superconducting Thermometers) is to search for particle dark matter via elastic scattering off nuclei. The experiment is located at the Laboratori Nazionali del Gran Sasso (LNGS), Italy, and it uses low-background cryogenic detectors with superconducting phase-transition thermometers for the direct detection of WIMP–nucleus scattering events.

Direct dark matter search with CRESST and EURECA

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.

Discrimination of recoil backgrounds in scintillating calorimeters

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 CaWO 4 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.

ZnWO 4 scintillators for cryogenic dark matter experiments

The scintillation properties of a zinc tungstate crystal, shaped as a hexagonal prism (height 40 mm, diagonal 40 mm) were determined. An energy resolution of 10.7% for the 662 keV γ-line of 137Cs was measured with the scintillator placed in a light collection setup similar to that used by the CRESST dark matter search. The light output and decay kinetics of ZnWO 4 were examined over the temperature range 7–300 K and confirmed to be competitive with those of CaWO 4. The radioactive contaminations of the ZnWO 4 scintillator measured in the Solotvina Underground Laboratory do not exceed 0.1–10 mBq/kg (depending on radionuclide). Our study highlights the excellent feasibility of this ZnWO 4 scintillator for a cryogenic dark matter experiment.

Cryogenic Composite Detectors for the Dark Matter Experiments CRESST and EURECA: A Thermal Detector Model

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.

Neutron scattering facility for characterization of CRESST and EURECA detectors at mK temperatures

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 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 α , e - and γ events is realized via the simultaneous measurement of a phonon and a scintillation signal generated in the 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. ZnWO 4, PbWO 4 and others as foreseen in the framework of the future multi-target tonne-scale experiment EURECA (European underground rare event calorimeter array).

Cryogenic composite detectors for the dark matter experiments CRESST and EURECA

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, 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.

Thermal detector model for cryogenic composite detectors for the dark matter experiments CRESST and EURECA

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 which takes into account the CDD has been developed.

The CRESST dark matter search

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.

Characterization of the Response of CaWO4 on Recoiling Nuclei from Surface Alpha Decays

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.

Properties of Tungsten Thin Films Produced with the RF-Sputtering Technique

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.