Results of Quenching Factor Measurements of CaWO_4 at mK Temperatures for the Direct Dark Matter Search Experiment CRESST

The CRESST experiment aims at a direct detection of WIMP dark matter (DM) using scintillating CaWO \(_4\) crystals operated as phonon detectors at mK temperatures. An important feature of the experiment is the active background discrimination technique exploiting the different light outputs depending on the kind of particle interaction. The reduced light yield of nuclear recoils compared to electron recoils is quantified by quenching factors (QFs). The precise measurement of the QFs and thus the identification of the individual recoiling nucleus in the multi-target material CaWO \(_4\) is crucial for neutron background discrimination and assuming a positive DM signal would allow to a certain extent WIMP-mass spectroscopy. At the Munich tandem accelerator a dedicated neutron scattering facility has been set up to measure the QFs of CaWO \(_4\) , in particular that of tungsten, at mK temperatures. Monoenergetic neutrons (11 MeV) produced by the accelerator are scattered off a CRESST-like detector module that is operated in a dilution refrigerator. In this setup, the recoiling nucleus (O, Ca and W) is identified by time-of-flight measurement in liquid-scintillator detectors placed at fixed scattering angles. The QF of W could be determined with unprecedented accuracy at mK temperatures and under realistic measurement conditions: \(QF_W=0.0196\pm 0.0022\) (preliminary value).     

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 CaWO₄ 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 CaWO₄ 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. ZnWO₄, PbWO₄ and others as foreseen in the framework of the future multi-target tonne-scale experiment EURECA (European underground rare event calorimeter array).     

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 München. 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.      

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.     

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, 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.     

Glued detectors for the CRESST-II experiment

The Cryogenic Rare Event Search with Superconducting Thermometers Phase II (CRESST-II) at the L.N.G.S in Italy is searching for Dark Matter using low-temperature calorimeters. These detectors allow to discriminate different particles by simultaneous measurement of phonons and scintillation light. The sensors used consist of superconducting tungsten thin-film thermometers, which measure the thermal effect of the phonons created in an attached absorber crystal. It has been observed that the scintillation of the CaWO₄ absorber degrades during the process of depositing the tungsten film. In order to prevent this, a new technique for producing the detectors was investigated. This technique might also be valuable by expanding the range of scintillator materials suitable for producing a Dark Matter detector.     

ZnWO4 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 ZnWO₄ were examined over the temperature range 7–300 K and confirmed to be competitive with those of CaWO₄. The radioactive contaminations of the ZnWO₄ 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 ZnWO₄ scintillator for a cryogenic dark matter experiment.     

Microemulsion-based synthesis and luminescence of nanoparticulate CaWO<Subscript>4</Subscript>, ZnWO<Subscript>4</Subscript>, CaWO<Subscript>4</Subscript>:Tb,and CaWO<Subscript>4</Subscript>:Eu

Blue emitting CaWO₄ and greenish blue emitting ZnWO₄ nanoparticles are synthesized via microemulsion techniques applying a cationic (CTAB) as well as a non-ionic surfactant (TritonX-100). The influence of the surfactant on particle size and shape is studied. Scanning electron microscopy and dynamic light scattering confirm the presence of uniform and non-agglomerated nanoparticles, 60–80 nm in diameter. Photoluminescence confirms [WO₄]²⁻-related broad-band emission with its maximum at 440 nm (CaWO₄) and 420 nm (ZnWO₄). The highest quantum yield (QY) is observed for nanoscaled CaWO₄ with a value of 23–25%. Doping of CaWO₄ and ZnWO₄ with Tb³⁺ and Eu³⁺ was performed and in the case of CaWO₄:Tb and CaWO₄:Eu and results in the emission of green and red light, again with comparably high QYs (17–19%).     

Anti-tumor activity of self-charged (Eu,Ca):WO3 and Eu:CaWO4 nanoparticles

Non-stoichiometric (Eu,Ca):WO₃ and Eu:CaWO₄ nanoparticles with anti-tumor activity are synthesized in a sol?Cgel method by adding excessive Eu^3?+? and Ca^2?+? ions to tungsten oxide crystal structure. Colorimetric assay shows that 10?nm (Eu,Ca):WO₃ and Eu:CaWO₄ nanoparticles can effectively inhibit growth of mammary cancer cells without any harm to normal cells. Nanoparticles are characterized by X-ray diffraction, high resolution transmission electron microscopy and fluorescence optical spectrometry. Nanomaterials, insoluble in synthesized water, have complicated self-charging surfaces that trap mammary cancer cells. Surface self-charging effect is suggested as the inhibition mechanism.     

Shape-controlled of CaWO4 microcrystals by self-assembly of nanocrystals via a simple sonochemical method

In this paper, we report the obtention of CaWO₄ crystals with controlled morphologies, such as spindle-like, nanorods, urchin-like, and sphere-like, were selectively synthesized via a simple sonochemical method without any surfactants or templates. The as-prepared samples were characterized by X-ray diffraction, transmission electron microscopy and high-resolution transmission electron microscopy. It is found that the ultrasonic time and pH value played an important role in the morphology-controlled synthesis of CaWO₄ crystals. The possible mechanism for the formation of CaWO₄ samples with different shapes was discussed in detail.     

Development of superconducting absorbers for CRESST light detectors

An important aspect of dark matter search experiments is the active background reduction by identification of the type of particle interacting in the detector. In CRESST this is achieved by a simultaneous detection of heat and light produced by an interaction in a scintillating crystal. The overall light collection efficiency is a crucial parameter in order to achieve enough sensitivity to measure the small fraction of the deposited energy that is emitted as scintillation light. For this purpose, a thin superconducting lead film deposited on sapphire substrates has been tested as an alternative light absorber to a silicon absorber. The first results already show a better light absorption of the lead film. Other superconducting films are also being analyzed.     

Crucibleless crystal growth and Radioluminescence study of calcium tungstate single crystal fiber

In this article, single phase and high optical quality scheelite calcium tungstate single crystal fibers were grown by using the crucibleless laser heated pedestal growth technique. The as-synthesized calcium tungstate powders used for shaping seed and feed rods were investigated by X-ray diffraction technique. As-grown crystals were studied by Raman spectroscopy and Radioluminescence measurements. The results indicate that in both two cases, calcined powder and single crystal fiber, only the expected scheelite CaWO₄ phase was observed. It was verified large homogeneity in the crystal composition, without the presence of secondary phases. The Radioluminescence spectra of the as-grown single crystal fibers are in agreement with that present in Literature for bulk single crystals, presented a single emission band centered at 420 nm when irradiated with β-rays.     

Effects of packaging SrI2(Eu) scintillator crystals

Recent renewed emphasis placed on gamma-ray detectors for national security purposes has motivated researchers to identify and develop new scintillator materials capable of high energy resolution and growable to large sizes. We have discovered that SrI₂(Eu) has many desirable properties for gamma-ray detection and spectroscopy, including high light yield of ∼90,000 photons/MeV and excellent light yield proportionality. We have measured <2.7% FWHM at 662 keV with small detectors (<1 cm³) in direct contact with a photomultiplier tube, and ∼3% resolution at 662 keV is obtained for 1 in.³ crystals. Due to the hygroscopic nature of SrI₂(Eu), similar to NaI(Tl), proper packaging is required for field use. This work describes a systematic study performed to determine the key factors in the packaging process to optimize performance. These factors include proper polishing of the surface, the geometry of the crystal, reflector materials and windows. A technique based on use of a collimated ¹³⁷Cs source was developed to examine light collection uniformity. Employing this technique, we found that when the crystal is packaged properly, the variation in the pulse height at 662 keV from events near the bottom of the crystal compared to those near the top of the crystal could be reduced to <1%. This paper describes the design and engineering of our detector package in order to improve energy resolution of 1 in.³-scale SrI₂(Eu) crystals.     

Room-temperature preparation of highly crystallized CaWO4 film by a galvanic cell method

Highly crystallized CaWO₄ film has been prepared directly by a galvanic cell method on tungsten substrates in calcium hydroxide aqueous solution without impressed current at room temperature (25 °C). The scanning electron microscopy (SEM) and X-ray diffraction (XRD) results reveal that the crystallized film has a scheelite-type tetragonal structure, uniform and homogeneous surface. The film shows only the blue emission of 447.5 nm with the excitation light of 250 nm at room temperature. The formation mechanism of CaWO₄ film under the simple electrochemical process has been discussed. This method could resolve the repulsion of the electric field on the anode for the mass transfer. The crystal growth in the solution is freer.     

Scintillation and optical properties of Pb-doped YCa4O(BO3)3 crystals

This communication reports optical properties and radiation responses of Pb²⁺ 0.5 and 1.0 mol%-doped YCa₄O(BO₃)₃ (YCOB) single crystals grown by the micro-pulling-down (μ-PD) method for neutron scintillator applications. The crystals had no impurity phases according to the results of X-ray powder diffraction. These Pb²⁺-doped crystals demonstrated blue-light luminescence at 330 nm because of Pb²⁺¹S₀-³P_0,1 transition in the photoluminescence spectra. The main emission decay component was determined to be about 250-260 ns under 260 nm excitation wavelength. When irradiated by a ²⁵²Cf source, the relative light yield of 0.5% Pb²⁺-doped crystal was about 300 ph/n that was determined using the light yield of a reference Li-glass scintillator.     

Optical homogeneity and photorefractive properties of stoichiometric and congruent lithium niobate crystals grown using charges of different origins

A stoichiometric lithium niobate crystal (LiNbO_{3 st}) and congruent lithium niobate crystals grown from a charge prepared using cyclohexanone as an extractant (LiNbO_{3 cong}(CHN)) and a charge prepared using cyclohexanone and carboxylic acid dimethylamides as extractants (LiNbO_{3 cong}(CHN + A)) have been characterized by photoinduced light scattering and laser conoscopy. The results demonstrate that the LiNbO_{3 cong}(CHN + A) crystals are rather homogeneous along their growth direction and possess good optical properties, similar to those of the LiNbO_{3 cong}(CHN) crystal. At the same time, the LiNbO_{3 cong}(CHN + A) crystal offers significantly better electro-optical properties (r _e = 29.3 pm/V).     

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.      

Optical Properties of Copper-Doped Lithium Niobate Crystals

We have studied the optical properties of copper-doped lithium niobate crystals using semiconductor light-emitting diodes and lasers emitting in the visible and ultraviolet spectral regions as excitation sources. The results demonstrate that, at an excitation frequency approaching the frequency of electronic absorption in copper ions, there is discrete resonance Raman scattering in the form of a frequency comb. The observed Raman satellites of discrete light scattering are due to polar longitudinal A₁ optical modes of the lithium niobate single crystal. Under excitation by a light-emitting diode with a wavelength of 520 nm, we observe a sharp increase in discrete light scattering intensity in comparison with the photoluminescence excited by shorter wavelength excitation sources.     

Scintillating Bolometers for Double Beta Decay Search

In the field of Double Beta Decay searches the possibility to have high resolution detectors in which a very large part of the natural background can be discriminated with respect to the tiny expected signal, results very appealing. This very interesting possibility can be fulfilled in the case of a scintillating crystal bolometer containing a DBD emitter whose transition energy exceeds the one of the natural 2615 keV gamma line of ²⁰⁸Tl. We present the results achieved in the development of bolometric light detectors for double beta searches. The detectors are 1 mm thick germanium disk coated with a layer of SiO₂ in order to increase the light collection. The adopted temperature sensors are NTD Ge thermistors optimized to work at temperatures between 9 and 13 mK. A light detector with a considerable large area (35 cm²) was constructed and run in a test measurement. A 140 g CdWO₄ crystal (¹¹⁶Cd has a DBD transition energy of 2802 keV) was operated as bolometer and the scintillation light was read by the light detector. The excellent results combined with extreme easy light detector assembly represent the first tangible proof demonstrating the feasibility of this kind of technique.      

Synthesis of stable suspensions of discrete colloidal zeolite (Na,TPA)ZSM-5 crystals

Synthesis of zeolite ZSM-5 in the form of stable suspensions of colloidal crystals from clear homogeneous solutions has been achieved from synthesis mixtures with low sodium and high TPAOH concentrations. The ultimate size of the discrete crystals is in the range 130–230 nm with a narrow particle size distribution. The crystal growth was monitored by the direct method of dynamic light scattering. An increasing alumina concentration was found to decrease the crystal growth rate, the number of crystals produced, and the ZSM-5 yield. The final crystal size was found to increase with increasing alumina concentration in the synthesis solution. The number of crystals produced decreased with an increased alkalinity in contradiction to what has been found for colloidal TPA-silicalite-1. This together with the presence of the two competing processes, crystal growth and crystal dissolution, which are both catalyzed by hydroxide ions, suggests the presence of an optimum alkalinity (depending on the alumina concentration).