Rare Event Searches Research Articles

A Low Nuclear Recoil Energy Threshold for Dark Matter Search with CRESST-III Detectors

The CRESST-III experiment (Cryogenic Rare Events Search with Superconducting Thermometers), located at the underground facility Laboratori Nazionali del Gran Sasso in Italy, uses scintillating CaWO_4 crystals as cryogenic calorimeters to search for direct dark matter interactions in detectors. A large part of the parameter space for spin-independent scattering off nuclei remains untested for dark matter particles with masses below a few GeV/c^2, despite many naturally motivated theoretical models for light dark matter particles. The CRESST-III detectors are designed to achieve the performance required to probe the low-mass region of the parameter space with a sensitivity never reached before. In this paper, new results on the performance and an overview of the CRESST-III detectors will be presented, emphasizing the results about the low-energy threshold for nuclear recoil of CRESST-III Phase 1 which started collecting data in August 2016.

Preliminary measurement of the spherical proportional counter prototype

A novel large-volume spherical proportional counter (SPC) aims at relatively large target masses with low threshold, low background, and possible particle identification, with an extremely simple and robust operation. In this paper, we present some progress on the study of such a kind of detector, with a prototype in Beijing for alpha neutron measurement and long-term running stability. Preliminary understanding was achieved with a SPC prototype. It can measure alpha and neutron and show the capability of particle identification. Long-term running of the detector shows that it is a good stable detector. It needs more tests and studies to reach lower energy region, better particle identification, more understanding of efficiency, etc. The SPC, in principle, is capable of detecting very low energy nuclear recoils (in the few tens keV energy region) and smaller energy with lower pressure, thus being a very good candidate for rare event searches.

A dual-phase xenon TPC for scintillation and ionisation yield measurements in liquid xenon

A small-scale, two-phase (liquid/gas) xenon time projection chamber (Xurich II) was designed, constructed and is under operation at the University of Zürich. Its main purpose is to investigate the microphysics of particle interactions in liquid xenon at energies below 50 keV, which are relevant for rare event searches using xenon as target material. Here we describe in detail the detector, its associated infrastructure, and the signal identification algorithm developed for processing and analysing the data. We present the first characterisation of the new instrument with calibration data from an internal ^{83mathrm {m}}Kr source. The zero-field light yield is 15.0 and 14.0 photoelectrons/keV at 9.4 and 32.1 keV, respectively, and the corresponding values at an electron drift field of 1 kV/cm are 10.8 and 7.9 photoelectrons/keV. The charge yields at these energies are 28 and 31 electrons/keV, with the proportional scintillation yield of 24 photoelectrons per one electron extracted into the gas phase, and an electron lifetime of 200 upmu s. The relative energy resolution, sigma /E, is 11.9 and 5.8% at 9.4 and 32.1 keV, respectively using a linear combination of the scintillation and ionisation signals. We conclude with measurements of the electron drift velocity at various electric fields, and compare these to literature values.

A Cryogenic Detector Characterization Facility in the Shallow Underground Laboratory at the Technical University of Munich

The Physics Department of the Technical University of Munich operates a shallow underground detector laboratory in Garching, Germany. It provides \(\sim 160\,{\mathrm{m}^2}\) of laboratory space which is shielded from cosmic radiation by \(\sim 6\,\mathrm{m}\) of gravel and soil, corresponding to a shielding of \(\sim 15\,{\mathrm{m.w.e.}}\). The laboratory also houses a cleanroom equipped with work- and wetbenches, a chemical fumehood as well as a spin-coater and a mask-aligner for photolithographic processing of semiconductor detectors. Furthermore, the shallow underground laboratory runs two high-purity germanium detector screening stations, a liquid argon cryostat and a \(^3\)He–\(^4\)He dilution refrigerator with a base temperature of \(\le 12-14\,\mathrm{mK}\). The infrastructure provided by the shallow laboratory is particularly relevant for the characterization of \(\hbox {CaWO}_4\) target crystals for the CRESST-III experiment, detector fabrication and assembly for rare event searches. Future applications of the laboratory include detector development in the framework of coherent neutrino nucleus scattering experiments (\(\nu \)-cleus) and studying its potential as a site to search for MeV-scale dark matter with gram-scale cryogenic detectors.

Application of Silicon Drift Detectors for the Readout of a CdWO4 Scintillating Crystal

In this paper, an application of silicon drift detectors (SDDs) as photodetectors for large inorganic scintillators is presented. In particular, a $2.3\times 2.3\times 1.5$ cm3 CdWO4 crystal has been coupled to a $3\times 2$ matrix of $8\times 8$ mm2 SDDs both cooled at 125 K, obtaining a scintillation detector whose performances are tested by measuring the $\gamma $ rays emitted by a 228Th source. The SDDs, characterized by high quantum efficiency and low electronic noise, are suitable candidates as photodetectors for high-resolution gamma-ray spectroscopy detectors. On the other hand, CdWO4 is an interesting crystal for the study of rare nuclear decays, such as the possible neutrinoless double beta decay ( $0\nu \beta \beta $ ) of 116Cd. With this prototype, a full-width at half-maximum resolution of 3.09 ± 0.05% at 2615 keV has been measured. This result provides a first validation of the application of this technology to rare event searches.

First scintillating bolometer tests of a CLYMENE R&D on Li[formula omitted]MoO[formula omitted] scintillators towards a large-scale double-beta decay experiment

A new R&D on lithium molybdate scintillators has begun within a project CLYMENE (Czochralski growth of Li2MoO4 crYstals for the scintillating boloMeters used in the rare EveNts sEarches). One of the main goals of the CLYMENE is a realization of a Li2MoO4 crystal growth line to be complementary to the one recently developed by LUMINEU in view of a mass production capacity for CUPID, a next-generation tonne-scale bolometric experiment to search for neutrinoless double-beta decay. In the present paper we report the investigation of performance and radiopurity of 158-g and 13.5-g scintillating bolometers based on a first large-mass (230 g) Li2MoO4 crystal scintillator developed within the CLYMENE project. In particular, a good energy resolution (2–7 keV FWHM in the energy range of 0.2–5 MeV), one of the highest light yield (0.97 keV/MeV) amongst Li2MoO4 scintillating bolometers, an efficient alpha particles discrimination (10σ) and potentially low internal radioactive contamination (below 0.2–0.3 mBq/kg of U/Th, but 1.4 mBq/kg of 210Po) demonstrate prospects of the CLYMENE in the development of high quality and radiopure Li2MoO4 scintillators for CUPID.

Development of a tagged source of Pb-206 nuclei

There is a particular class of unavoidable backgrounds that plague low-background experiments and rare event searches, particularly those searching for nuclear recoil event signatures: decaying daughters of the 238U nuclear decay chain, which result from radon plate-out on detector materials. One such daughter isotope, 210Po, undergoes α-decay and produces a recoiling 103 keV 206Pb nucleus. To characterize this important background in the context of noble element detectors, we have implemented a triggered source for these 206Pb recoils in a dual-phase xenon time projection chamber (Xe TPC) within the Davis Xenon R&D testbed system (DAX). By adhering 210Po to the surface of a PIN diode and electrically floating the diode on the cathode of the TPC, we tag the α signals produced in the PIN diode and trigger on the correlated nuclear recoils in the liquid xenon (LXe). We discuss our methods for 210Po deposition, electronic readout of the PIN diode signals at high voltage, and analysis methods for event selection.

Cryogenic Heat–Light Detection System for 1-cm³ Scintillating Crystals

We have developed a cryogenic heat and light detection system to investigate phonon and scintillation properties of scintillating crystals for rare event search experiments. The detector setup is designed to utilize a $1 \times 1\times 1$ cm3 scintillating crystal as a target material. A $1.5\times 1.5\times 0.05$ cm3 Ge wafer is used as the absorber of the light detection. Metallic magnetic calorimeters are employed to measure heat and scintillation-light signals of the scintillating crystal, simultaneously, at millikelvin temperatures. This measurement setup is motivated to characterize various types of scintillation crystals in a standard coupon size for a final selection of the crystal compounds to be used for a rare event search experiment. We present the first measurement for a calcium molybdate crystal doped with niobium in the test setup of heat–light detection. Clear particle identification was obtained in comparison of relative amplitude ratios of the phonon and scintillation signals. Moreover, alpha- and electron-induced events showed difference in their pulse shapes of phonon and scintillation signals. We discuss the usage of this setup for the AMoRE $0\nu \beta \beta $ search experiment.

Spectral shapes of forbidden argon β decays as background component for rare-event searches

The spectral shape of the electrons from the two first-forbidden unique decays of 39Ar and 42Ar were calculated for the first time to the next-to-leading order. Especially the spectral shape of the 39Ar decay can be used to characterize this background component for dark matter searches based on argon. Alternatively, due to the low thresholds of these experiments, the spectral shape can be investigated over a wide energy range with high statistics and thus allow a sensitive comparison with the theoretical predictions, in particular at low electron energies where the shape of the computed β spectrum has a slight dependence on the value of the weak axial-vector coupling constant.

Low-temperature relative reflectivity measurements of reflective and scintillating foils used in rare event searches

In this work we investigate the reflectivity of highly reflective multilayer polymer foils used in the CRESST experiment. The CRESST experiment searches directly for dark matter via operating scintillating CaWO4 crystals as targets for elastic dark matter–nucleon scattering. In order to suppress background events, the experiment employs the so-called phonon–light technique which is based on the simultaneous measurement of the heat signal in the main CaWO4 target crystal and of the emitted scintillation light with a separate cryogenic light detector. Both detectors are surrounded by a highly reflective and scintillating multilayer polymer foil to increase the light collection efficiency and to veto surface backgrounds. While this study is motivated by the CRESST experiment, the results are also relevant for other rare event searches using scintillating cryogenic bolometers in the field of the search of dark matter and neutrinoless double beta decay (0νββ). In this work a dedicated experiment has been set up to determine the relative reflectivity at 300 K and 20 K of three multilayer foils (“VM2000”, “VM2002”, “Vikuiti”) produced by the company 3M. The intensity of a light beam reflected off the foil is measured with a CCD camera. The ratio of the intensities at 300 K and 20 K corresponds to the relative reflectivity change. The measurements performed in this work show no variation of the reflectivity with temperature at a level of ∼1%.

Low-background gamma spectroscopy at the Boulby Underground Laboratory

The Boulby Underground Germanium Suite (BUGS) comprises three low-background, high-purity germanium detectors operating in the Boulby Underground Laboratory, located 1.1 km underground in the north-east of England, UK. BUGS utilises three types of detector to facilitate a high-sensitivity, high-throughput radio-assay programme to support the development of rare-event search experiments. A Broad Energy Germanium (BEGe) detector delivers sensitivity to low-energy gamma-rays such as those emitted by 210Pb and 234Th. A Small Anode Germanium (SAGe) well-type detector is employed for efficient screening of small samples. Finally, a standard p-type coaxial detector provides fast screening of standard samples. This paper presents the steps used to characterise the performance of these detectors for a variety of sample geometries, including the corrections applied to account for cascade summing effects. For low-density materials, BUGS is able to radio-assay to specific activities down to 3.6mBqkg−1 for 234Th and 6.6mBqkg−1 for 210Pb both of which have uncovered some significant equilibrium breaks in the 238U chain. In denser materials, where gamma-ray self-absorption increases, sensitivity is demonstrated to specific activities of 0.9mBqkg−1 for 226Ra, 1.1mBqkg−1 for 228Ra, 0.3mBqkg−1 for 224Ra, and 8.6mBqkg−1 for 40K with all upper limits at a 90% confidence level. These meet the requirements of most screening campaigns presently under way for rare-event search experiments, such as the LUX-ZEPLIN (LZ) dark matter experiment. We also highlight the ability of the BEGe detector to probe the X-ray fluorescence region which can be important to identify the presence of radioisotopes associated with neutron production; this is of particular relevance in experiments sensitive to nuclear recoils.

Input comparison of radiogenic neutron estimates for ultra-low background experiments

Ultra-low-background experiments address some of the most important open questions in particle physics, cosmology and astrophysics: the nature of dark matter, whether the neutrino is its own antiparticle, and does the proton decay.These rare event searches require well-understood and minimized backgrounds. Simulations are used to understand backgrounds caused by naturally occurring radioactivity in the rock and in every piece of shielding and detector material used in these experiments. Most important are processes like spontaneous fission and (α,n) reactions in material close to the detectors that can produce neutrons. A comparison study of the (α,n) reactions between two dedicated software packages is detailed. The cross section libraries, neutron yields, and spectra from the Mei–Zhang–Hime and the SOURCES-4A codes are presented. The resultant yields and spectra are used as inputs to direct dark matter detector toy models in GEANT4, to study the impact of their differences on background estimates and fits. Although differences in neutron yield calculations up to 50% were seen, there was no systematic difference between the Mei–Hime–Zhang and SOURCES-4A results. Neutron propagation simulations smooth differences in spectral shape and yield, and both tools were found to meet the broad requirements of the low-background community.

ICP-MS measurement of natural radioactivity at LNGS

Rare events search experiments, like those dedicated to the direct evidence of dark matter or neutrinoless double beta decay, are among the most exciting challenges of modern physics. The sensitivity of such experiments is driven by the background, which depends substantially on the radiopurity of the materials used for the experimental apparatus. Cutting edge measurement techniques are needed for a fast, sensitive and efficient screening of these materials and the certification of their production. Trace element measurements of high sensitivity and quick execution are mandatory also in other fields like tracing the geographical origin of food, temporal and geographical assignment of cultural heritage or monitoring environmental radioactivity. This work is an overview of the inorganic mass spectrometry facility available at Gran Sasso National Laboratory (LNGS) for radiopure material screening and is especially focused on its ICP-MS instrumentation. Analytical methods developed to achieve lowest detection limits in different types of matrix, like metals, polymers, crystals and composite materials, are also indicated. Detection limits of [Formula: see text] for [Formula: see text], [Formula: see text] for U and Th and [Formula: see text] for K are attained through dedicated operation conditions of the instrumentation. Details are given on the results obtained for different experiments ongoing or under construction at LNGS.

Cosmogenic activation of germanium used for tonne-scale rare event search experiments

We report a comprehensive study of cosmogenic activation of germanium used for tonne-scale rare event search experiments. The germanium exposure to cosmic rays on the Earth’s surface are simulated with and without a shielding container using Geant4 for a given cosmic muon, neutron, and proton energy spectrum. The production rates of various radioactive isotopes are obtained for different sources separately. We find that fast neutron induced interactions dominate the production rate of cosmogenic activation. Geant4-based simulation results are compared with the calculation of ACTIVIA and the available experimental data. A reasonable agreement between Geant4 simulations and several experimental data sets is presented. We predict that cosmogenic activation of germanium can set limits to the sensitivity of the next generation of tonne-scale experiments.

Ultra-low background mass spectrometry for rare-event searches

Inductively Coupled Plasma Mass Spectrometry (ICP-MS) allows for rapid, high-sensitivity determination of trace impurities, notably the primordial radioisotopes 238U and 232Th, in candidate materials for low-background rare-event search experiments. We describe the setup and characterisation of a dedicated low-background screening facility at University College London where we operate an Agilent 7900 ICP-MS. The impact of reagent and carrier gas purity is evaluated and we show that twice-distilled ROMIL-SpATM-grade nitric acid and zero-grade Ar gas delivers similar sensitivity to ROMIL-UpATM-grade acid and research-grade gas. A straightforward procedure for sample digestion and analysis of materials with U/Th concentrations down to 10 ppt g/g is presented. This includes the use of 233U and 230Th spikes to correct for signal loss from a range of sources and verification of 238U and 232Th recovery through digestion and analysis of a certified reference material with a complex sample matrix. Finally, we demonstrate assays and present results from two sample preparation and assay methods: a high-sensitivity measurement of ultra-pure Ti using open digestion techniques, and a closed vessel microwave digestion of a nickel–chromium-alloy using a multi-acid mixture.

Growth and characterization of Na2Mo2O7 crystal scintillators for rare event searches

Disodium dimolybdate (Na2Mo2O7) crystals were grown using the Czochralski technique. The thermal characteristics of the compound were analyzed using thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) measurements. The crystal structure of the grown sample was confirmed using X-ray diffraction (XRD). Luminescence properties were measured at room and low temperatures, using a light emitting diode (LED) source. Very weak luminescence was observed at room temperature; however, the luminescence intensity was enhanced at low temperatures. The crystal’s transmittance spectrum was measured for estimating its optical quality and energy band gap. The grown crystal exhibited a luminescence light yield of 55% compared with CaMoO4 crystals at 10 K, when excited by a 280-nm-wavelength LED source, but does not have the drawbacks of radioactive Ca isotopes. These results suggest that at cryogenic temperatures, Na2Mo2O7 crystal scintillators are promising for the detection of dark matter and neutrinoless double beta decay of 100Mo.

Identification of radiopure titanium for the LZ dark matter experiment and future rare event searches

The LUX-ZEPLIN (LZ) experiment will search for dark matter particle interactions with a detector containing a total of 10 tonnes of liquid xenon within a double-vessel cryostat. The large mass and proximity of the cryostat to the active detector volume demand the use of material with extremely low intrinsic radioactivity. We report on the radioassay campaign conducted to identify suitable metals, the determination of factors limiting radiopure production, and the selection of titanium for construction of the LZ cryostat and other detector components. This titanium has been measured with activities of 238Ue < 1.6 mBq/kg, 238Ul < 0.09 mBq/kg, 232The=0.28±0.03 mBq/kg, 232Thl=0.25±0.02 mBq/kg, 40K < 0.54 mBq/kg, and 60Co < 0.02 mBq/kg (68% CL). Such low intrinsic activities, which are some of the lowest ever reported for titanium, enable its use for future dark matter and other rare event searches. Monte Carlo simulations have been performed to assess the expected background contribution from the LZ cryostat with this radioactivity. In 1,000 days of WIMP search exposure of a 5.6-tonne fiducial mass, the cryostat will contribute only a mean background of 0.160 ± 0.001(stat) ± 0.030(sys) counts.

Lowering the CUORE energy threshold

The Cryogenic Underground Observatory for Rare Events (CUORE) is a ton-scale double beta decay experiment based on TeO2 cryogenic bolometers and is currently in the last construction stage at the Gran Sasso National Laboratory (LNGS). Its primary goal is to observe neutrino-less double beta decay of 130Te, however thanks to the ultra-low background and large projected exposure it could also be suitable for other rare event searches, as the detection of solar axions, neutrinos from type II supernovae or direct detection of dark matter. The sensitivity for these searches will depend on the performance achieved at the low energy threshold. For this reason a trigger algorithm based on continuous data filtering has been developed which will allow lowering the threshold down to the few keV region. The new trigger has been tested in CUORE-0, a single-tower CUORE prototype consisting of 52 TeO2 bolometers and recently concluded, and here we present the results in terms of trigger efficiency, data selection and low-energy calibration.

SINGLE: single photon sensitive cryogenic light detectors

Thermal detectors operated at few mK as calorimeters are a powerful tool for the study of rare particle physics processes. In order to implement particle identification, light detection can be effectively performed by means of other thermal detectors operated as light sensors. This configuration can be used also in large scale, thousand-channels setups, but the light sensors must be sensitive enough to detect few, possibly a single, photons. The SINGLE project described here aims at producing silicon based, large area devices that can be operated as thermal detectors with single-photon sensitivity, and demonstrate the reliability of the performance, scalability of the production process and integrability with present and next generation cryogenic experiments for the search for rare events.

Search for double β-decays of 124Xe with XENON100 & XENON1T

The rare nuclear process of two-neutrino double electron capture, where two electrons are simultaneously captured from the atomic shell, has not yet been observed for 124Xe. A detection of this decay would provide a new reference for nuclear matrix element calculations. Moreover, if a neutrinoless mode were discovered, it would prove a Majorana nature of neutrinos and would shed light on the effective neutrino mass. The XENON dark matter project, with its dual-phase xenon time projection chambers XENON100 and XENON1T, is well suited for this rare event searches with signatures in the keV-region. The search with the XENON100 detector, containing 29 g of 124Xe, is explained as well as the outlook of its successor XENON1T, which contains 2 kg of the isotope in its active volume.