Search

The direct detection of galactic dark matter particles requires ultra-low background conditions. NaI(Tl) crystals are applied in the search for these dark matter particles through their interactions in the detector by measuring the scintillation signal produced. The production of long-lived isotopes in materials due to the exposure to cosmic rays on Earth's surface can be an hazard for these ultra-low background demanding experiments, typically performed underground. Therefore, production rates of cosmogenic isotopes in all the materials present in the experimental set-up, as well as the corresponding cosmic rays exposure history, must be both well-known in order to assess the relevance of this effect in the achievable sensitivity of a given experiment. Here, analysis of the cosmogenic studies developed from the ANAIS experiment NaI(Tl) detectors are presented. Installed inside a convenient shielding at the Canfranc Underground Laboratory just after finishing surface exposure to cosmic rays and thanks to the prompt data taking developed, identification and quantification of isotopes with half-lives of the order of tens of days were allowed, and thanks to the long-term operation of the detectors long-lived isotopes have been also identified and quantified. Main results for the activation yields of iodine and tellurium isotopes, Na, Sn, Cd, and tritium are presented in this work, together with the estimate of the production rates for their activation by cosmic nucleons while on Earth's surface based on a selection of excitation functions over the entire energy range of cosmic nucleons. [ABSTRACT FROM AUTHOR]

large crystals are applied in the search for galactic dark matter particles through their elastic scattering off the target nuclei in the detector by measuring the scintillation signal produced. However, energies deposited in the form of nuclear recoils are small, which added to the low efficiency to convert that energy into scintillation, makes that events at or very near the energy threshold, attributed either to radioactive backgrounds or to spurious noise (nonbulk scintillation events), can compromise the sensitivity goals of such an experiment. DAMA/LIBRA experiment, using 250 kg target, reported first evidence of the presence of an annual modulation in the detection rate compatible with that expected for a dark matter signal just in the region below 6 keVee (electron equivalent energy). In the frame of the ANAIS (Annual modulation with NaI Scintillators) dark matter search project a large and long effort has been carried out in order to understand the origin of events at very low energy in large sodium iodide detectors and develop convenient filters to reject those nonattributable to scintillation in the bulk crystal. 40K is probably the most relevant radioactive contaminant in the bulk for detectors because of its important contribution to the background at very low energy. ANAIS goal is to achieve levels at or below 20 ppb natural potassium. In this paper we will report on our effort to determine the 40K contamination in several crystals, by measuring in coincidence between two (or more) of them. Results obtained for the 40K content of crystals from different providers will be compared and prospects of the ANAIS dark matter search experiment will be briefly reviewed. [ABSTRACT FROM AUTHOR]

In the last decade direct detection Dark Matter (DM) experiments have increased enormously their sensitivity and ton-scale setups have been proposed, especially using germanium and xenon targets with double readout and background discrimination capabilities. In light of this situation, we study the prospects for determining the parameters of Weakly Interacting Massive Particle (WIMP) DM (mass, spin-dependent (SD) and spin-independent (SI) cross-section off nucleons) by combining the results of such experiments in the case of a hypothetical detection. In general, the degeneracy between the SD and SI components of the scattering cross-section can only be removed using targets with different sensitivities to these components. Scintillating bolometers, with particle discrimination capability, very good energy resolution and threshold and a wide choice of target materials, are an excellent tool for a multitarget complementary DM search. We investigate how the simultaneous use of scintillating targets with different SD-SI sensitivities and/or light isotopes (as the case of 2 and ) significantly improves the determination of the WIMP parameters. In order to make the analysis more realistic we include the effect of uncertainties in the halo model and in the spin-dependent nuclear structure functions, as well as the effect of a thermal quenching different from 1. [ABSTRACT FROM AUTHOR]

To search for cold dark matter (CDM) particles, the data from two (Ge-76 and Ge-natural) detectors, fabricated in the first phase of the IGEX collaboration, were accumulated and analyzed for a period from May 1995 to May 1999. The use of effective passive and active shielding together with pulse shape discrimination allowed us to perform long-term measurements with an energy threshold of 2 keV at a level of the residual background 0.1 counts/(kg keV d). New restrictions on masses of weakly interacting massive particles and the cross section of their elastic scattering on nuclei have been obtained from the data corresponding to 810 d of live time. The derived exclusion plots compete with the best bounds obtained so far. Annual modulations of the CDM signal have been also investigated. It is shown that the planned operation of all IGEX detectors (at Canfranc and Baksan) with the recently achieved low-energy thresholds gives a chance to achieve the DAMA annual modulation sensitivity region in a nearest future. [ABSTRACT FROM AUTHOR]

The ANAIS (Annual modulation with NaI(Tl) Scintillators) experiment aims at the confirmation or refutation of the DAMA/LIBRA positive annual modulation signal in the low energy detection rate, using the same target and technique, at the Canfranc Underground Laboratory (LSC) in Spain. ANAIS-112, consisting of nine 12.5 kg NaI(Tl) modules produced by Alpha Spectra Inc. in a 3 × 3 matrix configuration, is taking data smoothly in "dark matter search" mode since August, 2017, after a commissioning phase and operation of the first detectors during the last years in various set-ups. A large effort has been carried out within ANAIS to characterize the background of sodium iodide detectors, before unblinding the data and performing the first annual modulation analysis. Here, the background models developed for all the nine ANAIS-112 detectors are presented. Measured spectra from threshold to high energy in different conditions are well described by the models based on quantified activities independently estimated following several approaches. In the region from 1 to 6 keV ee the measured, efficiency corrected background level is 3.58 ± 0.02 keV - 1 kg - 1 day - 1 ; NaI crystal bulk contamination is the dominant background source being 210 Pb 40 K 22 Na and 3 H contributions the most relevant ones. This background level, added to the achieved 1 keV ee analysis threshold (thanks to the outstanding light collection and robust filtering procedures developed), allow ANAIS-112 to be sensitive to the modulation amplitude measured by DAMA/LIBRA, and able to explore at three sigma level in 5 years the WIMP parameter region singled out by this experiment. [ABSTRACT FROM AUTHOR]

The annual modulation measured by the DAMA/LIBRA experiment can be explained by the interaction of dark matter WIMPs in NaI(Tl) scintillator detectors. Other experiments, with different targets or techniques, exclude the region of parameters singled out by DAMA/LIBRA, but the comparison of their results relies on several hypotheses regarding the dark matter model. ANAIS-112 is a dark matter search with 112.5 kg of NaI(Tl) scintillators at the Canfranc Underground Laboratory (LSC) to test the DAMA/LIBRA result in a model independent way. We analyze its prospects in terms of the a priori critical and detection limits of the experiment. A simple figure of merit has been obtained to compare the different experiments looking for the annual modulation observed by DAMA/LIBRA. We conclude that after 5 years of measurement, ANAIS-112 can detect the annual modulation in the 3 σ region compatible with the DAMA/LIBRA result. [ABSTRACT FROM AUTHOR]

ANAIS is a direct detection dark matter experiment aiming at the study of the annual modulation expected in the interaction rate. It uses same target and technique as the DAMA/LIBRA experiment, which reported a highly significant positive modulation compatible with that expected for dark matter particles distributed in the galactic halo. However, other very sensitive experiments do not find any hint of particles with the required properties, although comparison is model dependent. In 2017, ANAIS-112 experiment was installed at the Canfranc Underground Laboratory (LSC), in Spain, and after the commissioning run for calibration and general assessment, ANAIS-112 started data taking in dark matter mode on August 3rd, 2017. It consists of nine NaI(Tl) modules, amounting 112.5 kg of mass in total. Here we report on the experimental apparatus and detector performance after the first year of data taking. Total live time available amounts to 341.72 days, being the corresponding exposure 105.32 kg × year. ANAIS-112 has achieved an analysis energy threshold of 1 keVee and an average background in the region of interest, from 1 to 6 keVee, of 3.6 counts/keVee/kg/day after correcting by the event selection efficiencies. In these conditions, ANAIS-112 will be able to test the DAMA/LIBRA result at three sigma level in five years of data taking. [ABSTRACT FROM AUTHOR]

The direct detection of dark matter particles requires ultra-low background conditions at energies below a few tens of keV. Radioactive isotopes are produced via cosmogenic activation in detectors and other materials and those isotopes constitute a background source which has to be under control. In particular, tritium is specially relevant due to its decay properties (very low endpoint energy and long half-life) when induced in the detector medium, and because it can be generated in any material as a spallation product. Quantification of cosmogenic production of tritium is not straightforward, neither experimentally nor by calculations. In this work, a method for the calculation of production rates at sea level has been developed and applied to some of the materials typically used as targets in dark matter detectors (germanium, sodium iodide, argon and neon); it is based on a selected description of tritium production cross sections over the entire energy range of cosmic nucleons. Results have been compared to available data in the literature, either based on other calculations or from measurements. The obtained tritium production rates, ranging from a few tens to a few hundreds of nuclei per kg and per day at sea level, point to a significant contribution to the background in dark matter experiments, requiring the application of specific protocols for target material purification, material storing underground and limiting the time the detector is on surface during the building process in order to minimize the exposure to the most dangerous cosmic ray components. [ABSTRACT FROM AUTHOR]

Application of NaI(Tl) detectors in the search for galactic dark matter particles through their elastic scattering off the target nuclei is well motivated because of the long standing DAMA/LIBRA highly significant positive result on annual modulation, still requiring confirmation. For such a goal, it is mandatory to reach very low threshold in energy (at or below the keV level), very low radioactive background (at a few counts/keV/kg/day), and high detection mass (at or above the 100 kg scale). One of the most relevant technical issues is the optimization of the crystal intrinsic scintillation light yield and the efficiency of the light collecting system for large mass crystals. In the frame of the ANAIS (Annual modulation with NaI Scintillators) dark matter search project large NaI(Tl) crystals from different providers coupled to two photomultiplier tubes (PMTs) have been tested at the Canfranc Underground Laboratory. In this paper we present the estimates of the NaI(Tl) scintillation light collected using full-absorption peaks at very low energy from external and internal sources emitting gammas/electrons, and single-photoelectron events populations selected by using very low energy pulses tails. Outstanding scintillation light collection at the level of 15 photoelectrons/keV can be reported for the final design and provider chosen for ANAIS detectors. Taking into account the quantum efficiency of the PMT units used, the intrinsic scintillation light yield in these NaI(Tl) crystals is above 40 photons/keV for energy depositions in the range from 3 up to 25 keV. This very high light output of ANAIS crystals allows triggering below 1 keV, which is very important in order to increase the sensitivity in the direct detection of dark matter. [ABSTRACT FROM AUTHOR]

Abstract: The ANAIS (Annual Modulation with NaI(Tl) Scintillators) experiment aims at the confirmation of the DAMA/LIBRA signal using the same target and technique at the Canfranc Underground Laboratory. 250kg of ultrapure NaI(Tl) crystals will be used as a target, divided into 20 modules, each coupled to two photomultipliers. Two NaI(Tl) crystals of 12.5kg each, grown by Alpha Spectra from a powder having a potassium level under the limit of our analytical techniques, form the ANAIS-25 set-up. The background contributions are being carefully studied and preliminary results are presented: their natural potassium content in the bulk has been quantified, as well as the uranium and thorium radioactive chains presence in the bulk through the discrimination of the corresponding alpha events by PSA, and due to the fast commissioning, the contribution from cosmogenic activated isotopes is clearly identified and their decay observed along the first months of data taking. Following the procedures established with ANAIS-0 and previous prototypes, bulk NaI(Tl) scintillation events selection and light collection efficiency have been also studied in ANAIS-25. [Copyright &y& Elsevier]

Abstract: Dark matter direct search experiments with scintillators need an accurate knowledge of Light Relative Efficiency Factors (REF) between electron and nuclear recoils to estimate the energy of the recoiling nuclei from the scintillation signal (if the light signal is used with this purpose) or to implement an effective background rejection based on the comparison of the scintillation with ionization or heat signals (if the light signal is used for particle discrimination). The Light REF between α and γ particles is required in some nuclear physics applications of scintillators like rare α decay searches, internal radiopurity assessment and some double beta decay searches. Two scintillating bolometers of BGO and Al2O3 were operated at 20mK and exposed to fast neutrons, gamma rays, α particles and heavy nuclei. We measured their Light REF between γ and α particles and between electron and neutron induced nuclear recoils as a function of the deposited energy. We also measured the Light REF for O and Np ions in BGO. Results obtained for the different Light REFs were unsuccessfully compared with calculations based on a simple semi-empirical approach (with only one fitting parameter) proposed by Tretyak. [Copyright &y& Elsevier]

Highlights: [•] The interest of a NaI scintillating bolometer for Dark Matter search is discussed. [•] We study parylene-coated NaI(Tl) to overcome hygroscopicity problems. [•] We present parylene-coated NaI(Tl) scintillation spectra at 1.5, 4 and 77K. [•] We study light output from 1.5 to 300K and measure good light response at low T. [•] Tests of mechanical resistance to thermal cycles are not fully satisfactory. [Copyright &y& Elsevier]

Abstract: NaI (Tl) is a well known high light yield scintillator. Very large crystals can be grown to be used in a wide range of applications. In particular, such large crystals are very good-performing detectors in the search for dark matter, where they have been used for a long time and reported first evidence of the presence of an annual modulation in the detection rate, compatible with that expected for a dark matter signal. In the frame of the ANAIS (Annual modulation with NaI Scintillators) dark matter search project, a large and long effort has been carried out in order to characterize the background of sodium iodide crystals. In this paper we present in detail our background model for a 9.6kg NaI (Tl) detector taking data at the Canfranc Underground Laboratory (LSC): most of the contaminations contributing to the background have been precisely identified and quantified by different complementary techniques such as HPGe spectrometry, discrimination of alpha particles vs. beta/gamma background by Pulse Shape Analysis (PSA) and coincidence techniques; then, Monte Carlo (MC) simulations using Geant4 package have been carried out for the different contributions. Only a few assumptions are required in order to explain most of the measured background at high energy, supporting the goodness of the proposed model for the present ANAIS prototype whose background is dominated by 40K bulk contamination. At low energy, some non-explained background components are still present and additional work is required to improve background understanding, but some plausible background sources contributing in this range have been studied in this work. Prospects of achievable backgrounds, at low and high energy, for the ANAIS-upgraded detectors, relying on the proposed background model conveniently scaled, are also presented. [Copyright &y& Elsevier]

Abstract: Scintillating bolometers are particle detectors with a high particle discrimination power with many applications in nuclear and particle physics. This discrimination power is based on the different scintillation yield for different particles, and is strongly dependent on the target used. At the very low temperatures required for the operation of the bolometers, very few data about the scintillation yields are available. In this paper we present estimates of absolute light yields and energy partition among heat, light and trapping channels in Sapphire (Al2O3) and BGO (Bi4Ge3O12) scintillating bolometers operated at 20mK. The estimate relies on the observed negative correlation between the light and heat signals produced by γ-ray absorption in scintillating bolometers and on the study of the X-ray stimulated luminescence properties of BGO at temperatures down to 77K. [Copyright &y& Elsevier]

Abstract: A 46g BGO (Bi4Ge3O12) scintillating bolometer has been tested at 20mK as a prototype for dark matter direct detection in the frame of the ROSEBUD (Rare Objects SEarch with Bolometers UndergrounD) collaboration. The bolometer has been operated first in Orsay and then in an ultra-low background environment at the Canfranc Underground Laboratory. We analyze its response both in heat and light, and its particle discrimination capability. Light yield under γ-ray excitation at several energies (from 88.0 to 1633.3keV) and the relative light output for different particles (β/γ, α and neutrons) have been estimated. The analysis focuses on the discrimination of nuclear recoils, like those produced by hypothetical weakly interacting massive particles (WIMPs), against electron recoils, produced by β/γ radioactive background. [Copyright &y& Elsevier]

Abstract: Scintillating bolometers are very suitable detectors for dark matter searches. They offer a wide absorber choice allowing to tune up different masses and couplings of WIMPs, background rejection capability and high efficiency in the conversion into heat of nuclear recoil energy releases. In a previous work we have shown that undoped sapphire can be an excellent dark matter detector since, due to its high light yield at very low temperature, very low particle discrimination energy thresholds are achievable. In this Letter, we report the results of an experiment devoted to the estimate of its thermal relative efficiency factor (REF) for nuclear recoil versus gamma events which is a relevant parameter in WIMP search experiments. Taking advantage of the discrimination capability of scintillating bolometers, we have registered the energy spectrum of 206Pb recoiling nuclei from a 210Po alpha-source. Ensuing from the analysis we obtain an close to but slightly greater than unity. [Copyright &y& Elsevier]

A quantitative study of the neutron environment in the Canfranc Underground Laboratory has been performed. The analysis is based on a complete set of simulations and, particularly, it is focused on the IGEX-DM dark matter experiment. The simulations are compared to the IGEX-DM low energy data obtained with different shielding conditions. The results of the study allow us to conclude, with respect to the IGEX-DM background, that the main neutron population, coming from radioactivity from the surrounding rock, is practically eliminated after the implementation of a suitable neutron shielding. The remaining neutron background (muon-induced neutrons in the shielding and in the rock) is substantially below the present background level thanks to the muon veto system. In addition, the present analysis gives us a further insight on the effect of neutrons in other current and future experiments at the Canfranc Underground Laboratory. The comparison of simulations with the body of data available has allowed to set the flux of neutrons from radioactivity of the Canfranc rock (3.82 ± 0.44) × 10-6 cm-2 s-1, as well as the flux of muon-induced neutrons in the rock (1.73 ± 0.22(stat) ± 0.69(syst)) × 10-9 cm-2 s-1, and the rate of neutron production by muons in the lead shielding (4.8 ± 0.6(stat) ± 1.9 (syst)) × 10-9 cm-3 s-1. [Copyright &y& Elsevier]

The rare objects search with bolometers underground (ROSEBUD) is an experiment to search for weakly interacting massive particles (WIMPs) at the Canfranc Underground Laboratory with bolometric detectors. We present the progress achieved in the second phase of the experiment with three different absorbers (Al2O3, Ge and scintillating CaWO4). For the first time, different target nuclei have been used simultaneously in a common set-up, sharing identical external background conditions. Moreover, a discrimination of nuclear against electron recoils based on the simultaneous measurement of light and heat has been developed and applied to derive limits for WIMP candidates. [Copyright &y& Elsevier]

A method based on wavelet techniques has been developed and applied to background rejection in the data of the IGEX dark matter experiment. The method is presented and described in some detail to show how it efficiently rejects events coming from noise and microphonism through a mathematical inspection of their recorded pulse shape. The result of the application of the method to the last published 80 kg day data of IGEX is presented. [Copyright &y& Elsevier]

We report on the results of the first underground dark matter search with light versus heat discrimination. It has been performed at the Canfranc Underground Laboratory in the frame of the Rare Objects SEarch with Bolometers UndergrounD (ROSEBUD) Collaboration with a double bolometer consisting of a 54 g scintillating CaWO4 and a thin Ge (0.26 g) for light detection. Its capability to distinguish nuclear against electron recoils has been proved and applied to derive limits for weakly interacting massive particle (WIMP) dark matter candidates. [Copyright &y& Elsevier]

One IGEX 76Ge double-beta decay detector is currently operating in the Canfranc Underground Laboratory in a search for dark matter WIMPs, through the Ge nuclear recoil produced by the WIMP elastic scattering. A new exclusion plot, σ(m), has been derived for WIMP-nucleon spin-independent interactions. To obtain this result, 40 days of data from the IGEX detector (energy threshold Ethr∼4 keV), recently collected, have been analyzed. These data improve the exclusion limits derived from all the other ionization germanium detectors in the mass region from 20 to 200 GeV, where a WIMP supposedly responsible for the annual modulation effect reported by the DAMA experiment would be located. The new IGEX exclusion contour enters, by the first time, the DAMA region by using only raw data, with no background discrimination, and excludes its upper left part. It is also shown that with a moderate improvement of the detector performances, the DAMA region could be fully explored. [Copyright &y& Elsevier]

ANAIS is a direct detection dark matter experiment aiming at the testing of the DAMA/LIBRA annual modulation result, which, for about two decades, has neither been confirmed nor ruled out by any other experiment in a model independent way. ANAIS-112, consisting of 112.5 kg of sodium iodide crystals, has been taking data at the Canfranc Underground Laboratory, Spain, since August 2017. This Letter presents the annual modulation analysis of 1.5 years of data, amounting to 157.55 kg yr. We focus on the model independent analysis searching for modulation and the validation of our sensitivity prospects. ANAIS-112 data are consistent with the null hypothesis (p values of 0.67 and 0.18 for [2-6] and [1-6] keV energy regions, respectively). The best fits for the modulation hypothesis are consistent with the absence of modulation (Sm=-0.0044±0.0058 cpd/kg/keV and -0.0015±0.0063 cpd/kg/keV, respectively). They are in agreement with our estimated sensitivity for the accumulated exposure, which supports our projected goal of reaching a 3s sensitivity to the DAMA/LIBRA result in five years of data taking. [ABSTRACT FROM AUTHOR]