Rare-event Experiments Research Articles

A background simulation method for cosmogenic nuclides inside HPGe detectors for rare event experiments

Cosmogenic nuclides inside germanium detectors contribute background noise spectra quite different from ordinary external sources. We propose and discuss a nuclide decay and level transition model based on graph theory to understand the background contribution of the decay of cosmogenic nuclides inside a germanium crystal. In this work, not only the level transition process, but the detector response time was also taken into consideration to decide whether or not to apply coincidence summing-up. We simulated the background spectrum of the internal cosmogenic nuclides in a germanium detector, and found some unique phenomena caused by the coincidence summing-up effect in the simulated spectrum. Thus, the background spectrum of each cosmogenic nuclide can be quantitatively obtained.

The characteristics of a low background germanium gamma ray spectrometer at China JinPing underground laboratory

A low background germanium gamma ray spectrometer, GeTHU, has been installed at China JinPing Underground Laboratory (CJPL). The integral background count rate of the spectrometer was 0.629cpm between 40 and 2700keV, the origins of which were studied by Monte Carlo simulation. Detection limits and efficiencies were calculated for selected gamma peaks. Some samples of rare event experiments were measured and 137Cs contamination was found in boric acid. GeTHU will be mainly used to measure environmental samples and screen materials in dark matter and double beta decay experiments.

An experimental study of antireflective coatings in Ge light detectors for scintillating bolometers

Luminescent bolometers are double-readout devices able to measure simultaneously the phonon and the light yields after a particle interaction in the detector. This operation allows in some cases to tag the type of the interacting quantum, crucial issue for background control in rare event experiments such as the search for neutrinoless double beta decay and for interactions of particle dark matter candidates. The light detectors used in the LUCIFER and LUMINEU searches (projects aiming at the study of the double beta interesting candidates 82Se and 100Mo using ZnSe and ZnMoO4 scintillating bolometers) consist of hyper-pure Ge thin slabs equipped with NTD thermistors. A substantial sensitivity improvement of the Ge light detectors can be obtained applying a proper anti-reflective coatings on the Ge side exposed to the luminescent bolometer. The present paper deals with the investigation of this aspect, proving and quantifying the positive effect of a SiO2 and a SiO coating and setting the experimental bases for future tests of other coating materials. The results confirm that an appropriate coating procedure helps in improving the sensitivity of bolometric light detectors by an important factor (in the range 20% – 35%) and needs to be included in the recipe for the development of an optimized radio-pure scintillating bolometer.

Assessment of material radiopurity for Rare Event experiments using Micromegas

Micromesh gas amplification structures (Micromegas) can beused as readout of Time Projection Chambers in the field of RareEvent searches dealing with dark matter, double beta decay or solaraxions. The topological information of events offered by thesegaseous detectors is a very powerful tool for signal identificationand background rejection. However, in this kind of experiments theradiopurity of the detector components and surrounding materialsmust be thoroughly controlled in addition in order to keep theexperimental background as low as possible.A screening program based mainly on gamma-ray spectrometry using anultra-low background HPGe detector in the Canfranc UndergroundLaboratory is being developed for several years, with the aim tomeasure the activity levels of materials used in the Micromegasplanes and also in other components involved in a plausibleexperimental set-up: gas vessel, field cage, electronic boards,calibration system or shielding.The techniques and equipment used in these measurements will bedescribed and the main results will be presented and discussed. Inparticular, first results for the activity of Micromegas readouts ofthe microbulk type produced at CERN indicate that they are alreadycomparable to the cleanest readout systems in low backgroundexperiments and it should be possible to further improve theselevels after dedicated development.

Measurement of cosmic ray flux in the China JinPing underground laboratory

The China JinPing underground Laboratory (CJPL) is the deepest underground laboratory running in the world at present. In such a deep underground laboratory, the cosmic ray flux is a very important and necessary parameter for rare-event experiments. A plastic scintillator telescope system has been set up to measure the cosmic ray flux. The performance of the telescope system has been studied using the cosmic rays on the ground laboratory near the CJPL. Based on the underground experimental data taken from November 2010 to December 2011 in the CJPL, which has an effective live time of 171 days, the cosmic ray muon flux in the CJPL is measured to be (2.0±0.4)×10−10/(cm2·s). The ultra-low cosmic ray background guarantees an ideal environment for dark matter experiments at the CJPL.

Two-phase Cryogenic Avalanche Detectors with THGEM and hybrid THGEM/GEM multipliers operated in Ar and Ar+N2

Two-phase Cryogenic Avalanche Detectors (CRADs) with GEM and THGEM multipliers have become an emerging potential technique for charge recording in rare-event experiments. In this work we present the performance of two-phase CRADs operated in Ar and Ar+N2. Detectors with sensitive area of 10 × 10 cm2, reaching a litre-scale active volume, yielded gains of the order of 1000 with a double-THGEM multiplier. Higher gains, of about 5000, have been attained in two-phase Ar CRADs with a hybrid triple-stage multiplier, comprising of a double-THGEM followed by a GEM. The performance of two-phase CRADs in Ar doped with N2 (0.1–0.6%) yielded faster signals and similar gains compared to the operation in two-phase Ar. The applicability to rare-event experiments is discussed.

Advances in Cryogenic Avalanche Detectors

Cryogenic Avalanche Detectors (CRADs) are referred to as a new class of noble-gas detectors operated at cryogenic temperatures with electron avalanching performed directly in the detection medium, the latter being in gaseous, liquid or two-phase (liquid-gas) state. Electron avalanching is provided by Micro-Pattern Gas Detector (MPGD) multipliers, in particular GEMs and THGEMs, operated at cryogenic temperatures in dense noble gases. The final goal for this kind of detectors is the development of large-volume detectors of ultimate sensitivity for rare-event experiments and medical applications, such as coherent neutrino-nucleus scattering, direct dark matter search, astrophysical (solar and supernova) neutrino detection experiments and Positron Emission Tomography technique. This review is the first attempt to summarize the results on CRAD performances obtained by different groups. A brief overview of the available CRAD concepts is also given and the most remarkable CRAD physics effects are discussed.

CUORE: The Challenge of a 988 Bolometer Array

The design of CUORE (an array of 988 TeO2 bolometers working at ∼10 mK) was based on the information acquired with a 40.7 kg prototype, named Cuoricino, that allowed to obtain the present best limit on the half-life of 0νDBD of 130Te. In addition to providing important information on the different sources of spurious counts that limit the sensitivity for rare event experiments, Cuoricino allowed to study in detail the underground operation of a large array of bolometers (detector stability, noise sources and their reduction, off-line amplitude correction, …).

A simulation toolkit for electroluminescence assessment in rare event experiments

A good understanding of electroluminescence is a prerequisite when optimising double-phase noble gas detectors for Dark Matter searches and high-pressure xenon TPCs for neutrinoless double beta decay detection.A simulation toolkit for calculating the emission of light through electron impact on neon, argon, krypton and xenon has been developed using the Magboltz and Garfield programs. Calculated excitation and electroluminescence efficiencies, electroluminescence yield and associated statistical fluctuations are presented as a function of electric field. Good agreement with experiment and with Monte Carlo simulations has been obtained.

Photoelectron extraction efficiency from cesium iodide photocathodes in a pressurized atmosphere of argon and xenon up to 10 bar

Photoelectron extraction efficiency is a key parameter in gaseous detectors design requiring CsI photocathodes, particularly in pressurized applications for rare event experiments. Within this work we report on our measurements of the photoelectron extraction efficiency from a CsI photocathode into argon and xenon up to 10 atm. It is demonstrated that the photoelectron extraction efficiency is independent from pressure when scaling the electric field accordingly. This measurement provides experimental validation to the Monte Carlo predictions.

A Monte Carlo study for the shielding of γ backgrounds induced by radionuclides for CDEX

The CDEX (China Dark matter EXperiment) Collaboration will carry out a direct search for WIMPs (Weakly Interacting Massive Particles) using an Ultra-Low Energy Threshold High Purity Germanium (ULE-HPGe) detector at the CJPL (China JinPing deep underground Laboratory). A complex shielding system was designed to reduce backgrounds and a detailed GEANT4 Monte Carlo simulation was performed to study the achievable reduction of γ rays induced by radionuclides and neutron backgrounds by D(γ,n)p reaction. Furthermore, the upper level of allowed radiopurity of shielding materials was estimated under the constraint of the expected goal. Compared with the radiopurity reported by other low-background rare-event experiments, it indicates that the shielding used in the CDEX can be made out of materials with obtainable radiopurity.

Composite macro-bolometers for the rejection of surface radioactive background in rare-event experiments

Experiments searching for rare events, such as neutrinoless double beta decay and interactions of dark matter candidates, require extremely low levels of background. When these experiments are performed using macro-bolometers, radioactive contamination near the surfaces is of particular concern. For a bolometric neutrinoless double beta decay experiment, it can cause counts in the spectral region where the signal is expected, while for a dark matter experiment which exploits ionization signals for particle identification, it originates an incomplete charge collection simulating a nuclear recoil. In order to control the effects of surface contamination, we developed a novel technique that uses composite macro-bolometers to identify energy depositions that occur close to the surfaces of materials immediately surrounding the detector. The composite macro-bolometer proposed and studied here consists of a main energy absorber that is thermally coupled to and entirely surrounded by thin absorbers that act as active shields. Surface energy depositions can be rejected by the analysis of simultaneous signals in the main absorber and the shields. In this paper, we describe a full thermal model and experimental results for three prototype detectors. The detectors consist of Ge, Si, or TeO 2 thin absorbers as active shields, all with TeO 2 crystals as main absorbers. In all cases, the surface event rejection capability is clearly demonstrated. In addition, simulations and preliminary results show that it is possible to detect energy depositions that occurred on the shields without separate readout channels for them. The energy depositions in the shields are distinguished from those in the main absorber through pulse shape discrimination. This simplification makes this technique a viable method for the rejection of surface energy depositions in next-generation bolometric double beta decay searches, such as possible extensions or upgrades of the CUORE experiment.

Direct observation of avalanche scintillations in a THGEM-based two-phase Ar avalanche detector using Geiger-mode APD

A novel concept of optical signal recording in cryogenic two-phase avalanche detectors, with Geiger-mode Avalanche Photodiodes (G-APD) measuring avalanche-scintillation photons in a thick Gas Electron Multiplier (THGEM), has been studied in view of its potential applications in rare-event experiments. The effective detection of avalanche scintillations in THGEM holes has been demonstrated in two-phase Ar with a bare G-APD without wavelength shifter, i.e. insensitive to VUV emission of Ar. At gas-avalanche gain of 400 and under ±70° viewing-angle, the G-APD yielded 640 photoelectrons (pe) per 60 keV X-ray converted in liquid Ar; this corresponds to 0.7 pe per initial (prior to multiplication) electron. The avalanche-scintillation light yield measured by the G-APD was about 0.7 pe per avalanche electron, extrapolated to 4π acceptance. The avalanche scintillations observed occurred presumably in the near infrared (NIR) where G-APDs may have high sensitivity. The measured scintillation yield is similar to that observed by others in the VUV. Other related topics discussed in this work are the G-APD's single-pixel and quenching resistor characteristics at cryogenic temperatures.

Geiger mode APD performance in a cryogenic two-phase Ar avalanche detector based on THGEMs

Characteristic properties of a Geiger Mode APD (G-APD) in a THGEM-based cryogenic two-phase Ar avalanche detector were studied in view of potential applications in rare-event experiments. G-APD signal amplitude and noise characteristics at cryogenic temperatures turned out to be superior to those at room temperature. The effective detection of avalanche scintillations from THGEM-multiplier holes in two-phase Ar has been demonstrated using a G-APD without wavelength shifter. At an avalanche gain of 60, the avalanche scintillation yield measured by the G-APD was as high as 0.9 photoelectrons per avalanche electron, extrapolated to 4 π acceptance.

Thick GEM versus thin GEM in two-phase argon avalanche detectors

The performance of thick GEMs (THGEMs) was compared to that of thin GEMs in two-phase Ar avalanche detectors, in view of their potential application in coherent neutrino-nucleus scattering, dark-matter search and in other rare-event experiments. The detectors comprised a 1 cm thick liquid-Ar layer followed by either a double-THGEM or a triple-GEM multiplier, operated in the saturated vapor above the liquid phase. Three types of THGEMs were studied: those made of G10 and Kevlar and that with resistive electrodes (RETHGEM). Only the G10-made THGEM showed a stable performance in two-phase Ar with gains reaching 3000. Successful operation of two-phase Ar avalanche detectors with either thin- or thick-GEM multipliers was demonstrated at low detection thresholds, of 4 and 20 primary electrons respectively. Compared to the triple-GEM the double-THGEM multiplier yielded slower anode signals; this allowed applying a pulse-shape analysis to effectively reject noise signals. Noise rates of both multipliers were evaluated in two-phase Ar; with detection thresholds of 20 electrons and applying pulse-shape analysis noise levels as low as 0.007 Hz per 1 cm2 of active area were reached.

Measurement of low temperature specific heat of crystalline TeO2 for the optimization of bolometric detectors

The optimization of bolometric detectors, like those that will be developed for the rare event experiment CUORE, requires a complete knowledge of the detector's thermal parameters. Since the CUORE detecting elements will consist of TeO2 crystals, we have measured the specific heat of this material down to 60 mK with the thermal relaxation method. Previous available data were taken at temperatures higher than 0.6 K. Our results are clearly consistent with a lattice dominated specific heat. The Debye temperature, evaluated to be (232±7) K, is in excellent agreement with the elastic constant values measured by other authors. The knowledge of the Debye temperature allows a simple prediction of the pulse amplitude of presently working bolometers.

An active background discrimination technique using a multiple detector event by event recording system

For rare event experiments (environmental, solar neutrino, ββ-decay and dark matter) a careful selection of the construction materials of the detectors with respect to low radioactive contamination is fundamental. Non-destructive low-level γ-ray spectrometry serves this purpose best. To improve the sensitivity of a germanium detector, background reduction is a prerequisite. This paper describes an active background discrimination technique, which in principle is a coincidence method. Instead of an active veto, an event-by-event recording technique is used to improve the signature of each event. Two detectors, one low level HPGe-detector in a lead shield and a large area plastic scintillation detector (above the Ge-detector and outside the lead shield) are operated simultaneously (the system allows up to 12 ADC inputs). For each event the detector identification number, the measured pulse-height and the time of occurrence of the event are stored. The coincidence or anti-coincidence conditions can be chosen by software after the measurement, the conditions can be optimised and altered without losing information. The technique allows to distinguish between fast muon induced events, radioimpurity events and, under special detector conditions, Compton events. These background events can then be subtracted, leading to a smaller statistical uncertainty of the net signal from the sample. From the recorded data the muon flux can be extracted as well. Experiments at ground level and in a depth of 500 m water equivalent are described.