Rare Event Search Experiments Research Articles

Development of a Method to Measure Trace Levels of Uranium and Thorium in Scintillation Films

Abstract We have established a method to measure picograms-per-gram (pg g−1) levels of 238U and 232Th in scintillation films by combining the dry ashing method and inductively coupled plasma mass spectrometry. Trace amounts of 238U and 232Th were measured in up to 2 g of scintillation film with almost 100% collection efficiency. This paper details the experimental procedure, including the pretreatment of the samples and labware, detection limit of the method, collection efficiencies of 238U and 232Th, and measurement of 238U and 232Th in a polyethylene naphthalate film. This method is also applicable to 238U and 232Th measurements in other low-background organic materials for rare-event search experiments.

Muon-induced background in a next-generation dark matter experiment based on liquid xenon

Muon-induced neutrons can lead to potentially irreducible backgrounds in rare event search experiments. We have investigated the implication of laboratory depth on the muon-induced background in a future dark matter experiment capable of reaching the so-called neutrino floor. Our simulation study focused on a xenon-based detector with 70 tonnes of active mass, surrounded by additional veto systems plus a water shield. Two locations at the Boulby Underground Laboratory (UK) were analysed as examples: an experimental cavern in salt at a depth of 2850 m w. e. (similar to the location of the existing laboratory), and a deeper laboratory located in polyhalite rock at a depth of 3575 m w. e. Our results show that no cosmogenic background events are likely to survive standard analysis cuts for 10 years of operation at either location. The largest background component we identified comes from beta-delayed neutron emission from 17\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$^{17}$$\\end{document}N which is produced from 19\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$^{19}$$\\end{document}F in the fluoropolymer components of the experiment. Our results confirm that a dark matter search with sensitivity to the neutrino floor is viable (from the point of view of cosmogenic backgrounds) in underground laboratories at these levels of rock overburden. This work was conducted in 2019–21 in the context of a feasibility study to investigate the possibility of developing the Boulby Underground Laboratory to host a next-generation dark matter experiment; however, our findings are also relevant for other underground laboratories.

Radon concentration measurement with a high-sensitivity radon detector at the Yemilab

The radiation emitted from radon is a critical background in rare event search experiments conducted at the Yemi Underground Laboratory (Yemilab) in Jeongseon, Korea. A Radon Reduction System (RRS) has been developed and installed in Yemilab to reduce radon concentration in the air. The RRS primarily provides a purified air of 50 m3/h to the cleanroom used to assemble crystal detectors in the AMoRE, a neutrinoless double beta decay search experiment. RRS can reduce the radon level by a factor of 300, so a high-sensitivity radon detector was required. A highly sensitive radon detector was constructed using a 70 L chamber with a large PIN photodiode to measure radon concentration in the purified air. The radon detector shows an excellent resolution of 72 keV (FWHM) for 6.003 MeV alphas from 218Po decay and a sensitivity down to 23.8 ± 2.1 mBq/m3 with a boil-off N2 gas sample. The radon concentration level from the RRS measured by the radon detector was below 0.29 Bq/m3 with a reduction factor of about 300.

Prototyping a High Purity Germanium cryogenic veto system for a bolometric detection experiment

The use of High Purity Germanium detectors operated in ionization mode at cryogenic temperatures is investigated as an external background mitigation solution for bolometers used in rare-event search experiments. A simple experimental setup, running a 52-g Li2WO4 bolometer sandwiched in-between two 2-cm thick High Purity Germanium cylindrical detectors in a dry cryostat, shows promising rejection to environmental gammas and atmospheric muons backgrounds. The acquired data are used together with a Monte Carlo simulation of the setup to extract the main contributions to the external backgrounds expected in an above ground experiment, such as e.g. current and future experimental efforts targeting the detection of coherent elastic neutrino-nucleus scattering at reactor facilities. Based on all these results, a 4π coverage similar veto system achieving a O(10 keV) energy threshold is expected to achieve a ≳73% and a ≳92% rejection power for gamma-like and muon-like events, respectively.

Study of cosmogenic activation in 76Ge enriched germanium detectors during fabrication and transportation above ground

Rare event search experiments using germanium detectors are operated in underground laboratories to minimize the background induced by cosmic rays. However, the cosmogenic activation in germanium crystals on the ground during fabrication and transportation generates long half-life radionuclides and contributes a considerable background. We simulated the production rates of cosmogenic radionuclides in germanium and calculated the specific activities of cosmogenic radionuclides according to the scheduled fabrication and transportation processes of 76Ge enriched germanium detectors. The impact of cosmogenic background in germanium crystals for the next generation CDEX experiment was assessed with the scheduled exposure history above ground.

A plastic scintillation muon veto for sub-Kelvin temperatures

Rare-event search experiments located on-surface, such as short-baseline reactor neutrino experiments, are often limited by muon-induced background events. Highly efficient muon vetos are essential to reduce the detector background and to reach the sensitivity goals. We demonstrate the feasibility of deploying organic plastic scintillators at sub-Kelvin temperatures. For the NUCLEUS experiment, we developed a cryogenic muon veto equipped with wavelength shifting fibers and a silicon photo multiplier operating inside a dilution refrigerator. The achievable compactness of cryostat-internal integration is a key factor in keeping the muon rate to a minimum while maximizing coverage. The thermal and light output properties of a plastic scintillation detector were examined. We report first data on the thermal conductivity and heat capacity of the polystyrene-based scintillator UPS-923A over a wide range of temperatures extending below one Kelvin. The light output was measured down to 0.8 K and observed to increase by a factor of 1.61 ± 0.05 compared to 300 K. The development of an organic plastic scintillation muon veto operating in sub-Kelvin temperature environments opens new perspectives for rare-event searches with cryogenic detectors at sites lacking substantial overburden.

Rapid analysis of 226Ra in ultrapure gadolinium sulfate octahydrate

Abstract Numerous particle physics experiments utilize gadolinium (Gd), a rare earth element with the most significant neutron capture cross-section among all elements, to detect anti-neutrinos via inverse beta decays or to remove neutron-induced background events. For example, to load Gd into water Cherenkov detectors, Gd2(SO4)3 · 8H2O is dissolved and rare event search experiments are required to screen for radioactive impurities in Gd2(SO4)3 · 8H2O before dissolution. This study developed a new method to rapidly measure the radium-226 (226Ra) concentration in Gd2(SO4)3 · 8H2O. This method requires only 3 days to measure a batch of samples, as opposed to the usual method using high-purity germanium detectors, which takes approximately 20 days after arrival. The detection limit for the measurement of 226Ra concentration in Gd2(SO4)3 · 8H2O is 0.43 mBq/kg. This method has been already used for Gd2(SO4)3 · 8H2O screening at the Super-Kamiokande Gd (SK-Gd) project, and it can be applied to future experiments.

Reduction in radioactivity-induced backgrounds using a novel active veto detector for rare event search experiments

The results of a newly developed annular cryogenic phonon-mediated active veto detector are discussed which shows a significant reduction of radioactivity-induced backgrounds in rare event search experiments. The veto detector is made up of germanium weighing ∼500 g with an outer diameter of 76 mm and an inner diameter of 28 mm. The detector can host a 25 mm diameter germanium inner target detector of mass ∼10 g. A GEANT4 simulation with the active veto and inner target detector shows a gamma background reduction of 50 - 80% which is further improved (> 90%) with 4pi veto coverage. Experimental measurements with the detector assembly agree well with the simulation.

Li2100deplMoO4 Scintillating Bolometers for Rare-Event Search Experiments

We report on the development of scintillating bolometers based on lithium molybdate crystals that contain molybdenum that has depleted into the double-β active isotope 100Mo (Li2100deplMoO4). We used two Li2100deplMoO4 cubic samples, each of which consisted of 45-millimeter sides and had a mass of 0.28 kg; these samples were produced following the purification and crystallization protocols developed for double-β search experiments with 100Mo-enriched Li2MoO4 crystals. Bolometric Ge detectors were utilized to register the scintillation photons that were emitted by the Li2100deplMoO4 crystal scintillators. The measurements were performed in the CROSS cryogenic set-up at the Canfranc Underground Laboratory (Spain). We observed that the Li2100deplMoO4 scintillating bolometers were characterized by an excellent spectrometric performance (∼3–6 keV of FWHM at 0.24–2.6 MeV γs), moderate scintillation signal (∼0.3–0.6 keV/MeV scintillation-to-heat energy ratio, depending on the light collection conditions), and high radiopurity (228Th and 226Ra activities are below a few µBq/kg), which is comparable with the best reported results of low-temperature detectors that are based on Li2MoO4 using natural or 100Mo-enriched molybdenum content. The prospects of Li2100deplMoO4 bolometers for use in rare-event search experiments are briefly discussed.

Pulse shape simulation and discrimination using machine learning techniques

An essential metric for the quality of a particle-identification experiment is its statistical power to discriminate between signal and background. Pulse shape discrimination (PSD) is a basic method for this purpose in many nuclear, high-energy and rare-event search experiments where scintillation detectors are used. Conventional techniques exploit the difference between decay-times of the pulses from signal and background events or pulse signals caused by different types of radiation quanta to achieve good discrimination. However, such techniques are efficient only when the total light-emission is sufficient to get a proper pulse profile. This is only possible when adequate amount of energy is deposited from recoil of the electrons or the nuclei of the scintillator materials caused by the incident particle on the detector. But, rare-event search experiments like direct search for dark matter do not always satisfy these conditions. Hence, it becomes imperative to have a method that can deliver a very efficient discrimination in these scenarios. Neural network based machine-learning algorithms have been used for classification problems in many areas of physics especially in high-energy experiments and have given better results compared to conventional techniques. We present the results of our investigations of two network based methods viz. Dense Neural Network and Recurrent Neural Network, for pulse shape discrimination and compare the same with conventional methods.

Triggerless DAQ system for a light based detector

The utilization of various types of photon detectors, generally referred to as light based detectors, is widespread in many fields of high energy physics, nuclear physics, astrophysics and medical imaging to measure scintillation photons or Cherenkov light. Both the traditional photomultiplier tubes (PMTs), still the most commonly used, and the silicon photomultipliers (SiPMs) in rapid development in the last years show good performance. The sensitivity down to single photoelectron (PE), the wide dynamic range and the fast timing characteristics can be exploited thanks to a careful optimization of the readout electronics, trigger and DAQ.In applications where a high efficiency is required at low energy threshold (as for rare-event search experiments), in a typical trigger-based DAQ architecture, lowering the number of coincident signals for the event selection is mandatory with the consequent heavy increase of data rate that could not be sustained by the DAQ chain.We have developed a triggerless data collection scheme for a light based detector that relies on a new generation of waveform digitizer board, developed by CAEN and able to manage an independent data acquisition on each channel. This paper describes the implementation and the performance of the triggerless DAQ system.

A gaseous time projection chamber with Micromegas readout for low-radioactive material screening

PurposeLow-radioactive material screening is becoming essential for rare event search experiments, such as neutrinoless double beta decay and dark matter searches in underground laboratories. A gaseous time projection chamber (TPC) can be used for such purposes with large active areas and high efficiency.MethodsA gaseous TPC with a Micromegas readout plane of approximately \(20 \times 20\) \(\hbox {cm}^2\) is successfully constructed for surface alpha contamination measurements.ResultsWe have characterized the energy resolution, gain stability, and tracking capability with calibration sources.ConclusionWith the unique track-related background suppression cuts of the gaseous TPC, we have established that the alpha background rate of the TPC is (\(0.13\pm 0.03\))\(\times 10^{-6}\) Bq/\(\hbox {cm}^2\), comparable to the leading commercial solutions.

Radon dynamic adsorption coefficients of two activated charcoals at different temperatures in nitrogen environment

Profiting from their intrinsic low radioactivity, CarboACT and Saratech, the two kinds of activated charcoal, are commonly adopted in radon removal and radon enrichment process related to modern rare-event search experiments. For supporting CDEX collaboration on radon issue, a flow-through experimental system was established and radon dynamic adsorption coefficients (kα) of CarboACT and Saratech were systematically measured in N2 condition within the temperature range of −85 °C∼200 °C. The experimental results show that radon dynamic adsorption coefficients of CarboACT and Saratech increase exponentially with the decrease of temperature, and different elution curves of them were observed. Extrapolated to 77K, the kα-values of CarboACT and Saratech at liquid nitrogen temperature could be firstly estimated at 1.2 × 1013 (L/g) and 9.5 × 1011 (L/g), respectively. The overall image of the adsorption capacities of these two activated charcoals is valuable for rare-event search studies related to radon issue.

A thermal neutron detection system based on boron-coated straw detector with prompt gamma coincidence

Accurate measurement of the neutron flux is of crucial importance for rare event search experiments in underground laboratories. The intrinsic radioactive background of the detector becomes the limiting factor for the detection of extremely low neutron flux. In this study, a thermal neutron detection system aimed at low flux measurement was developed based on boron-coated straw (BCS) neutron detectors. The neutron events can be distinguished from the detector background by coincidence measurement of neutron and the prompt gamma ray. With state-of-the-art BCS neutron detectors and NaI(Tl) gamma detector, a system sensitivity of ∼120 cps/nv was achieved, comparable to that of the commonly used 3He counters. Based on the selected coincidence criteria, the background events were rejected to 0.1% with 45.3% of the neutrons preserved. The background accidental coincidence count rate of the system was measured as 2.3×10-5 cps, corresponding to a lower limit of measurable thermal neutron flux of 1.9×10-7 n/cm2/s. The performance of the system can be further improved by using other gamma scintillator with lower neutron absorption (such as BGO) and adding extra shielding for ambient gamma rays.

Energy calibration of EJ-301 scintillation detector using unfolding methods for fast neutron measurement

We simulated a 2′′×2′′ EJ-301 organic liquid scintillation detector in the GEANT4 framework to obtain its response to standard gamma and neutron sources. The resolution scale and Birks’ constant for the EJ-301 scintillator have been estimated by comparing the simulation with experimental data. We calibrated the detector for fast neutron measurements with the help of an Am–Be source using two unfolding methods: RooUnfold and Gravel unfold. The neutron spectra obtained from both unfolding methods are similar to the Am–Be ISO (International Organization for Standardization) spectrum. The calibrated detector can be used in an underground site to understand the neutron background in rare event search experiments.

The upgraded low-background germanium counting facility Gator for high-sensitivity γ-ray spectrometry

We describe the upgrade and performance of the high-purity germanium counting facility Gator, which is dedicated to low-background γ-ray spectrometry. Gator is operated at the Gran Sasso Underground Laboratory in Italy, at an average depth of 3600 meter water equivalent, and employed for material screening and selection in ultra-low background, rare-event search experiments in astroparticle physics. The detector is equipped with a passive shield made of layers of copper, lead and polyethylene, and the sample cavity is purged with gaseous nitrogen maintained at positive pressure for radon suppression. After upgrading its enclosure, the background rate is (82.0 ± 0.7) counts/(kg·day) in the energy region 100 keV to 2700 keV, a 20% reduction compared to the previously reported rate. We show the stability of various operation parameters as a function of time. We also summarize the sample analysis procedure, and demonstrate Gator's sensitivity by examining one material sample, a candidate photosensor for the DARWIN experiment.

A novel active veto prototype detector with an inner target for improved rare event searches

We report the fabrication and performance of an annular, cryogenic, phonon-mediated veto detector that can host an inner target detector, allowing substantial reduction in radiogenic backgrounds for rare event search experiments. A germanium veto detector of mass ∼500 g with an outer diameter of 76 mm and an inner diameter of 28 mm was produced inside of which was mounted a 25 mm diameter germanium inner target detector of mass ∼10 g. The detector was designed using inputs from a GEANT4 based simulation, where it was modeled to be sandwiched between two germanium detectors. The simulation showed that the background rates (dominated by gamma interactions) could be reduced by > 90%, and that such an arrangement is sufficient for aggressive background reduction needed for neutrino and dark matter search experiments. Operating at mK temperatures at the experimental site, the veto detector prototype achieved a baseline resolution of 1.24 ± 0.02 keV while hosting a functional inner target detector. The baseline resolution of the inner target detector was 147 ± 2 eV. The experimental results of an identical detector arrangement are in excellent agreement with the simulation.

Measurements of gamma ray, cosmic muon and residual neutron background fluxes for rare event search experiments at an underground laboratory

Ambient radiation background contributed by the penetrating cosmic ray particles and the radionuclides present in the rock materials have been measured at an underground laboratory at Jaduguda, Jharkhand, India, located inside a mine at 555 m depth. The laboratory is being set up to explore rare event search processes, such as direct dark matter search, neutrinoless double beta decay, axion search, supernova neutrino detection, etc., that require specific knowledge of the nature and extent of the radiation environment in order to assess the sensitivity reach and also to plan for its reduction for the targeted experiment. The gamma ray background, which is mostly contributed by the primordial radionuclides and their decay chain products, have been measured inside the laboratory and found to be dominated by rock radioactivity for Eγ≲3MeV. Shielding of these residual gamma rays for the experiment was also evaluated. The cosmic muon flux, measured inside the laboratory using large area plastic scintillator telescope, was found to be: (2.257±0.261±0.042)×10−7cm−2s−1, which agrees reasonably well with simulation results. The neutron background flux has been measured for the radiogenic neutrons and found to be: (1.61±0.03)×10−4cm−2s−1 for no threshold cut. Detailed GEANT4 simulation for the radiogenic neutrons and the cosmogenic neutrons have been carried out. Effects of multiple scattering of both the types of neutrons within the surrounding rock and the cavern walls were studied and the results for the radiogenic neutrons are found to be in reasonable agreement with experimental results. Neutron fluxes contributed by those neutrons of cosmogenic origin have been reported as function of the energy threshold.

Identification of neutron sources and background levels in the polyethylene room of the China Jinping Underground Laboratory

The neutron backgrounds induced by supplementary experimental materials can result in contaminations in rare event search experiments. To address this, we present the neutron background levels arising from ambient materials in the polyethylene room of the China Jinping Underground Laboratory; particularly, we compare simulated spectra with measured neutron spectra unfolded using a genetic algorithm. The genetic algorithm optimizes the continuity of the energy spectra and obtains a reasonable spectral result. A good agreement between the unfolded and simulated spectra is achieved. Moreover, estimated neutron background levels of representative ambient materials such as polyethylene, aluminum, and lead are obtained using an exposure time of 511.27 days via a 28 liter 0.5%-gadolinium-doped liquid scintillator detector. The identification of rare neutron sources can aid in background reduction in next-generation large-scale rare event experiments.

Measurements and Simulation of background radiation for rare event search experiments at an underground laboratory

Experimental measurements and simulation of various radiation backgrounds at a new underground site have been presented. The experimental cosmic muon flux, measured at this site with the help of a muon telescope set-up, agree well with simulation results after accounting for the geometric effect of the detector assembly. The gamma ray background inside the laboratory space was measured to assess and optimize the required thickness of Lead for shielding. Neutrons, produced in the surrounding rock due to radioactive decay, followed by nuclear reactions with the rock constituents, and the faster neutrons, produced through interaction of the charged cosmic particles, propagate through the rock material to reach inside the laboratory. The neutron background flux was measured and GEANT4 simulations were carried out. The results were found to be in reasonable agreement with measurements.