Low Background Measurements Research Articles

Direct measurements of cosmic – Ray iron and nickel with CALET on the International Space Station

Iron and nickel cosmic ray nuclei play a key role in the understanding of the acceleration and propagation mechanisms of charged particles in our Galaxy. In fact, iron and nickel are the most abundant nuclei among the heavy elements and provide favorable conditions for a low background measurement thanks to the negligible contamination from spallation of higher mass elements. CALET, operating on the ISS since 2015, has excellent capabilities of charge discrimination up to nickel and can measure the energy of cosmic ray nuclei thanks to a lead tungstate calorimeter providing a direct and precise measurement of heavy charged nuclei spectra. In this contribution, a direct measurement of iron and nickel nuclei spectra in the energy range from 10 GeV/n to 2 TeV/n and from 8.8 GeV/n to 240 GeV/n, respectively is presented. More than five years of data collected by CALET were used. A detailed study of systematic uncertainties is also illustrated. The measured spectra are compared with the ones measured by other experiments and are compatible with a single power law fit in the energy region from 50 GeV/n to 2 TeV/n and from 20 GeV/n to 240 GeV/n for iron and nickel respectively. Also, the ratio between nickel and iron spectra is reported.

Spectroscopic performance evaluation and modeling of a low background HPGe detector using GEANT4

Low background gamma spectrometry employing HPGe detectors is a sensitive technique for measuring low-level radioactivity in environmental applications, material screening, and for rare decay searches. This work presents spectroscopic performance evaluation and modeling of a low background measurement setup developed at IIT Ropar in Punjab, India, to measure trace natural radioactive elements, with a particular interest in studying low-level radioactivity in soil and/or rock samples to generate specific inputs for low background experiments. The performance test and characterization of a low background cryocooled HPGe detector with relative efficiency of ∼33% have been carried out. An effective detector model has been developed using GEANT4 Monte Carlo simulation to determine the response of the detector over an energy range of 80.9–1408 keV and compared with the experimental performance of the detector. The response of the detector obtained using Monte Carlo simulations agrees reasonably well within 93% level of confidence, indicating only 7% deviation in the comparison. The present setup offers improved detection limits of primordial radionuclides (U/Th and K) to measure radioactive contamination in environmental matrices, which has been used elsewhere (Thakur, 2023).

Characterization of a 4παβ(LS)-γ(HPGe) prototype system for low-background measurements

A ground-level prototype system for low-background measurements was developed and tested. The system consists of a high-purity germanium (HPGe) detector used for detecting γ rays and coupled to a liquid scintillator (LS) used for detecting α and β particles. Both detectors are surrounded by shielding materials and anti-cosmic detectors (“veto”) used to suppress background events. The energy and timestamp of detected α, β and γ emissions are recorded event-by-event and analyzed offline. By requiring timing coincidence between the HPGe and LS detectors, background events originating from outside the volume of the measured sample can be effectively rejected. The system performance was evaluated using liquid samples containing known activities of an α emitter (241Am) or a β emitter (60Co) whose decays are accompanied by γ rays. The LS detector was found to provide a solid angle of almost 4π for α and β particles. Compared to the traditional γ-singles mode, operating the system in coincidence mode (i.e., α-γ or β-γ) reduced the background counts by a factor of ∼100. Consequently, the minimal detectable activity for 241Am and 60Co was improved by a factor of 9, being 4 mBq and 1 mBq for an 11-d measurement, respectively. Furthermore, by applying a spectrometric cut in the LS spectrum that corresponds to α emission from 241Am, a background reduction factor of ∼2400 (compared to γ-singles mode) was achieved. Beyond low-background measurements, this prototype exhibits additional compelling features, such as the ability to focus on certain decay channels and study their properties. This concept for a measurement system may be of interest to laboratories that monitor environmental radioactivity, studies involving environmental measurements and/or trace-level radioactivity.

A differential light addressable potentiometric sensor using solid-state reference

The differential light addressable potentiometric sensor (LAPS) can provide low-background measurement for chemicals. For the conventional differential LAPS system, the acquisition of reference signal requires liquid participation. In this study, a differential LAPS with solid-state reference (SSR) is presented for pH measurement, where SSR based on the structure of metal-insulator-semiconductor is used as reusable reference signal source. In order to facilitate the differential signal matching and realize the effect of large reduction of background signal, an intelligent method to match the amplitude and phase between LAPS output and SSR output by controlling the illumination excitation of SSR is proposed. Effect of SSR as reference signal source for differential LAPS is studied, which is including pH sensing and chemical image performance. As an auxiliary liquid-free chip, SSR can assist the differential LAPS to realize low-background pH measurement conveniently and flexibly.

Background shielding by dense samples in low-level gamma spectrometry

In low activity gamma spectrometric measurements of large, dense samples, the bulk sample material shields the HPGe crystal from external background sources. If not accounted for in studies that utilise background-subtraction methods, this effect may result in systematic errors in the sample activity and detection limit estimation. We introduce a Monte Carlo based method to minimise the impact of this effect on sample gamma spectra. It is validated using simulated detector backgrounds and applied to a measurement of low-activity Gd2SO43⋅8H2O. One main prerequisite for the correct application of this method is to know in advance the nuclides which contribute to the detector background spectrum and their spatial distribution. With a thorough understanding of the detector backgrounds, the method improves the accuracy of sensitive low-background measurements of low-activity samples. Even without knowing the background sources and their distribution, conservative results may still be presented that account for the potential systematic errors introduced by this background shielding effect.

PICOLON dark matter search project

PICOLON (Pure Inorganic Crystal Observatory for LOw-energy Neutr(al)ino) aims to search for cosmic dark matter by high purity NaI(Tl) scintillator. We developed extremely pure NaI(Tl) crystal by hybrid purification method. The recent result of 210Pb in our NaI(Tl) is less than 5.7 μBq/kg. We will report the test experiment in the low-background measurement at Kamioka Underground Laboratory. The sensitivity for annual modulating signals and finding dark matter particles will be discussed.

Low background measurement in CANDLES-III for studying the neutrinoless double beta decay of Ca48

We developed a CANDLES-III system to study the neutrino-less double beta (0$\nu\beta\beta$) decay of $^{48}$Ca. The proposed system employs 96 CaF$_{2}$ scintillation crystals (305 kg) with natural Ca ($^{\rm nat.}$Ca) isotope which corresponds 350\,g of $^{48}$Ca. External backgrounds were rejected using a 4$\pi$ active shield of a liquid scintillator surrounding the CaF$_2$ crystals. The internal backgrounds caused by the radioactive impurities within the CaF$_2$ crystals can be reduced effectively through analysis of the signal pulse shape. We analyzed the data obtained in the Kamioka underground for a live-time of 130.4\,days to evaluate the feasibility of the low background measurement with the CANDLES-III detector. Using Monte Carlo simulations, we estimated the background rate from the radioactive impurities in the CaF$_{2}$ crystals and the rate of high energy $\gamma$-rays caused by the (n, $\gamma$) reactions induced by environmental neutrons. The expected background rate was in a good agreement with the measured rate, i.e., approximately 10$^{-3}$ events/keV/yr/(kg of $^{\rm nat.}$Ca), in the 0$\nu\beta\beta$ window. In conclusion, the background candidates were estimated properly by comparing the measured energy spectrum with the background simulations. With this measurement method, we performed the first search for 0$\nu\beta\beta$ decay in a low background condition using a detector with a Ca isotope, in which the Ca present was not enriched, in a scale of hundreds of kg. The $^{48}$Ca isotope has a high potential for use in 0$\nu\beta\beta$ decay search, and is expected to be useful for the development of a next-generation detector for highly sensitive measurements.

Underground Measurement of 6Li(p,γ)7Be and 6Li(p,3He)4He Performed at LUNA

Proton-induced reactions on 6Li play an important role in nuclear astrophysics studies in relation to primordial lithium abundances. Whilst big bang nucleosynthesis theory excludes the existence of primordial 6Li, the 6Li/7Li abundance ratio observed in pre-main sequence stars is ≃ 0.5. The 6Li(p,3He)4He and 6Li(p,γ)7Be reactions are the main processes that contribute to 6Li destruction in stars. Both reactions were recently studied at LUNA via proton bombardment of 6Li-enriched targets, with complementary target composition studies performed at HZDR. Improvements on the precision of the low-energy S-factor values are expected from this study. Notably, the low-background measurement at LUNA will assist the search for a recently claimed 6Li(p,γ)7Be low energy resonance at E r ≃ 195 keV. I present the LUNA experimental setup and preliminary results of the ongoing analysis.

Low Background Radiation Detection Techniques and Mitigation of Radioactive Backgrounds

The study of rare fundamental physics phenomena, such as double-beta decay, rare nuclear decays and dark matter, requires very low levels of background radiation in order to observe a signal. To achieve the required background levels, experiments are located deep underground as these facilities provide significant rock overburden and commensurate reduction in the cosmic ray flux and cosmic ray-spallation induced products. An overview of the sources of these backgrounds will be presented. Taking advantage of the deep underground laboratory spaces, there have been a growing number of underground measurements in other fields, including environmental monitoring, benchmarking of other physical techniques, Life Science studies in low background environments, and material selection. The exceptional sensitivity and high resolution of high-purity germanium detectors allows for very sensitive measurements using gamma-ray spectrometry. Their use has been increasing as they allow for non-destructive measurements of experiment components, which can be directly used if they meet specified background requirements. This paper will discuss the current most sensitive ultra-low background germanium detectors in operation and explain how to achieve the best level of background reduction to attain the best sensitivities. In addition, an overview of several complementary low background measurement methods will be discussed. A proposed program to cross calibrate germanium detectors at several laboratories will be described and a searchable database used to store radioactivity measurements of experimental materials will be introduced.

NEW OPPORTUNITIES OF NUCLEAR PHYSICS RESEARCH OF THE OBJECTS OF ENVIRONMENT IN THE COURT EXPERTISE ACTIVITY

In this paper are studied the possibility of using nuclear techniques, in particular, low-background measurements of the radionuclide content of natural (isotopes of natural series U / Th, 40K) and man-made origin for the identification of natural objects as non-nuclear materials. In particular, this can be the samples of soil, river sludge or rock, presented as evidential objects for court expertise. Тhe proposed technique explores new data on the nature of the nucleosynthesis of the Earth’s crust, the distribution and conditions of radioactive equilibrium of the daughter’s isotopes of the U / Th series. Recent studies show the ability to use lowbackground research data to establish standards for the content / ratio of the clarks of chemical elements U / Th / K, which may be sufficient identification signs of a given territory like fingerprints for a person. This is especially relating to such objects of the environment as samples of soils, rocks, etc. that are genetically related to a given territory. Considering these environmental objects as a subject of court examination, we can talk about their important role as an evidence-based argument, which in many cases allows us to identify the origin of the “criminal element”. Therefore, the methods of identification of environmental objects and their binding to a certain territory are very important. The paper presents the first examples of such studies on samples of soil, river sludge and rock, taken from protected areas of Transcarpathia, the influence of manmade factors on which is minimal. It justifies the choice of the minimum number of radionuclide tags of a given territory, the method of sampling to obtain the radiation characteristics of a given area, and the statistical stability of the obtained data on the content of the studied radionuclides for different series of obtained samples is investigated. Systematic studies of this kind make it possible to carry out radiation mapping of territories as the development of the ideas of Vernadsky about the importance of taking into account ground-based radiation and to get a more accurate binding of the objects under study to a specific region. The possibility of using the nuclear dating technique in forensic analysis according to low-background measurements about the content of natural U / Th series in environmental objects is shown. The results of nuclear dating of samples of the Carpathian rocks using the uranium method are presented. Summing up, it can be said that nuclear techniques can be used not only to analyze radioactive materials, but also to complement effectively traditional physicochemical studies of the non-nuclear materials as forensic objects.

Research and development toward KamLAND2-Zen

The KamLAND2-Zen experiment is a future plan of KamLAND-Zen, which searches for neutrino-less double-beta decay (0vββ) of 136Xe, with low background measurements. The goal of this experiment is to reach a sensitivity of 〈mββ 〉 < 20 meV which covers the inverted hierarchy region. It can be achieved by increasing the light yield with light collecting mirrors, new brighter liquid scintillator and higher quantum efficiency photomultiplier tubes (PMTs) and scintillation vessel. In this paper, we describe the R&Ds for KamLAND2-Zen experiment.

Purification of the NaI(Tl) crystal for dark matter search project PICOLON

Direct search for dark matter is one of the most important problems in astrophysics. Significant signal for dark matter will be a hint to clarify the origin of the universe. Only DAMA/LIBRA experiment with NaI(Tl) detector has ever suggested the presence of dark matter signal. Verifying the DAMA/LIBRA result by a NaI(Tl) detector is urgent and important task. We have tried to purify NaI(Tl) crystal to search for dark matter. In this presentation, the present status of purification will be discussed. The concentration of potassium is successfully reduced to desired sensitivity. The 210Pb, which is difficult to reduce, has been reduced effectively. Present status of low background measurement in Kamioka observatory will be shown.

Background analysis for the SNO+ experiment

SNO+ is a large multi-purpose liquid scintillator based experiment, with the main physics goal of searching for the neutrinoless double-beta decay of 130Te. The first of the three SNO+ phases started in May 2017, with the detector filled with ultra-pure water. The deep underground location (6000 m.w.e.) and the low background levels allowed new physics searches, together with the measurement of the 8B solar neutrino flux in the 5-15 MeV energy range. Recently, SNO+ has began the transition to the scintillator phase. The double-beta decay phase is expected to start in early 2020, when the ultra-pure liquid scintillator will be loaded with 3.9 tonnes of natural tellurium, for a half-life sensitivity larger than 2×1026 years in 5 years of data taking. This article focuses on the low background measurement of the initial water phase.

Low Background Measurement Capabilities at SNOLAB

Experiments currently searching for dark matter and studying properties of neutrinos require very low levels of radioactive backgrounds both in their own construction materials and in the surrounding environment. These low background levels are required so that the current and next generation experiments can achieve the required sensitivities for their searches. This presentation will describe the low background measurement facilities currently operating at SNOLAB and will discuss plans and options to expand these facilities to allow for the increased sensitivity required by the next generation of experiments.

A fixed-target platform for serial femtosecond crystallography in a hydrated environment

For serial femtosecond crystallography at X-ray free-electron lasers, which entails collection of single-pulse diffraction patterns from a constantly refreshed supply of microcrystalline sample, delivery of the sample into the X-ray beam path while maintaining low background remains a technical challenge for some experiments, especially where this methodology is applied to relatively low-ordered samples or those difficult to purify and crystallize in large quantities. This work demonstrates a scheme to encapsulate biological samples using polymer thin films and graphene to maintain sample hydration in vacuum conditions. The encapsulated sample is delivered into the X-ray beam on fixed targets for rapid scanning using the Roadrunner fixed-target system towards a long-term goal of low-background measurements on weakly diffracting samples. As a proof of principle, we used microcrystals of the 24 kDa rapid encystment protein (REP24) to provide a benchmark for polymer/graphene sandwich performance. The REP24 microcrystal unit cell obtained from our sandwiched in-vacuum sample was consistent with previously established unit-cell parameters and with those measured by us without encapsulation in humidified helium, indicating that the platform is robust against evaporative losses. While significant scattering from water was observed because of the sample-deposition method, the polymer/graphene sandwich itself was shown to contribute minimally to background scattering.

Study of characteristics of CdZnTe detector

The results of investigation of detector made of cadmium zinc telluride (CdZnTe or CZT) crystal with a working volume of 1500 mm3 are presented. The detector is shown to have good spectrometric characteristics in registering gamma radiation from 241Am, 57Co, 137Cs and 60Co sources and can be effectively used for low-background measurements.

Low-Background Method of Isotope Markers for Measuring the Efficiency of Intercalation of Graphite by Potassium Atoms

The result of low-background measurements of the gamma activity of graphite-potassium intercalated sample is presented. A germanium gamma spectrometer, used for measurements, was located in the low-background chamber of Baksan Neutrino Observatory. For 384 hours of exposure, 768 decays of K-40 isotope nuclei were registered. This activity corresponds to 85 μg/cm2 potassium atoms embedded in to graphite lattice. A computer simulation of the intercalation process and the gamma-ray spectrum set is also presented. The accuracy of the potassium concentration determination can be brought to 10–11–10–12 g/g for mixture enriched with K-40 isotope.

Background studies of high energy γ rays from (n,γ) reactions in the CANDLES experiment

High energy γ rays with several MeV produced by (n,γ) reactions can be a trouble for low background measurements in the underground laboratories such as double beta decay experiments. In the CANDLES project, which aimed to observe the neutrino-less double beta decay from 48Ca, γ rays caused by (n,γ) reactions were found to be the most significant background. The profile of the background was studied by measurements with a neutron source and a simulation with a validity check of neutron processes in Geant4. The observed spectrum of γ rays from (n,γ) reactions was well reproduced by the simulated spectra, which were originated from the surrounding rock and a detector tank made of stainless steel. The environmental neutron flux was derived by the observed event rate of γ rays from (n,γ) reactions using the simulation. The thermal and non-thermal neutron flux were found to be (1.3±0.6)×10−6 cm−2s−1 and (1.1±0.5)×10−5 cm−2s−1, respectively. It is necessary to install an additional shield to reduce the background from (n,γ) reaction to the required level.

Low Background Measurement by Means of NaI(Tl) Scintillator—Improvement of Sensitivity for Cosmic Dark Matter—

Low Background Measurement by Means of NaI(Tl) Scintillator—Improvement of Sensitivity for Cosmic Dark Matter—

Comparative study of the double- K -shell-vacancy production in single- and double-electron-capture decay

Background: A double-$K$-electron capture is a rare nuclear-atomic process in which two $K$ electrons are captured simultaneously from the atomic shell. A ``hollow atom'' is created as a result of this process. In single-$K$-shell electron-capture decays, there is a small probability that the second electron in the $K$ shell is excited to an unoccupied level or can (mostly) be ejected to the continuum. In either case, a double vacancy is created in the $K$ shell. The relaxation of the double-$K$-shell vacancy, accompanied by the emission of two $K$-fluorescence photons, makes it possible to perform experimental studies of such rare processes with the large-volume proportional gas chamber.Purpose: The purpose of the present analysis is to estimate a double-$K$-shell vacancy creation probability per $K$-shell electron capture ${P}_{KK}$ of $^{81}\mathrm{Kr}$, as well as to measure the half-life of $^{78}\mathrm{Kr}$ relative to $2\ensuremath{\nu}2K$ capture.Method: Time-resolving current pulse from the large low-background proportional counter (LPC), filled with the krypton sample, was applied to detect triple coincidences of ``shaked'' electrons and two fluorescence photons.Results: The number of $K$-shell vacancies per the $K$-electron capture, produced as a result of the shake-off process, has been measured for the decay of $^{81}\mathrm{Kr}$. The probability for this decay was found to be ${P}_{KK}=(5.7\ifmmode\pm\else\textpm\fi{}0.8)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}5}$ with a systematic error of ${(\mathrm{\ensuremath{\Delta}}{P}_{KK})}_{\text{syst}}=\ifmmode\pm\else\textpm\fi{}0.4\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}5}$. For the $^{78}\mathrm{Kr}(2\ensuremath{\nu}2K)$ decay, the comparative study of single- and double-capture decays allowed us to obtain the signal-to-background ratio up to 15/1. The half-life ${T}_{1/2}^{2\ensuremath{\nu}2K}(g.s.\ensuremath{\rightarrow}g.s.)=[1.{9}_{\ensuremath{-}0.7}^{+1.3}(\text{stat})\ifmmode\pm\else\textpm\fi{}0.3(\text{syst})]\ifmmode\times\else\texttimes\fi{}{10}^{22}\phantom{\rule{4pt}{0ex}}y$ is determined from the analysis of data that have been accumulated over 782 days of live measurements in the experiment that used samples consisted of 170.6 g of $^{78}\mathrm{Kr}$.Conclusions: The data collected during low background measurements using the LPC were analyzed to search the rare atomic and nuclear processes. We have determined ${P}_{KK}^{\text{exp}}$ for the $EC$ decay of $^{81}\mathrm{Kr}$, which are in satisfactory agreement with ${Z}^{\ensuremath{-}2}$ dependence of ${P}_{KK}$ predicted by Primakoff and Porter. This made possible to more accurately determine the background contribution in the energy region of our interest for the search for the $2K$ capture in $^{78}\mathrm{Kr}$. The general procedure of data analysis allowed us to determine the half-life of $^{78}\mathrm{Kr}$ relative to $2\ensuremath{\nu}2K$ transition with a greater statistical accuracy than in our previous works.