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An Erratum to this paper has been published: https://doi.org/10.1134/S0021364022390011

The resonant excitation of the 83Kr first excited nuclear level (E = 9.4 keV) by solar axions whose fluxes depend on the axion–electron coupling constant gAe is sought. The γ- and X-ray photons and the conversion and Auger electrons from the excited-level relaxation are detected with a gas proportional counter of a low-background detector in the underground Baksan Neutrino Observatory (Institute for Nuclear Research, Russian Academy of Sciences). As a result, a new constraint $${\text{|}}{{g}_{{Ae}}}(g_{{AN}}^{3} - g_{{AN}}^{0}){\text{|}}$$ ≤ 1.50 × 10–17 (90% C.L.) has been obtained for the axion–electron and axion–nucleon coupling constants, which corresponds to new constraints on the axion mass mA ≤ 320 eV and mA ≤ 4.6 eV in the KSVZ and DFSZ axion models, respectively.

Natural background radiation is a permanent multicomponent factor. It has an influence on biological organisms, but effects of its deprivation still remain unclear. The aim of our work was to study for the first time responses of D. melanogaster to conditions of the Deep Underground Low-Background Laboratory DULB-4900 (BNO, INR, RAS, Russia) at the transcriptome level by RNA-seq profiling. Overall 77 transcripts demonstrated differential abundance between flies exposed to low and natural background radiation. Enriched biological process functional categories were established for all genes with differential expression. The results showed down-regulation of primary metabolic processes and up-regulation of both the immune system process and the response to stimuli. The comparative analysis of our data and publicly available transcriptome data on D. melanogaster exposed to low and high doses of ionizing radiation did not reveal common DEGs in them. We hypothesize that the observed changes in gene expression can be explained by the influence of the underground conditions in DULB-4900, in particular, by the lack of stimuli. Thus, our study challenges the validity of the LNT model for the region of background radiation doses below a certain level (~16.4 nGy h-1) and the presence of a dose threshold for D. melanogaster.

180mTa, which is the second excited state of 180Ta at Ex = 77 keV, is naturally occurring and is the only known stable isomer. The half-life of 180mTa is considered to be an important parameter for nuclear synthesis models for heavy elements. However, the decay of 180mTa has never been observed even though several groups tried to measure it. We will search for gamma transitions from 180mTa decays in a tantalum sample by using an array of fourteen HPGe detectors recently installed in an underground laboratory at Yangyang, Korea. In preparation for the measurement, Monte-Carlo simulation studies were conducted to optimize the tantalum sample configuration. Based on the simulation study, we decided on a configuration composed of a 2 mm thick disk with diameter of 200 mm and six 2 mm thick rectangular plates with dimension of 158 × 195 mm2. The finalized tantalum sample configuration gives 2.0 and 7.5 coincidence events per year for the EC and the β-decay of 180mTa, respectively. In this study, we used T1/2 =2.0 × 1017 years and T1/2 = 5.8 × 1016 years for the EC and β-decay which are the present best lower-limits as reported by B. Lehnert et al.

Because of a high energy of the neutrinoless double-beta decay (0ν2β) of the isotope 150Nd and a high value of the daughter-nucleus charge Zf, 150Nd is one of the most promising isotopes for 0ν2β-decay searches. A 150Nd-containing detector on the basis of a liquid organic scintillator permits employing large isotope masses. Requirements on the radiation purity of the neodymium sample used are determined. The possible design of a large-scale detector of this type and expected results are considered.

The GERmanium Detector Array (GERDA) collaboration searched for neutrinoless double-$\beta$ decay in $^{76}$Ge with an array of about 40 high-purity isotopically-enriched germanium detectors. The experimental signature of the decay is a monoenergetic signal at Q$_{\beta\beta}$ = 2039.061(7)keV in the measured summed energy spectrum of the two emitted electrons. Both the energy reconstruction and resolution of the germanium detectors are crucial to separate a potential signal from various backgrounds, such as neutrino-accompanied double-$\beta$ decays allowed by the Standard Model. The energy resolution and stability were determined and monitored as a function of time using data from regular $^{228}$Th calibrations. In this work, we describe the calibration process and associated data analysis of the full GERDA dataset, tailored to preserve the excellent resolution of the individual germanium detectors when combining data over several years., Comment: 12 pages, 7 figures

The GERmanium Detector Array (GERDA) experiment searched for the lepton-number-violating neutrinoless double-$\beta$ ($0\nu\beta\beta$) decay of $^{76}$Ge, whose discovery would have far-reaching implications in cosmology and particle physics. By operating bare germanium diodes, enriched in $^{76}$Ge, in an active liquid argon shield, GERDA achieved an unprecedently low background index of $5.2\times10^{-4}$ counts/(keV$\cdot$kg$\cdot$yr) in the signal region and met the design goal to collect an exposure of 100 kg$\cdot$yr in a background-free regime. When combined with the result of Phase I, no signal is observed after 127.2 kg$\cdot$yr of total exposure. A limit on the half-life of $0\nu\beta\beta$ decay in $^{76}$Ge is set at $T_{1/2}>1.8\times10^{26}$ yr at 90% C.L., which coincides with the sensitivity assuming no signal., Comment: 7 pages, 3 figures, submitted to Physical Review Letters

We present the first search for bosonic superweakly interacting massive particles (super-WIMPs) as keV-scale dark matter candidates performed with the GERDA experiment. GERDA is a neutrinoless double-β decay experiment which operates high-purity germanium detectors enriched in Ge76 in an ultralow background environment at the Laboratori Nazionali del Gran Sasso (LNGS) of INFN in Italy. Searches were performed for pseudoscalar and vector particles in the mass region from 60 keV/c2 to 1 MeV/c2. No evidence for a dark matter signal was observed, and the most stringent constraints on the couplings of super-WIMPs with masses above 120 keV/c2 have been set. As an example, at a mass of 150 keV/c2 the most stringent direct limits on the dimensionless couplings of axionlike particles and dark photons to electrons of gae<3×10−12 and α′/α<6.5×10−24 at 90% credible interval, respectively, were obtained.

We present the first search for bosonic superweakly interacting massive particles (super-WIMPs) as keV-scale dark matter candidates performed with the GERDA experiment. GERDA is a neutrinoless double-s decay experiment which operates high-purity germanium detectors enriched in ^{76}Ge in an ultralow background environment at the Laboratori Nazionali del Gran Sasso (LNGS) of INFN in Italy. Searches were performed for pseudoscalar and vector particles in the mass region from 60 keV/c^{2} to 1 MeV/c^{2}. No evidence for a dark matter signal was observed, and the most stringent constraints on the couplings of super-WIMPs with masses above 120 keV/c^{2} have been set. As an example, at a mass of 150 keV/c^{2} the most stringent direct limits on the dimensionless couplings of axionlike particles and dark photons to electrons of g_{ae}

The Advanced Molybdenum-based Rare process Experiment in its second phase (AMoRE-II) will search for neutrinoless double-beta (0{\nu}\b{eta}\b{eta}) decay of 100Mo in 200 kg of molybdate crystals. To achieve the zero-background level in the energy range of the double-beta decay Q-value of 100Mo, the radioactive contamination levels in AMoRE crystals should be low. 100EnrMoO3 powder, which is enriched in the 100Mo isotope, is used to grow the AMoRE crystals. A shielded array of fourteen high-purity germanium detectors with 70% relative efficiency each was used for the measurement of background activities in a sample of 9.6-kg powder. The detector system named CAGe located at the Yangyang underground laboratory was designed for measuring low levels of radioactivity from natural radioisotopes or cosmogenic nuclides such as 228Ac, 228Th, 226Ra, 88Y, and 40K. The activities of 228Ac and 228Th in the powder sample were 0.88 \pm 0.12 mBq/kg and 0.669 \pm 0.087 mBq/kg, respectively. The activity of 226Ra was measured to be 1.50 \pm 0.23 mBq/kg. The activity of 88Y was 0.101 \pm 0.016 mBq/kg. The activity of 40K was found as 36.0 \pm 4.1 mBq/kg., Comment: 24 pages, 10 figures, 5 tables

Results of a comparative analysis of the $^{214}$Po ($T_{1/2}= 163.47\pm0.03$ $\mu$s), $^{213}$Po ($T_{1/2}=3.705 \pm 0.001$ $\mu$s) and $^{212}$Po ($T_{1/2}=294.09\pm0.07$ ns) half-life annular variation parameters are presented. It is shown that two independent sequential sets of the $^{214}$Po $\tau$-values $(\tau\equiv T_{1/2})$ obtained in the spaced laboratories can be described by sinusoidal functions. The sinusoid curve with amplitude $A=(5.0 \pm1.5) \cdot 10^{-4}$, period $\omega=(365\pm 8)$ days, and phase $\phi=(170 \pm 7)$ days approximates the set of $^{214}$Po $\tau$ values obtained at BNO INR RAS during the $\sim$973 days starting on January 4, 2012. The function approximates a set of $\tau$-values with a time duration of $\sim1460$ days obtained at the KhNU has an amplitude $A=(4.9\pm1.8)\cdot10^{-4}$, a period $\omega= (377\pm13)$ days and a phase $\phi=(77\pm10)$ days. The $^{213}$Po $\tau$-value set with a time duration of $\sim1700$ days can be described by a sinusoidal function with an amplitude $A=(3.9\pm1.2)\cdot10^{-4}$, a period $\omega= (370\pm13)$ days and a phase $\phi=(130\pm9)$ days. The $^{212}$Po $\tau$-value set with a time duration of $\sim670$ days can be described by a sinusoidal function with an amplitude $A=(7.5\pm1.6)\cdot10^{-4}$, a period $\omega= (375\pm13)$ days and a phase $\phi=(40\pm10)$ days., Comment: 6 pages, 5 figures, Prepared for the proceedings of The 5th International Conference on Particle Physics and Astrophysics (ICPPA-2020)

A device with the parent 229Th source was constructed to search for variations of the daughter 213Po half-life (T1/2 = 4.2 μs). A solar-daily variation with amplitude ASo = (5.3 ± 1.1) × 10–4, a lunar-daily variation with amplitude AL = (4.8 ± 2.1) × 10–4, and a sidereal-daily variation with amplitude AS = (4.2 ± 1.7) × 10–4 were found upon proceeding the data series over a 622-day interval (from July 2015 to March 2017). The 213Po half-life mean value is found to be T1/2 = 3.705 ± 0.001 μs. The reference value is corrected by 12%.

A brief description of the results of joint analysis of data obtained in long-term measurements of krypton samples differing in 78Kr content is presented. Low-background high-pressure proportional gas counters were used as detectors. The comparative analysis of experimental data on single and double K-capture provided the first estimate of the probability of production of a double K-shell vacancy in the process of K-shell electron capture in 81Kr: $${{P}_{{KK}}} = \left[ {5.7 \pm 0.8\,\,({\text{stat}}{\text{.}}) \pm 0.4\,\,({\text{syst}}{\text{.}})} \right] \times {{10}^{{ - 5}}}$$ . A new result for the half-life of 78Kr with respect to the 2K(2ν)-mode was also obtained: $${{T}_{{1/2}}} = \left[ {1.9_{{ - 0.7}}^{{ + 1.3}}\,\,({\text{stat}}{\text{.}}) \pm 0.3\,\,({\text{syst}}{\text{.}})} \right] \times {{10}^{{22}}}$$ years (90% CL).

A new low-background laboratory has been constructed in the Pyhasalmi mine in central Finland and was commissioned in the beginning of 2017. The underground experimental hall is located at a depth of 1436 m corresponding to about 4100 m of water equivalent (m.w.e.). We describe the equipment of the laboratory and the backgrounds arising from the natural rock radioactivity and the flux of cosmic ray muons at the experimental hall location. The high-sensitivity experiments operating at the low-background laboratory are briefly described.

The results of the experimental search for two-neutrino 2K-capture in 124Xe with a large copper proportional counter obtained by processing the data for an exposure of 37.7 kg day are presented. The experimental setup is located at the Underground Low-Background Laboratory of the Baksan Neutrino Observatory at a depth of 4900 m.w.e. The combination of methods of selection of useful signals with a unique set of characteristics and the event topology taken into account allowed us to suppress the background in the energy region of interest. A new half-life limit for 2K(2ν)-capture in 124Xe was determined: T1/2 ≥ 7.7 × 1021 years (90% CL).

Counting characteristics of the system for recording rare events of the 71Ge decay in the radiochemical gallium Baksan Experiment on Sterile Transitions (BEST) are presented. Events are recorded simultaneously along eight counting channels in the energy range 0.3–16 keV, which include 71Ge decays in the region of L and K peaks with a total efficiency of up to 75%. The data analysis is based on the recording of the full shape of the leading edge of the charge pulses using a digital oscilloscope.

Resonance excitation of the 83Kr first nuclear level (E = 9.4 keV) by solar axions formed via the Primakoff mechanism is sought. The γ- and X-ray photons and the conversion and Auger electrons arising from the excited-level relaxation are detected with a gas proportional counter of a low-background detector in the underground Baksan Neutrino Observatory. The following experimental constraint is obtained for the product of the axion–photon coupling constant and the axion mass:|gAγ × mA| ≤ 6.3 × 10 -17 In the framework of the hadronic-axion model, this corresponds to a new axion-mass constraint of mA ≤ 12.7 eV at 95% C.L.

A setup for measuring natural-radioactivity backgrounds and ultralow concentrations of the isotope 14C in samples of a liquid organic scintillator was created at the low-background laboratory of the Baksan Neutrino Observatory (Institute for Nuclear Research, Russian Academy of Sciences) at a depth of 4900 mwe. The concentration of the radiocarbon 14C in a sample of a scintillator based on domestically produced linear alkylbenzene was measured, and it was found that 14C/12C (3.3 ± 0.5) × 10−17.

At the Baksan Neutrino Observatory (Institute for Nuclear Research, Russian Academy of Sciences, Moscow) deployed in the Caucasus mountains, it is proposed to create, at a depth corresponding to 4760 mwe, a large-volume neutrino detector on the basis of a liquid scintillator with a target mass of 10 kt. The detector in question is intended for recording natural fluxes of neutrinos whose energy may be as low as 100MeV. Neutrino fluxes from various sources are considered in the present study, and the expected effect in the proposed detector is estimated. The detector hat is being developed at the Baksan Neutrino Observatorywill become part of the world network of neutrino detectors for studying natural neutrino fluxes.

Features of a thermal neutron test detector with thin scintillator [ZnS(Ag)+6LiF] are described. Background of the detector and its registration efficiency were defined as a result of measurements. The thermal neutron flux at different locations, and for different conditions around the Baksan Neutrino Observatory are reported.

The results of investigation of 127Xe radioactive isotope production in the xenon sample enriched in 124Xe, 126Xe, 128Xe are presented. The isotope is supposed to be the source of the background events in the low-background experiment on search for 2K-capture of 124Xe. In this work we consider two channels of 127Xe production: the neutron knock-out from 128Xe nucleus by cosmogenic muons and the neutron capture by 126Xe nucleus. For the first channel the upper limit of the cross section of 127Xe production was found to be σ ≤ 0.007 × 10−24 cm2 at 95% C.L. For the second channel the value obtained for the cross section was found to be equal to σ = (2.74 ± 0.4) × 10−24 cm2, which coincides well, within the statistical error, with reference value.

The results of a search for two neutrino mode of double K-capture of 124Xe using a large copper low-background proportional counter are presented. Data collected during 3220 hours of measurements with 58.6 g of 124Xe provides us to a new limit on the half-life of 124Xe regarding 2K-capture at the level: T1/2 ≥ 2.0 × 1021 years at a 90% confidence level.

We report on the study of thermal neutron flux using monitors based on mixture of ZnS(Ag) and LiF enriched with a lithium-6 isotope at the deep underground laboratory DULB-4900 at the Baksan Neutrino Observatory. An annual modulation of thermal neutron flux in DULB-4900 is observed. Experimental evidences were obtained of correlation between the long-term thermal neutron flux variations and the absolute humidity of the air in laboratory. The amplitude of the modulation exceed 5% of total neutron flux.

Axions with the energy of $$9.4$$ keV emitted in the М1 transition in 83Kr nuclei in the Sun have been searched for using the resonant absorption reaction $$A + \,{{\,}^{{83}}}{\kern 1pt} {\text{Kr}} \to \,{{\,}^{{83}}}{\text{K}}{{{\text{r}}}^{ \star }} \to \,{{\,}^{{83}}}{\kern 1pt} {\text{Kr}} + \gamma ,e$$ (9.4 keV). A krypton-filled proportional counter located in a low-background setup is used to detect $$\gamma $$ rays and electrons. A new limit on the axion–nucleon coupling constant $$\left| {g_{{AN}}^{3} - g_{{AN}}^{0}} \right| \leqslant 8.4 \times {{10}^{{ - 7}}}$$ is obtained. In the hadronic axion model, it leads to a new limit on the axion mass $${{m}_{A}} \leqslant 65$$ eV (95% CL). A possibility of searching for solar axions with 57Fe-containing semiconductor crystals is described.

Axions with energies of 9.4 keV putatively produced in the Sun via an M1-transition in the 83Kr nucleus are searched for using the reaction of resonant absorption: A +83Kr →83Kr* →83Kr + γ, e (9.4 keV). The γ-quanta and electrons are detected with a proportional counter filled with krypton and placed in a low-background setup. A new upper limit on the axion–nucleon coupling constant is imposed: |g AN 3 g AN 0 |≤1.02 106. For the hadronic axion, this implies a new upper limit on the axion mass: m A ≤ 79eV at the 95% confidence level.

The technique used in the experiment in the search for the process of 2K2ν-capture in 124Xe using large high-pressure proportional counters is briefly described. The measurements with a xenon sample enriched to 21% (or ~57 g) of the 124Xe isotope carried out for 3340 hours yielded a zero signal. The resultant upper limit on the 124Xe half-life with respect to the 2ν-mode of the nuclear capture of two K-shell electrons amounts to T 1/2>4.6 1021 yr at the 90% confidence level.

The GERmanium Detector Array (Gerda) experiment at the Gran Sasso underground laboratory (LNGS) of INFN is searching for neutrinoless double-beta (0νββ) decay of 76Ge. The technological challenge of Gerda is to operate in a “background-free” regime in the region of interest (ROI) after analysis cuts for the full 100 kg·yr target exposure of the experiment. A careful modeling and decomposition of the full-range energy spectrum is essential to predict the shape and composition of events in the ROI around Qββ for the 0νββ search, to extract a precise measurement of the half-life of the double-beta decay mode with neutrinos (2νββ) and in order to identify the location of residual impurities. The latter will permit future experiments to build strategies in order to further lower the background and achieve even better sensitivities. In this article the background decomposition prior to analysis cuts is presented for Gerda Phase II. The background model fit yields a flat spectrum in the ROI with a background index (BI) of $$ {16.04}_{-0.85}^{+0.78}\cdotp {10}^{-3} $$ 16.04 − 0.85 + 0.78 · 10 − 3 cts/(keV·kg·yr) for the enriched BEGe data set and $$ {14.68}_{-0.52}^{+0.47}\cdotp {10}^{-3} $$ 14.68 − 0.52 + 0.47 · 10 − 3 cts/(keV·kg·yr) for the enriched coaxial data set. These values are similar to the one of Phase I despite a much larger number of detectors and hence radioactive hardware components.

The detectors based on the liquid scintillator (LS) monitored by an array of photo-multiplier tubes (PMT) are often used in low energy experiments such as neutrino oscillation studies and search for dark matter. Detectors of this kind operate in an energy range spanning from hundreds of keV to a few GeV providing a few percent resolution at energies above 1 MeV and allowing to observe fine spectral features. This article gives a brief overview of relevant physical processes and introduces a new universal simulation tool LSMC (Liquid Scintillator Monte Carlo) for simulation of LS-based detectors equipped with PMT arrays. This tool is based on the Geant4 framework and provides supplementing functionality for ease of configuration and comprehensive output. The usage of LSMC is illustrated by modeling and optimization of a compact detector prototype currently being built at Baksan Neutrino Observatory.

Advanced Mo-based Rare process Experiment (AMoRE) – II led by Center for Underground Physics (CUP) will search for neutrinoless double beta ($0v\beta\beta$) decay of $^{100}$Mo, using molybdate crystals such as $^{40}Ca^{100}MoO_3$ (CMO) crystals [1]. Because the theorized 0νββ decay would happen rarely, it is important to reach zero-background level around the region of interest near 3 MeV. Therefore, it is necessary to check the raw material of the crystal, the $^{100}MoO_3$ powder, for trace levels of radioactive contamination. The High Purity Germanium (HPGe) detector group operates an array of 14 HPGe detectors, referred to as CUP Array of Germanium (CAGe), at the YangYang underground Laboratory (Y2L) in Korea. By measuring $^{100}MoO_3$ powder samples in the CAGe, we obtained activity levels of $^{226}$Ra (1.6 ± 0.3 mBq/kg), $^{40}$K (12 ± 3 mBq/kg), 228Ac (344 ± 71 $\mu$Bq/kg), $^{228}$Th (244 ± 50 $\mu$Bq/kg), and $^{88}$Y(33 ± 8 $\mu$Bq/kg). Data analysis and experimental methods are discussed in this report.

At the Baksan Neutrino Observatory (Institute for Nuclear Research of the Russian Academy of Sciences, Moscow) deployed in the Caucasus mountains, it is proposed to create, at a depth corresponding to about 4700 mwe (meter water equivalent), a large-volume neutrino detector on the basis of a liquid scintillator with a target mass of 10 kt. This article describes the current state of the first stage of the project, namely a prototype detector with a scintillator mass of 0.5 t. The design of the detector, the equipment and methods used are described.

Resonance excitation of the 83Kr first nuclear level (E = 9.4 keV) by solar axions formed via the Primakoff mechanism and via an M1-transition in the 83Kr nucleus in the Sun is searched. The γ-and x-ray photons and the conversion and Auger electrons arising from the excited-level relaxation are detected with a proportional counter in the underground low-background facility of Baksan Neutrino Observatory. The following experimental constraint is obtained for the product of the axion-photon coupling constant and the axion mass: |g Aγ m A | ≼ 6.16 × 10−17. In the framework of the hadronic-axion model, this corresponds to a new axion-mass constraint of mA ≼ 12.6 eV at 95% coincidence level (C.L.). In case of pure hadronic interaction the upper limit for axion-nucleon coupling is | g AN 3 − g AN 0 | ≼ 8.3 × 10−7 at 95% C.L., which corresponds to upper limit for hadrionic axion mass: mA ≼ 64 eV at 95% C.L. with the generally accepted values S = 0.5 and z = 0.56.

Searches for new physics push experiments to look for increasingly rare interactions. As a result, detectors require increasing sensitivity and specificity, and materials must be screened for naturally occurring, background-producing radioactivity. Furthermore, the detectors used for screening must approach the sensitivities of the physics-search detectors themselves, thus motivating iterative development of detectors capable of both physics searches and background screening. We report on the design, installation, and performance of a novel, low-background, fourteen-element high-purity germanium detector named the CAGe (CUP Array of Germanium), installed at the Yangyang underground laboratory in Korea.