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In experiments devoted to studying nonstandard neutrino properties by means of high-intensity artificial neutrino sources, a determination of source activities to a high degree of precision is required. A calorimetric system that makes it possible to measure the source activity to a precision better than 1 $${\%}$$ was created for measuring the activity of the neutrino source on the basis of $${}^{51}$$ Cr in the BEST experiment. The structure of the calorimetric system and basic factors that affect the accuracy of the measurements are considered. The results obtained by measuring the activity of the source manufactured for the BEST experiment are presented. At the beginning of the experiment, this activity was 3.41 MCi.

The Baksan Experiment on Sterile Transitions (BEST) probes the gallium anomaly and its possible connections to oscillations between active and sterile neutrinos. Based on the Gallium-Germanium Neutrino Telescope (GGNT) technology of the SAGE experiment, BEST employs two zones of liquid Ga target to explore neutrino oscillations on the meter scale. Oscillations on this short scale could produce deficits in the $^{71}$Ge production rates within the two zones, as well as a possible rate difference between the zones. From July 5th to October 13th 2019, the two-zone target was exposed to a primarily monoenergetic, 3.4-MCi $^{51}$Cr neutrino source 10 times for a total of 20 independent $^{71}$Ge extractions from the two Ga targets. The $^{71}$Ge production rates from the neutrino source were measured from July 2019 to March 2020. At the end of these measurements, the counters were filled with $^{71}$Ge doped gas and calibrated during November 2020. In this paper, results from the BEST sterile neutrino oscillation experiment are presented in details. The ratio of the measured $^{71}$Ge production rates to the predicted rates for the inner and the outer target volumes are calculated from the known neutrino capture cross section. Comparable deficits in the measured ratios relative to predicted values are found for both zones, with the $4 \sigma$ deviations from unity consistent with the previously reported gallium anomaly. If interpreted in the context of neutrino oscillations, the deficits give best fit oscillation parameters of $\Delta m^2=3.3^{+\infty}_{-2.3}$ eV$^2$ and sin$^2 2\theta=0.42^{+0.15}_{-0.17}$, consistent with $\nu_e \rightarrow \nu_s$ oscillations governed by a surprisingly large mixing angle.

The Baksan Experiment on Sterile Transitions (BEST) was designed to investigate the deficit of electron neutrinos, $\nu_{e}$, observed in previous gallium-based radiochemical measurements with high-intensity neutrino sources, commonly referred to as the \textit{gallium anomaly}, which could be interpreted as evidence for oscillations between $\nu_e$ and sterile neutrino ($\nu_s$) states. A 3.414-MCi \nuc{51}{Cr} $\nu_e$ source was placed at the center of two nested Ga volumes and measurements were made of the production of \nuc{71}{Ge} through the charged current reaction, \nuc{71}{Ga}($\nu_e$,e$^-$)\nuc{71}{Ge}, at two average distances. The measured production rates for the inner and the outer targets respectively are ($54.9^{+2.5}_{-2.4}(\mbox{stat})\pm1.4 (\mbox{syst})$) and ($55.6^{+2.7}_{-2.6}(\mbox{stat})\pm1.4 (\mbox{syst})$) atoms of \nuc{71}{Ge}/d. The ratio ($R$) of the measured rate of \nuc{71}{Ge} production at each distance to the expected rate from the known cross section and experimental efficiencies are $R_{in}=0.79\pm0.05$ and $R_{out}= 0.77\pm0.05$. The ratio of the outer to the inner result is 0.97$\pm$0.07, which is consistent with unity within uncertainty. The rates at each distance were found to be similar, but 20-24\% lower than expected, thus reaffirming the anomaly. These results are consistent with $\nu_e \rightarrow \nu_s$ oscillations with a relatively large $\Delta m^2$ ($>$0.5 eV$^2$) and mixing sin$^2 2\theta$ ($\approx$0.4)., Comment: Paper updated to final version

The Baksan Large Neutrino Telescope is a liquid scintillation neutrino detector with a target mass of 10 kt, which is proposed to be constructed at the Baksan Neutrino Observatory of the Russian Academy of Sciences at a depth of 4700 m.w.e. In addition to the main full-scale detector, this project includes prototypes with target masses of 0.5, 5 and 100 t. This article evaluates photomultipliers (PMTs) Hamamatsu R7081-100 WA-S70 that are used in the first two phases of the project as the main ones. Also this article evaluates PMTs Hamamatsu R5912-100 WA-S70 used for the muon veto system for the 5-t detector prototype. To evaluate the PMTs, the quantum efficiency, single photoelectron response, timing characteristics, anode dark count rate, linearity and long afterpulses were measured.

The possibility of employing a 65Zn source in the gallium experiment BEST-2 with the aim of constraining the region of allowed oscillation parameters is examined. The source activity required for the BEST-2 experiment is calculated, and the size of the source and its effect on the results of oscillation measurements are discussed along with the possibilities for producing such a source. Various schemes for implementing respective measurements are considered.

The paper describes the capabilities of the three experiments on the search for oscillations on a short baseline—BEST, SOX, and DANSS. It is shown that only the first two experiments have the capabilities for localization of the oscillation parameters.

Within the framework of the BEST experiment project, a calorimetric system was developed to measure the activity of high-intensity (several MCi) neutrino sources based on 51Cr. In the range of thermal capacities of 250–520 W, the uncertainty of the heat release measurement is less than 0.25%. Taking into account the uncertainty of the energy release value for the 51Cr decay (0.23%), the activity of the neutrino source can be determined with an accuracy of ∼0.5%.

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.

A method of determining the activity of intensive distributed -sources on the measurement of the continuous spectrum of radiation, for example the internal bremsstrahlung, is developed. The recurrent formula for reconstructing of a continuous spectrum, registered in a Ge detector, at distorting it in the detector. The method of precise measurements of the spectrum of 51Cr internal bremsstrahlung using two point sources of low activity is described.

New counting system for 71Ge decays rare events registration is developed for realization of the BEST experiment to search for sterile neutrinos. Description of the system, its technical and counting data are presented in the article. Comparison with already existing counting system for SAGE is given also.

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.

Baksan Experiment on Sterile Neutrino (BEST) is presently at the stage of production of the artificial neutrino source Cr-51, the gallium exposure will start in July and proceed for three months. While aiming specifically at investigating the Gallium neutrino anomaly (SAGE and GALLEX experiments), BEST can do more and it is tempting to estimate its ability in testing sterile neutrino explanation of antineutrino (reactor) anomalies. We observe a moderate sensitivity to the region in model parameter space (sterile neutrino mass and mixing with active electron neutrino) outlined by the old reactor antineutrino anomaly and the best fit of DANSS experiment, while the Neutrino-4 favorite region falls right in the BEST ballpark. In particular, by analyzing SAGE+GALLEX and Neutrino-4 $\chi^2$ distributions we find that Neutrino-4 results are fully consistent with the Gallium anomaly, the significance of the combined anomaly almost reaches 4$\sigma$ level. If the BEST confirms the Neutrino-4 results, the joint analysis will indicate more than 5$\sigma$ evidence for the sterile neutrino of eV-scale mass., Comment: 4 pages; v2: journal version

The activity of the 51Cr source in the BEST experiment searching for sterile neutrinos is planned to be determined by measuring the spectrum of internal bremsstrahlung. The measurements of activity and the spectrum of internal bremsstrahlung of point low-activity 51Cr sources with Ge semiconductor detectors are discussed. The accuracy and errors of this measurement method are examined.

The possibility of the BEST experiment on electron neutrino disappearance with intense artificial sources of electron neutrino 51Cr is considered. BEST has the great potential to search for transitions of active neutrinos to sterile states with Δm2 ∼ 1 eV2 and to set the limits on short baseline electron neutrino disappearance oscillation parameters. The possibility of the further constraints the oscillation parameters region with using 65Zn source is discussed.

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.

The paper presents the first results of development of manufacturing technology of metallic chromium targets from highly enriched isotope 50Cr for irradiation in a high flux nuclear reactor to obtain a compact high intensity neutrino source with low content of radionuclide impurities and minimum losses of enriched isotope. The main technological stages are the hydrolysis of chromyl fluoride, the electrochemical reduction of metallic chromium, the hot isostatic pressing of chromium powder and the electrical discharge machining of chromium bars. The technological stages of hot isostatic pressing of chromium powder and of electrical discharge machining of Cr rods have been tested.

The SAGE experiment, which measures the flux of solar neutrinos, contains approximately 50 tonnes of liquid gallium metal. Low-energy neutrinos interact with the isotope Ga 71 in the Ga and convert it into Ge 71 at a rate of approximately one atom per day. In each measurement of the solar neutrinos about 210 μg of a stable Ge isotope in a Ga matrix is added to the Ga and the mixture is stirred to homogeneity. After a 30-day exposure of the gallium to solar neutrinos, any germanium is chemically extracted from the gallium. The Ge is converted into the gas germane and the number of Ge 71 atoms that it contains is measured in a proportional counter. After 6 months of counting, the Ge is removed from the counter, dissolved in an aqueous solution, and the isotopic content of Ge in this solution is measured by ICP-MS. The efficiency of extraction of Ge from the Ga is calculated from the measured mass and isotopic content of added and extracted Ge. In 75 recent exposures of the Ga to solar neutrinos the average Ge extraction efficiency was 97.7 ± 1.1%. This uncertainty in extraction efficiency is much less than the 3.3% uncertainty obtained by measurements of Ge mass alone, i.e., without isotopic analysis.

A technique for the measurement of activities of intense β sources by measuring the continuous gamma-radiation (internal bremsstrahlung) spectra is developed. A method for reconstructing the spectrum recorded by a germanium semiconductor detector is described. A method for the absolute measurement of the internal bremsstrahlung spectrum of 51Cr is presented.

Experimental study of nonstandard neutrino properties using high-intensity artificial neutrino sources requires the activity of the sources to be determined with high accuracy. In the BEST project, a calorimetric system for measurement of the activity of high-intensity (a few MCi) neutrino sources based on 51Cr with an accuracy of 0.5–1% is created. In the paper, the main factors affecting the accuracy of determining the neutrino source activity are discussed. The calorimetric system design and the calibration results using a thermal simulator of the source are presented.

A current status of the project of a large volume scintillation telescope at the Baksan neutrino observatory is presented. The main research activities of the BLVST are low- energy neutrino physics, astrophysics and geophysics. To detect geoneutrinos, large-scale new- generation scintillator detectors located at large depths in the regions with a low background level from nuclear reactors are required. The Baksan Neutrino Observatory is geographically located in one of these places. Recently resumed R&D activities are aimed at the creation of new-generation telescope with a target mass of 10 kt at a depth of 4800 m.w.e. A small scale prototype is already under construction.

The proposal for an experiment on a 2-zone gallium target of a solar Galliumgermanium neutrino telescope at the Baksan neutrino observatory of the INR RAS with a 65Zn source for short baseline neutrino oscillations search is considered. The possibilities of determining the parameters of oscillations, the necessary characteristics of the neutrino source, the possibility of its production and its use are described. The expected results of measurements with a 65Zn source are compared with the results expected in the experiment BEST with the 51Cr source.

O(1) eV sterile neutrino can be responsible for a number of anomalous results of neutrino oscillation experiments. This hypothesis may be tested at short base-line neutrino oscillation experiments, several of which are either ongoing or under construction. Here we concentrate on the so-called Gallium anomaly, found by SAGE and GALLEX experiments, and its foreseeable future tests with BEST experiment at Baksan Neutrino Observatory. We start with a revision of the neutrino-gallium cross section, that is performed by utilizing the recent measurements of the nuclear final state spectra. We accordingly correct the parameters of Gallium anomaly and refine the BEST prospects in testing it and searching for sterile neutrinos. We further evolve the previously proposed idea to investigate the anomaly with Zn-65 artificial neutrino source as a next option available at BEST, and estimate its sensitivity to the sterile neutrino model parameters following the Bayesian approach. We show that after the two stages of operation BEST will make 5$\sigma$-discovery of the sterile neutrinos, if they are behind the Gallium anomaly., Comment: 7 pages, 8 figures; v2: 8 pages, 9 figures, journal version

The potential of the SAGE and BOREXINO experiments with 51Cr artificial neutrino sources for searches for and determination of parameters of short-baseline oscillations.

The results of gallium measurements of solar neutrino and measurements with artificial sources of neutrinos are presented. Conclusions are drawn from these results, and the potential of the SAGE experiment for studying transitions of active neutrinos to sterile states for Δm 2 > 0.5 eV2 and a sensitivity of a few percent to the disappearance of electron neutrinos is examined.

We present a first direct Q -value measurement of the Ga 71 ( ν , e − ) Ge 71 reaction using the TITAN mass-measurement facility at ISAC/TRIUMF. The measurements were performed in a Penning trap on neon-like 71 Ga 21+ and 71 Ge 22+ using isobar separation of the on-line produced mother and daughter nuclei through threshold charge breeding in an electron-beam ion trap. In addition, isoionic samples of 71 Ga 21+ and 71 Ge 21+ were stored concurrently in the Penning trap and provided a separate Q -value measurement. Both independent measurements result in a combined Q -value of 233.5 ± 1.2 keV , which is in agreement with the previously accepted Q -value for the ν cross-section calculations. Together with a recent measurement of the ν -response from the excited states in 71 Ge, we conclude that there are no further uncertainties in the nuclear structure, which could remove the persistent discrepancy between the SAGE and GALLEX calibration measurements performed with neutrinos from reactor-produced 51 Cr and 37 Ar sources and the theoretical expectation.

Numerous anomalous results in neutrino oscillation experiments can be attributed to interference of ~1 eV sterile neutrino. The specially designed to fully explore the Gallium anomaly Baksan Experiment on Sterile Transitions (BEST) starts next year. We investigate the sensitivity of BEST in searches for sterile neutrino mixed with electron neutrino. Then, performing the combined analysis of all the Gallium experiments (SAGE, GALLEX, BEST) we find the regions in model parameter space (sterile neutrino mass and mixing angle), which will be excluded if BEST agrees with no sterile neutrino hypothesis. For the opposite case, if BEST observes the signal as it follows from the sterile neutrino explanation of the Gallium (SAGE and GALLEX) anomaly, we show how BEST will improve upon the present estimates of the model parameters., Comment: 6 pages,4 figures

A 71Ga(3He, t)71Ge charge-exchange experiment was performed to extract with high precision the Gamow–Teller (GT) transition strengths to the three lowest-lying states in 71Ge, i.e., the ground state ( 1 / 2 − ) , the 175 keV ( 5 / 2 − ) and the 500 keV ( 3 / 2 − ) excited states. These are the relevant states, which are populated via a charged-current reaction induced by neutrinos from reactor-produced 51Cr and 37Ar sources. A precise measurement of the GT transition strengths is an important input into the calibration of the SAGE and GALLEX solar neutrino detectors and addresses a long-standing discrepancy between the measured and evaluated capture rates from the 51Cr and 37Ar neutrino calibration sources, which has recently spawned new ideas about unconventional neutrino properties.

The response function of a fast neutron capture-gated detector with a simple geometry has been simulated with the aim of studying the mechanism of its formation. The Geant4 package and the DGEOM program developed by the Institute for Nuclear Research especially for studying processes of fast neutron moderation in an organic medium have been used for numerical simulation of the response function. The response functions and the detection efficiencies of an actual detector for neutron energies of 1.2, 2.4, 5.0, and 14.6 MeV have been reconstructed and analyzed. The behavior of the response function has been comprehensively analyzed in the neutron energy range of 1–14 MeV. It is shown that our simulation adequately represents the experimental data and that the response function of the detector has a double-peak structure deteriorating its energy resolution. Taking into account the interaction with carbon for neutrons with energies of

We report on the development of a fast neutron detector using a liquid scintillator doped with enriched Li-6. The lithium was introduced in the form of an aqueous LiCl micro-emulsion with a di-isopropylnaphthalene-based liquid scintillator. A Li-6 concentration of 0.15 % by weight was obtained. A 125 mL glass cell was filled with the scintillator and irradiated with fission-source neutrons. Fast neutrons may produce recoil protons in the scintillator, and those neutrons that thermalize within the detector volume can be captured on the Li-6. The energy of the neutron may be determined by the light output from recoiling protons, and the capture of the delayed thermal neutron reduces background events. In this paper, we discuss the development of this 6Li-loaded liquid scintillator, demonstrate the operation of it in a detector, and compare its efficiency and capture lifetime with Monte Carlo simulations. Data from a boron-loaded plastic scintillator were acquired for comparison. We also present a pulse-shape discrimination method for differentiating between electronic and nuclear recoil events based on the Matusita distance between a normalized observed waveform and nuclear and electronic recoil template waveforms. The details of the measurements are discussed along with specifics of the data analysis and its comparison with the Monte Carlo simulation.

Photo-emf in the range hv = 0.76 − 1.35 eV was found in p-i-n structures produced from undoped GaAs crystals with known parameters; the current sensitivities in the impurity and intrinsic (hv > 1.35 eV) regions were comparable. It was proven that the impurity photovoltaic effect results from EL2 and EL3 structural defects creating deep donor levels in the forbidden zone. Calculations were performed that justified the possibility of observing this effect on the investigated structures.

Development of a capture gated spectrometer on the basis of a liquid organic scintillator doped with enriched 6Li is discussed. Particular interest is evoked by the good pulse height resolution of the spectrometer for 14-MeV neutrons, which is expected to be very high, ∼10–15%. This resolution is attained by compensating for the nonlinearity of the light yield in the scintillator owing to the use of separate optically isolated sections, which independently detect scintillations from each recoil proton. The detector is sensitive to fluence rates ranging from 10−4 to 102 cm−2 s−1 above a threshold of 500 keV under conditions of uncorrelated γ-ray background at a level of up to 102 s−1 (E > 100 keV). A pilot model of the detector based on a scintillator without a lithium dopant has been produced and tested. The detector efficiency is governed by the scintillator volume (∼1.2 l); for 3-MeV neutrons, its value is 0.2–0.5%. The response of the pilot detector to neutrons from a Pu-α-Be source with energies of up to 10 MeV has been measured. Initial testing indicates a low threshold at an ∼600-keV energy of a recoil proton. A good spectral response is obtained using the criterion that three optical sections of the detector operate at a time. This spectrometer can find application in low-background experiments in basic physics research, as well as in space research and nuclear medicine for measuring the parameters of the neutron flux.

Bulk GaAs crystals were grown by synthesis solute diffusion (SSD) technique in a wide range of growth temperatures between 990 and 1150 °C. Electrical properties of these crystals were studied by means of van der Pauw, admittance spectroscopy, deep levels transient spectroscopy and photoinduced current spectroscopy techniques. It was shown that the main defects determining the properties were the GaAs antisites acceptors and the A center acceptors with the levels, respectively, Ev +0.078 eV and Ev +0.43 eV. The conductivity of the grown crystals was p-type and showed a pronounced maximum at a level of 104–105 Ω cm for growth temperatures between 1020 and 1080 °C. If the crystals were additionally compensated either by unintentional Si donors contamination from quartz crucibles or by intentional light Te doping one could get semi-insulating material with the room temperature resistivity higher than 106 Ω cm. The Fermi level in such crystals was pinned near Ec −0.8 eV, i.e. close to the EL2 donors. Measurements by deep levels transient spectroscopy on n-type doped crystals or by low frequency capacitance–voltage on semi-insulating crystals showed that the density of EL2 in these samples was in the low 1014 cm−3 and that thus the EL2 donors were not the main compensating agents.

The activity of an artificial neutrino source based on 37Ar was measured by a specially developed method of directly counting 37Ar decays in a proportional counter. This source was used to irradiate the target of the SAGE radiochemical gallium—germanium neutrino telescope at the Baksan Neutrino Observatory (Institute for Nuclear Research, Russian Academy of Sciences, Moscow), whereupon the measurements were performed at the Institute of Reactor Materials (Zarechny, Sverdlovsk oblast, Russia). The method used to prepare gaseous samples for measurements in proportional counters and the counting procedure are described. The measured activity of the 37Ar neutrino source is 405.1 ± 3.7 kCi (corrected for decays that occurred within the period between the instant of activity measurement and the commencement of the irradiation of Ga target at 04:00 Moscow time, 30.04.2004).

In April 2004, a neutrino source was produced by irradiating a 330-kg piece of pressed calcium oxide at the fast-neutron reactor BN-600 (Zarechny, Russia) for six months. The 37Ar isotope was obtained via the (n, α) reaction on 40Ca, and 37Ar was extracted from an aqueous solution of nitric acid in which the solid target was dissolved. After that, 37Ar was purified and sealed into a capsule. This source was used to measure the neutrino-capture rate in metalic gallium for neutrinos from 37Ar decay, which have an energy close to that of the main line of solar 7Be neutrinos (863 keV). The target of the SAGE Gallium-Germanium Neutrino Telescope was irradiated by using this source at the Baksan Neutrino Observatory (Institute for Nuclear Research, Russian Academy of Sciences). The source activity was measured by several methods during its production, in the course of irradiation, and after its completion. The weighted mean of the activity for six measurements was 409 ± 2 kCi at the beginning of irradiation of the gallium target (04:00 Moscow time, 30.04.2004). The scatter in the activity values obtained by different methods does not exceed 5%.

We discuss the development of a spectrometer based on full energy absorption using liquid scintillator doped with enriched 6Li. Of specific interest, the spectrometer is expected to have good pulse height resolution, estimated to lie in the range 10–15% for 14-MeV neutrons. It should be sensitive to flux rates from 10−6 cm−2 s−1 to 106 cm−2 s−1 above a threshold of 500 keV in an uncorrelated γ background of up to 104 s−1. We have constructed a pilot version of the detector using undoped liquid scintillator, and we report its present status. The detector’s efficiency is determined by the volume of the scintillator (∼1.21) and is estimated to be 0.2–0.5% for 3-MeV neutrons. The good pulse height resolution is achieved by compensation of the nonlinear light yield of the scintillator due to the use of optically separated segments, which collect scintillations from each recoil proton separately. We demonstrate here the response of the detector to neutrons from a Pu-α-Be source, whose energies range up to 10 MeV. Initial testing indicates a low threshold (≈600 keV) and good spectral response after requiring a multiplicity of three segments. Such a spectrometer has applications for low-background experiments in fundamental physics research, characterizations of neutron flux in space, and the health physics community.

A radiochemical 71 Ga- 71 Ge experiment to determine the primary flux of neutrinos from the Sun began measurements of the solar neutrino flux at the Baksan Neutrino Observatory in 1990. The number of 71 Ge atoms extracted from 30 tons of gallium in 1990 and from 57 tons of gallium in 1991 was measured in 12 runs during the period of January 1990 to December 1991. The combined 1990 and 1991 data-sets give a value of 58 + 17/ —24 (stat) ± 14 (syst) SNU. This is to be compared with 132 + 7/ —5 SNU predicted by the Standard Solar Model.

A precision measurement of the Ga-71(He-3,t)Ge-71 charge-exchange reaction was performed. By using a rather complete set of theoretical form factors to describe the cross-section angular distributions over a large angular range, the Gamow-Teller strength distribution up to the effective neutron-separation energy in Ge-71 was extracted. The data and the analysis constrain the Ga-71(nu(e),e(-)) solar neutrino rate in a neutrino nonoscillation scenario. For nonoscillating neutrinos we report a solar neutrino capture rate of 122.4 +/- 3.4(stat) +/- 1.1(sys) SNU, which is lower than the presently accepted value of 132 +/- 18 SNU, though not in disagreement given the quoted errors.

A technique to search for 2K-capture of $^{78}$Kr with large low-background proportional counter filled with an enriched in $^{78}$Kr up to 99.8% sample of Krypton at a pressure of 4.51 is described in this paper. The results of first measurements are presented. Analysis of data collected during 159 hours yielded new limit to the half-life of $^{78}$Kr with regard to 2K-capture (T$_{1/2}\geq6\cdot10^{21}$ yr (90% C.L.)). Sensitivity of the facility to the process for one year of measurement was evaluated to be $\texttt{S}=1.0\cdot10^{22}$ yr (90% C.L.)., Comment: 4 pages, 5 figures; talk at the NANP'05 Conference; submitted to Phys. At. Nucl

A design of an artificial neutrino source with an activity of 400 kCi and a technology for filling it with 37Ar are described. The compliance of this source to Russian and international safety standards is confirmed by the test results.

The results of development of the technology and the design and operation of a facility intended for production of a radionuclide-37Ar-based neutrino source with an activity of 0.4 Mci are presented. This facility ensures extraction of 37Ar from 330 kg of calcium oxide irradiated in the ƂH-600 reactor and the subsequent purification and collection of 37Ar into the source. The efficiency of 37Ar extraction and collection into the source’s ampoule is 98.4 ± 0.2%. The content of 37Ar in the source’s ampoule is 96.5% (at the moment it is filled). The first measurements of the source’s activity yielded a value of 416±3 kCi. The neutrino source developed on the basis of 37Ar was used to calibrate the SAGE gallium-germanium solar neutrino telescope.

A method for measuring the detector counting rates of up to 106 s−1 with an accuracy better than 1% using a digital oscilloscope is described. An example of measuring the counting rate of 37Ar decays in proportional counters is considered.

The characteristics of detectors based on bulk semi-insulating GaAs (SI-GaAs) have been studied by α particle detection and spectrometry. A distinctive feature of these detectors is the dependence of the width of the space charge region W on reverse bias voltage U. The rate of increase in W(U) is ∼1 µm/V, which permits formation of a sensitive region a few millimeters thick. The main obstacle to applying kilovolt-range bias voltages U is the reverse current noise. The characteristics of diode structures in which a rectifying barrier to SI-GaAs was formed by metal deposition (Schottky diodes) and by growing heterostructures with heavily doped AlGaAs or GaAsSb epitaxial layers were compared. Nonequilibrium carrier transport in epitaxial structures capable of sustaining bias voltages above 1 kV was investigated in both weak (below 1 kV/cm) and strong (10–30 kV/cm) electric fields. In both cases, the carrier lifetimes were found to be about a few nanoseconds. Such low values are due to the high concentration of trapping centers (EL2-type native defects), which limits the carrier transport. An analysis of the spectral line shape revealed that the lifetime is almost constant throughout the detector volume. The charge introduced by a particle was found to be enhanced in fields of ∼30 kV/cm. This effect can be qualitatively explained by focusing the electric field lines at the vertex of the α-particle track, which leads to an increase in the local field strength to ∼10−5 V/cm and impact ionization by nonequilibrium electrons.