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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 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 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.

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.

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.

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.

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

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.

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 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.

The effect of external γ radiation on the process of counting 71Ge decays in the proportional counters of the SAGE experiment measuring the solar-neutrino flux is considered. The systematic uncertainty in the SAGE result due to radon decays inside the air volume surrounding the counters is estimated. The background counting rate in the proportional counters that is caused by γ radiation from the enclosing shield is also determined.

The solar neutrino capture rate measured by the Russian-American Gallium Experiment on a metallic gallium target SAGE during the time from January 1990 through December 2000 is 77.0 −6.2 −3.0 +6.2 +3.5 SNU, where the uncertainties are statistical and systematic, respectively. The experimental procedures and data analysis are presented.

We present measurements of the solar neutrino capture rate on metallic gallium in the Soviet-American gallium experiment (SAGE) over a period of slightly more than half the 22-year solar cycle. A combined analysis of 92 runs over the twelve-year period from January 1990 until December 2001 yields a capture rate of 70.8 −5.2 +5.3 (stat) −3.2 +3.7 (sys) SNU for solar neutrinos with energies above 0.233 MeV. This value is slightly more than half the rate predicted by the standard solar model, 130 SNU. We present the results of new runs since April 1998 and analyze all runs combined by years, months, and bimonthly periods beginning in 1990. A simple analysis of the SAGE results together with the results of other solar neutrino experiments gives an estimate of (4.6±1.2)× 1010 neutrinos cm−2 s−1 for the flux of the electron pp neutrinos that reach the Earth without changing their flavor. The flux of the pp neutrinos produced in thermonuclear reactions in the Sun is estimated to be (7.6 ± 2.0) × 1010 neutrinos cm−2 s−1, in agreement with the value of (5.95±0.06)×1010 neutrinos cm−2 s−1 predicted by the standard solar model.

Data on a direct determination of systematic uncertainties caused by the background production of germanium isotopes in the radiochemical SAGE experiment measuring the solar-neutrino flux are analyzed. The result obtained for the rate of 68Ge production is 6.5(1±1.0) times greater than the expected one; the rate of 69Ge production does not exceed preliminary estimates. The above result for 68Ge corresponds to the systematic uncertainty that is caused by the interaction of cosmic-ray muons and which is equal to 5.8% (4.5 SNU) at a solar-neutrino-capture rate of 77.0 SNU. An experiment is proposed that would test the effect of cosmic-ray muon influence on the SAGE systematic uncertainty and which would be performed at the location of the underground scintillation telescope facilities of the Baksan Neutrino Observatory (Institute for Nuclear Research, Russian Academy of Sciences).

A method for estimating the systematic uncertainty associated with radon in the SAGE experiment1) aimed at observing the solar-neutrino flux is described. For the gallium target used in this experiment, the systematic uncertainty in the measured neutrino-capture rate of 75 SNU2) is below 0.3 SNU.

The parameters of the gas mixture for the proportional counters have been optimized using calibrations with 55Fe and Cd-Se sources. As a result, the accuracy in determining the energy region of 71Ge decays detected in the SAGE experiment with the aim of measuring the solar neutrino flux has been improved.

The fast-neutron flux in one of the deep underground facilities situated at a depth of 4900 mwe in the Baksan Neutrino Observatory are measured. The relative neutron-shielding properties of several commonly available natural materials are also investigated. Preliminary results obtained with a highly sensitive fast-neutron spectrometer at a sensitivity level of about 10−7 neutron cm−2 s−1 are presented and discussed.

The solar-neutrino-capture rate measured by the Russian-American Gallium Experiment on metallic gallium during the period from January 1990 to December 1997 is (67.2 −7.0−3.0 +7.2+3.5 ) SNU, where the uncertainties are statistical and systematic, respectively. This result represents a 7σ depression in the neutrino flux in relation to the predicted standard-solar-model rates. The experimental procedures used and data analysis are presented.