A. M. Gangapshev

The Large Enriched Germanium Experiment for Neutrinoless Double Beta Decay (LEGEND)

The observation of neutrinoless double-beta decay (0νββ) would show that lepton number is violated, reveal that neu-trinos are Majorana particles, and provide information on neutrino mass. A discovery-capable experiment covering the inverted ordering region, with effective Majorana neutrino masses of 15 - 50 meV, will require a tonne-scale experiment with excellent energy resolution and extremely low backgrounds, at the level of ∼0.1 count /(FWHM·t·yr) in the region of the signal. The current generation 76Ge experiments GERDA and the Majorana Demonstrator, utilizing high purity Germanium detectors with an intrinsic energy resolution of 0.12%, have achieved the lowest backgrounds by over an order of magnitude in the 0νββ signal region of all 0νββ experiments. Building on this success, the LEGEND collaboration has been formed to pursue a tonne-scale 76Ge experiment. The collaboration aims to develop a phased 0νββ experimental program with discovery potential at a half-life approaching or at 1028 years, using existing resources as appropriate to expedite physics results.

Working characteristics of the New Low-Background Laboratory(DULB-4900)

The Baksan Neutrino Observatory (BNO) of Institute for Nuclear Research of the Russian Academy of Science is one of major old underground laboratories in the world for experiments in the field of a nuclear physics, particle and astroparticle physics. A concise technical characteristic of a new lowbackground laboratory DULB-4900 of the BNO is presented. The laboratory is located at a distance of 3700 m from the main entry to the Observatory tunnel, in the hall of 6 6 40 m 3 . Thickness of the mountain rock over DULB corresponds to 4900 m:w:e:, thereby decreasing cosmic ray flux by 10 7 times. The technique and the results of background measurements in the Hall, ordinary box and low-background box are presented. 222 Rn contamination in the laboratory air has been measured by direct detection of -radiation of its daughter 214 Bi distributed over the volume of the lowbackground box. The results of the data analysis are presented.

Results of experiments devoted to searches for 2K capture on 78Kr and for the double-beta decay of 136Xe with the aid of proportional counters

A brief description of two low-background setups deployed at the Baksan Neutrino Observatory (Institute for Nuclear Research, Russian Academy of Sciences) and intended for searches for two types of double-beta decay of inert-gas isotopes—2K capture on 78Kr and the double-beta decay of 136Xe—is given. The two setups in question have similar structures and employ identical large high-pressure copper proportional counters as detectors. Upon a treatment of data from measurements with krypton samples differing in the content of the isotope 78Kr, the spectrum for an enriched sample revealed an excess of events at a statistical-significance level of about two standard deviations (2σ). If one attributes this excess to 2K(2ν) capture on 78Kr, the respective half-life is T1/2 = 1.4−0.7+2.3 × 1022 yr at a 90% C.L. A treatment of data from measurements with xenon samples differing in content of the isotope 136Xe led to the appearance of an excess of events in the spectrum for an enriched sample at a statistical-significance level of about 2.2σ. If one assumes that this excess is due to the two-neutrino double-beta decay of 136Xe, then the respective half-life is T1/2 = 5.8−1.8+4.7 × 1021 yr.

Sources of the systematic errors in measurements of 214Po decay half-life time variations at the Baksan deep underground experiments

The design changes of the Baksan low-background TAU-1 and TAU-2 set-ups allowed to improve a sensitivity of 214Po half-life (τ) measurements up to the 2.5 × 10−4 are described. Different possible sources of systematic errors influencing on the τ-value are studied. An annual variation of 214Po half-life time measurements with an amplitude of A = (6.9 ± 3) × 10−4 and a phase of φ = 93 ± 10 days was found in a sequence of the week-collected τ-values obtained from the TAU-2 data sample with total duration of 480 days. 24 hours’ variation of the t-value measurements with an amplitude of A = (10.0 ± 2.6) × 10−4 and phase of φ = 1 ± 0.5 hours was found in a solar day 1 hour step t-value sequence formed from the same data sample. It was found that the 214Po half-life averaged at 480 days is equal to 163.45 ± 0.04 μs.

New stage of search for 2K(2ν) capture of 78Kr

A technique to search for 2K capture of 78Kr with a large low-background proportional counter filled with a sample of krypton enriched in 78Kr up to 99.8% at a pressure of 4.51 atm is described in this paper. The results of first measurements are presented. An analysis of data collected for 159 h yielded a new limit to the half-life of 78Kr with regard to 2K capture (T1/2 ≥ 1.5 × 1021 yr (90% C.L.)). The sensitivity of the facility to the process for one year of measurement was evaluated to be S = 1.0 × 1022 yr (90% C.L.).

Results of a search for 2β decay of 136Xe with high-pressure copper proportional counters in Baksan Neutrino Observatory

A search for 2β decay of 136Xe with two high-pressure copper proportional counters was carried out in the Baksan Neutrino Observatory. The experiment is based on comparison of spectra measured with natural and enriched xenon. No evidence was found for 2β(2ν) and 2β(0ν) decay. The decay half-life limit based on data measured for 8000 h is T1/2 ≥ 8.5 × 1021 yr for 2ν mode and T1/2 ≥ 3.1 × 1023 yr for 0ν mode (90% C.L.).

First result of the experimental search for the 2K-capture of 124Xe with the copper proportional counter

First result of experiment for searching of 2K-capture of 124Xe with the large-volume copper proportional counter (LPC) is given. The 12 liters sample with 63.3% (44 g) of 124Xe was used in measurements. The limit on the half-life of 124Xe with regard to 2K(2ν)-capture for the ground state of 124Te (0+ → 0+, g.s.) has been found: T1/2 ≥ 4.67 × 1020 yr (90% C.L.). A sample with volume 52 liters comprising of 124Xe (10.6L - 58.6 g) and 126Xe (14.1L - 79.3 g) will used at the next step of the experiment to increase a sensitivity of 2K-caption of 124Xe registration. In this case sensitivity to the investigated process will be at the level of S = 1.46 × 1021 yr (90% C.L.) for 1 year measurement.

Status of the Germanium Detector Array (GERDA) in the search of neutrinoless ββ decays of 76Ge at LNGS

The Germanium Detector Array (GERDA) in the search for neutrinoless ββ decays of 76Ge at LNGS will operate bare germanium diodes enriched in 76Ge in an (optional active) cryogenic fluid shield to investigate neutrinoless ββ decay with a sensitivity of T1/2 > 2 × 1026 yr after an exposure of 100 kg yr. Recent progress includes the installation of the first underground infrastructures at Gran Sasso, the completion of the enrichment of 37.5 kg of germanium material for detector construction, prototyping of low-mass detector support and contacts, and front-end and DAQ electronics, as well as the preparation for construction of the cryogenic vessel and water tank.

A technique for searching for the 2K capture in 124Xe with a copper proportional counter

An experimental technique for searching for the 2K capture in 124Xe with a large low-background copper proportional counter is described. Such an experiment is conducted at the Baksan Neutrino Observatory of the Institute for Nuclear Research of the Russian Academy of Sciences. The experimental setup is located in the Low-Background Deep-Level Laboratory at a depth of 4900 m.w.e., where the flux of muons of cosmic rays is suppressed by a factor of 107 relative to that at the Earth’s surface. The setup incorporates a proportional counter and low-background shielding (18 cm of copper, 15 cm of lead, and 8 cm of borated polyethylene). The results of processing the data obtained in 5 months of live measurement time are presented. A new limit on the half-life of 124Xe with respect to the 2K capture is set at the level of 2.5 × 1021 years.

Large-Volume Detector at the Baksan Neutrino Observatory for Studies of Natural Neutrino Fluxes for Purposes of Geo- and Astrophysics

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.