A concise technical characteristic of a new low-background laboratory DULB-4900 of the BNO INR RAS is presented. 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 γ-radiation of its daughter 214 Bi distributed over the volume of the low-background box. The results of the data analysis are presented.

A method and results of an experimental test of the time stability of the half-life of alpha-decay 214 Po nuclei are presented. Two underground installations aimed at monitoring the time stability have been constructed. Time of measurement exceeds 1038 days for one set up and 562 days for the other. It was found that amplitude of possible annual variation of 214 Po half-life does not exceed 0.2% of the mean value. The limit on the deviation of the decay curve from exponent at 0.034 · T 1/2 < t < 0.1 · T 1/2 range has been found.

Features of data accumulated at 1817 hours in the experimental search for 2K(2ν)-capture decay mode of 78 Kr are discussed. The new limit for this decay half-life is found to be T 1/2 ≥ 2.3 × 10 20 yr. (90% C.L.).

Search for ββ-decay of 136 Xe with two high pressure proportional counters is carried out in Baksan Neutrino Observatory. The experiment is based on comparison of spectra measured with natural and enriched xenon. The measured half life is equal to T 1/2 = 5.5 +4.6 −1.7 · 10 21 yr (67% C.L.) for ββ2ν decay mode. No evidence has been found for neutrinoless ββ-decay. The decay half life limit based on data measured during 17280 h is T 1/2 (ββ0ν) ≥ 4.9 · 10 23 yr (90% C.L.).

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 ≥ 6 · 10 21 yr (90% C.L.)). Sensitivity of the facility to the process for one year of measurement was evaluated to be S = 1.0 · 10 22 yr (90% C.L.).