Investigation of rare nuclear decays with BaF crystal scintillator contaminated by radium.

The radioactive contamination of a BaF scintillation crystal with mass of 1.714 kg was measured over 101 hours in the low-background DAMA/R&D set-up deep underground (3600 m w.e.) at the Gran Sasso National Laboratories of INFN (LNGS, Italy). The half-life of Po (present in the crystal scintillator due to contamination by radium) was measured as $ T_{1/2}({}^{212}Po)$ = 298.8±0.8( stat.)±1.4( syst.) ns by the analysis of the events' pulse profiles. The Rn nuclide is known as 100% decaying via the emission of the $ \alpha$ particle with T = 3.82 d; however, its $ \beta$ decay is also energetically allowed with $ Q_{\beta}=24\pm 21$ keV. Search for decay chains of events with specific pulse shapes characteristic for $ \alpha$ or for $ \beta/\gamma$ signals and with known energies and time differences allowed us to set, for the first time, the limit on the branching ratio of Rn relatively to $ \beta$ decay as $ B_{\beta} < 0.13$ % at 90% C.L. (equivalent to limit on partial half-life $ T_{1/2}^{\beta}> 8.0$ y). The half-life limits of Pb, Rn and Ra relatively to 2 $ \beta$ decays are also improved in comparison with the earlier results. [ABSTRACT FROM AUTHOR]