Investigation of rare nuclear decays with BaF₂ crystal scintillator contaminated by radium

Abstract.

The radioactive contamination of a BaF₂ 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 ²¹²Po (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 ²²²Rn nuclide is known as 100% decaying via the emission of the $\alpha$ particle with T _1/2 = 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 ²²²Rn 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 ²¹²Pb, ²²²Rn and ²²⁶Ra relatively to 2$\beta$ decays are also improved in comparison with the earlier results.