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Investigation of rare nuclear decays with BaF$_2$ crystal scintillator contaminated by radium

The radioactive contamination of a BaF$_2$ 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 $^{212}$Po (present in the crystal scintillator due to contamination by radium) was measured as $T_{1/2}(^{212}$Po) = 298.8$\pm$0.8(stat.)$\pm$1.4(syst.) ns by analysis of the events' pulse profiles. The $^{222}$Rn nuclide is known as 100% decaying via emission of $α$ particle with $T_{1/2}$ = 3.82 d; however, its $β$ decay is also energetically allowed with $Q_β= 24\pm21$ keV. Search for decay chains of events with specific pulse shapes characteristic for $α$ or for $β/γ$ signals and with known energies and time differences allowed us to set, for the first time, the limit on the branching ratio of $^{222}$Rn relatively to $β$ decay as $B_β< 0.13$% at 90% C.L. (equivalent to limit on partial half-life $T_{1/2}^β> 8.0$ y). Half-life limits of $^{212}$Pb, $^{222}$Rn and $^{226}$Ra relatively to $2β$ decays are also improved in comparison with the earlier results.