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

Abstract

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 $\hat{I}\pm$ particle with $T_{1/2}$ = 3.82 d; however, its ${\hat{I}}^2$ decay is also energetically allowed with $Q_{\hat{I}}^2=24\pm 21$ keV. Search for decay chains of events with specific pulse shapes characteristic for $\hat{I}\pm$ or for ${\hat{I}}^2/{\hat{I}}^3$ 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 ${\hat{I}}^2$ decay as $B_{\hat{I}}^2<0.13$% at 90% C.L. (equivalent to limit on partial half-life $T_{1/2}^2>8.0$ y). Half-life limits of ${}^{212}$Pb, ${}^{222}$Rn and ${}^{226}$Ra relatively to $2{\hat{I}}^2$ decays are also improved in comparison with the earlier results.

10 pages, 9 figures, 2 tables