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

TL;DR

This study measured contamination levels and decay properties of radium isotopes in a BaF$_2$ crystal scintillator, setting new limits on rare decay modes and improving existing half-life constraints.

Contribution

First measurement of radium contamination in BaF$_2$ scintillator and new limits on rare decay modes of radium isotopes.

Findings

Measured $^{212}$Po half-life as 298.8 ns.

Set limit on $^{222}$Rn beta decay branching ratio below 0.13%.

Improved half-life limits for $^{212}$Pb, $^{222}$Rn, and $^{226}$Ra.

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 $\alpha$ particle with $T_{1/2}$ = 3.82 d; however, its $\beta$ decay is also energetically allowed with $Q_\beta = 24\pm21$ 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 $^{222}$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). Half-life limits of $^{212}$Pb, $^{222}$Rn and $^{226}$Ra relatively to $2\beta$ decays are also improved in comparison with the earlier results.