Experimental searches for rare alpha and beta decays

TL;DR

This review summarizes recent experimental advances in detecting rare alpha and beta decays with extremely long half-lives, highlighting new observations enabled by improved techniques and underground experiments.

Contribution

It reports the first observations of several rare alpha decays and the lowest known Q-value beta decay, demonstrating progress in experimental detection of extremely rare nuclear decays.

Findings

First observation of alpha decays of $^{151}$Eu, $^{180}$W, $^{190}$Pt, and $^{209}$Bi.

Detection of beta decay of $^{115}$In to an excited state with the lowest known Q-value.

Identification of other rare decays in isotopes like $^{48}$Ca, $^{50}$V, and $^{96}$Zr.

Abstract

The current status of the experimental searches for rare alpha and beta decays is reviewed. Several interesting observations of alpha and beta decays, previously unseen due to their large half-lives ($10^{15} - 10^{20}$ yr), have been achieved during the last years thanks to the improvements in the experimental techniques and to the underground locations of experiments that allows to suppress backgrounds. In particular, the list includes first observations of alpha decays of $^{151}$Eu, $^{180}$W (both to the ground state of the daughter nuclei), $^{190}$Pt (to excited state of the daughter nucleus), $^{209}$Bi (to the ground and excited states of the daughter nucleus). The isotope $^{209}$Bi has the longest known half-life of $T_{1/2} \approx 10^{19}$ yr relatively to alpha decay. The beta decay of $^{115}$In to the first excited state of $^{115}$Sn (E$_{exc} = 497.334$ keV), recently observed for the first time, has the $Q_\beta$ value of only $(147 \pm 10)$ eV, which is the lowest $Q_\beta$ value known to-date. Searches and investigations of other rare alpha and beta decays ($^{48}$Ca, $^{50}$V, $^{96}$Zr, $^{113}$Cd, $^{123}$Te, $^{178m2}$Hf, $^{180m}$Ta and others) are also discussed.