First observation of single beta decay of $^{96}$Zr
The single beta decay of $^{96}$Zr has been detected for the first time using a 211 cm$^3$ low-background HPGe detector and an external source consisting of two samples of enriched zirconium (atomic fraction of $^{96}$Zr is 88.28%, total mass is 140.65 g). During the search for the $\beta$ decay of $^{96}$Zr, the $\beta$ decay of the daughter nucleus $^{96}$Nb to the excited states of $^{96}$Mo has been observed. The $\gamma$-ray cascade produced by the $^{96}$Mo nucleus while de-exciting to the ground state has been detected with the HPGe detector. The experiment has been carried out at the Baksan Neutrino Observatory. It has produced 12625.34 h of data. The half-life of the single beta decay of $^{96}$Zr is measured to be $T_{1/2} = [2.27^{+0.53}_{-0.36}(stat) \pm 0.27(syst)]\times10^{20}$ yr.