Measurement of the Electron capture of ${}^{76}$As into the first excited state of ${}^{76}$Ge

Abstract

The neutrinoless double beta decay of ${}^{76}$Ge is searched for in the large-scale experiment LEGEND. The measurement of the half-life of this process would give access to the neutrino mass using the nuclear matrix element. Experimentally the contribution of the ${}^{76}$As ground state to the nuclear matrix element can be investigated via the branching ratios of its ${\beta }^{-}$ and electron capture decay. While energetically, the electron capture of ${}^{76}$As into the first excited state of ${}^{76}$Ge is possible and was measured once before this work, the electron capture into the ${}^{76}$Ge ground state was not observed yet. The present study investigates the branching of ${}^{76}$As that is produced via ${}^{75}$As(n,$\gamma$) on a thin As${}_2$O${}_3$ sample. A silicon drift detector measures characteristic X-rays emitted by the germanium atoms caused by an inner vacancy after the electron capture. A high-purity germanium detector is used to measure the 562.9keV $\gamma$-rays emitted after electron capture into the excited state. Investigation of coincident signals in both detectors leads to the branching ratio of the ${}^{76}$As electron capture into the first excited state of ${}^{76}$Ge of ${\nu }_{{\mathrm{EC}}^{\ast }}=(0.0572\pm 0.0029(stat.)\pm 0.0074(syst.))\%$. This is the first measurement with the full uncertainty budget quantified.