Measurement of the electron capture of 76 As into the first excited state of 76 Ge

The neutrinoless double beta decay of ⁷⁶Ge 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 ⁷⁶As ground state to the nuclear matrix element can be investigated via the branching ratios of its β⁻ and electron capture decay. While energetically, the electron capture of ⁷⁶As into the first excited state of ⁷⁶Ge is possible and was measured once before this work, the electron capture into the ⁷⁶Ge ground state was not observed yet. The present study investigates the branching of ⁷⁶As that is produced via ⁷⁵As(n,$γ$) on a thin As₂O₃ 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 562.9$\,$keV $γ$-rays emitted after electron capture into the excited state. Investigation of coincident signals in both detectors leads to the branching ratio of the ⁷⁶As electron capture into the first excited state of ⁷⁶Ge of νEC^ = (0.0572 ± 0.0029 (stat.) ± 0.0074(syst.))%. This is the first measurement with the full uncertainty budget quantified.