In-Trap Decay Spectroscopy of Radioactive Nuclei at TITAN∕TRIUMF for a Determination of 2νββ Matrix Elements

To extract the effective neutrino Majorana mass from an observed 0νββ decay, precise knowledge of the involved nuclear matrix element is required. This poses a challenge to nuclear structure models. In particular, calculations based on the proton‐neutron Quasiparticle Random Phase Approximation show significant discrepancies with experimental electron capture (EC) rates of the intermediate nucleus even when tuned on 2νββ decay rates. However, EC branching ratios (BR) are in many cases not well known because the strengths of these decay branches are at the sensitivity limit of traditional experimental techniques. Here, we present a novel approach at TITAN/TRIUMF to measure EC‐BR and to overcome shortcomings of previous methods. We also present results of a first proof‐of‐principle experiment which represents the very first observation of an electron capture in a Penning trap.