Precise 113Cd β decay spectral shape measurement and interpretation in terms of possible 𝑔�𝐴� quenching

Abstract

Highly forbidden β decays provide a sensitive test to nuclear models in a regime in which the decay goes through high spin-multipole states, similar to the neutrinoless double-β decay process. There are only 3 nuclei (50V, 113Cd, 115In) which undergo a 4th forbidden non-unique β decay. In this work, we compare the experimental 113Cd spectrum to theoretical spectral shapes in the framework of the spectrumshape method. We measured with high precision, with the lowest energy threshold and the best energy resolution ever, the β spectrum of 113Cd embedded in a 0.43 kg CdWO4 crystal, operated over 26 days as a bolometer at low temperature in the Canfranc underground laboratory (Spain). We performed a Bayesian fit of the experimental data to three nuclear models (IBFM-2, MQPM and NSM) allowing the reconstruction of the spectral shape as well as the half-life.The fit has two free parameters, one of which is the effective weak axial-vector coupling constant, geff A , which resulted in geff A between 1.0 and 1.2, compatible with a possible quenching. Based on the fit,wemeasured the half-life of the 113Cd β decay including systematic uncertainties as 7.73+0.60−0.57 ×1015 yr, in agreement with the previous experiments. These results represent a significant step towards a better understanding of low-energy nuclear processes.