Total β-electron spectra of 214Pb and 214Bi

Beta electrons (electrons emitted in β decays) are a common background in rare-events experiments searching for beyond-thestandard-model physics in double beta decays, in dark-matter detection, etc. One particular culprit for the background events are the 220,222Rn radioactive chains leading to β − decays of 212,214Pb and 212,214Bi, common contaminants in the rare-events experiments. We have computed the total β-electron spectra of these decays using a state-of-the-art β-decay theory. The β decays involve numerous first-forbidden non-unique transitions depending strongly on the wave functions of the decay mother and daughter nuclei through the many involved nuclear matrix elements (NME). These NME have been computed by using the interacting shell model (ISM) with the khpe effective Hamiltonian. This Hamiltonian leads to a nice description of the energy spectra of the involved nuclei. We are able to reproduce the measured branching ratios of all the decay transitions by adopting the small relativistic NME (sNME) as a fit parameter. Here we report detailed results for the decays of 214Pb and 214Bi. We are confident that these calculations help the present and future rare-events experiments better identify their β-electron backgrounds stemming from the radon β-decay chains.