Evidence for the exchange effect in the β decay of 241 Pu

The exchange effect has been previously given as a possible explanation for a significant deviation from an allowed shape observed at low energy in the ${}^{241}$Pu $\ensuremath{\beta}$ spectrum. Calculations set out here confirm that this atomic effect explains a large part of this deviation. The equations needed to calculate the exchange effect are detailed, as well as the evaluation of the confluent hypergeometric function for complex arguments of large magnitudes. After a review of the possible other effects that could explain the remaining discrepancy at low energy, the screening correction using effective nuclear charges seems to be the best explanation. For radionuclides with high $Z$, this work has demonstrated the necessity to take into account the spatial variation of the nuclear charge experienced by the ejected electron to accurately correct for the screening effect.