Evidence for the Exchange Effect Down to Very Low Energy in the Beta Decays of $$^{63}$$ 63 Ni and $$^{241}$$ 241 Pu

Abstract

Metallic magnetic calorimeters have been used for several years for the determination of the shapes of beta spectra. Experimental spectra of the beta decays of 63 Ni and 241 Pu exhibit, at very low energy, strong deviations from standard the- oretical calculations including corrections for screening, radiative, and nuclear size effects. These discrepancies can be attributed to the atomic exchange effect. In partic- ular the spectrum of 63 Ni measured with an electroplated 63 Ni source is in excellent agreement with theory including the exchange effect. This underlines the quality of theexperimentalspectrumaswellasthecorrectnessofthecalculationoftheexchange effect.