Exchange effects in. beta. decays of many-electron atoms

The effect on the {beta} spectrum of exchange between bound and continuum electrons is discussed for decay of a many-electron atom. Results of calculations of the exchange distortion of the {beta} spectrum are presented using both a screened hydrogenic and a Hartree-Fock approximation for the electron wave functions. The results of the two approaches agree well and show that the inclusion of exchange leads to an enhancement of the {beta} spectrum, particularly at low electron energy. This corrects an error in a previous calculation that found that exchange decreased the {beta} intensity. For the low-energy {beta}{sup {minus}} emitters {sup 106}Ru and {sup 241}Pu, the present calculation indicates that the enhancement due to exchange is of the order of several percent over much of the spectrum, becoming larger at very low electron energy. Exchange with 1{ital s} electrons dominates in the high-energy part of the spectrum, but exchange with {ital ns} ({ital n}{ge}2) electrons becomes significant in the low-energy region. The inclusion of exchange leads to an increase in the phase-space integral by 6.4% for {sup 106}Ru and 7.5% for {sup 241}Pu. Results are presented for exchange effects in other {beta} spectra, including those of {sup 14}C and {sup 35}S, inmore » which experimentally measured distortions have been interpreted as evidence for a heavy antineutrino of mass 17 keV. The distortions due to exchange are found to be significantly smaller than the measured distortions.« less