Fermi theory of nuclear beta decay and heavy neutrino searches.
We examine the data analysis of nuclear β decay of 3 H, 14 C, 35 S, and 63 Ni, using a relativistic Fermi function for a Hulthen screened field. This Fermi function does not differ much from that used by Hime, Jelley, and Simpson in the cases of light nuclei, but in the cases of sulfur and nickel reduces significantly the claimed spectral excess and, hence, the mixing of the heavy neutrino. We also discuss radiative corrections to 35 S data