Low Energy Beta-Ray Spectra:Pm147S35

The beta-spectra of ${\mathrm{S}}^{35}$ and ${}_{61}{}^{}{}_{}{}^{}\mathrm{Pm}_{}^{147}{}_{}{}^{}$ have been measured in order to study further the nature of any low energy deviation from the Fermi theory of beta-decay. Measurements were made with thin, relatively uniform sources in both the 40-cm radius of curvature spectrometer and also in a small 180 degree focusing Helmholtz coil spectrometer designed specifically for low energy spectra. The thinnest sources were less than 10 micrograms/${\mathrm{cm}}^2$. Using Zapon counter windows ranging from 15 to 3 micrograms/${\mathrm{cm}}^2$ and also a windowless counter technique, Fermi plots were obtained which showed how the measured distribution of particles at low energy depends on both source and counter window thickness.