Study of the Beta-Spectra ofC14andS35

Using sources of high specific activity, the beta-spectra of ${\mathrm{C}}^{14}$ and ${\mathrm{S}}^{35}$ have been investigated in a large magnetic spectrometer. Since the distributions are measured under essentially identical conditions, a direct comparison of the data is possible. The ${\mathrm{S}}^{35}$ spectrum is in agreement with that predicted by the Fermi theory for energies from $W=1.15\mathrm{}$ ${\mathrm{mc}}^2$ to the end point at $W_0=1.331\mathrm{}$ ${\mathrm{mc}}^2$. Below this region, there is a deviation from the theory the exact nature of which is unfortunately somewhat masked by distortions due to source thickness and backing. The ${\mathrm{C}}^{14}$ spectrum has a shape which is slightly different from that which would be predicted for an allowed transition. The experimental shape is not consistent with that calculated for a second forbidden transition using Fermi selection rules. The extrapolated end point for ${\mathrm{C}}^{14}$ is at $W_0=1.306\mathrm{}$ ${\mathrm{mc}}^2$. From the shape of the ${\mathit{S}}^{35}$ spectrum in the neighborhood of the end point, an upper limit is obtained for the mass of the neutrino as less than one percent the mass of the electron.