Comparison of the Beta Spectra ofB12andN12

Abstract

The conserved-vector-current (CVC) theory predicts a deviation from the allowed shape of the ${\mathrm{B}}^{12}$ and ${\mathrm{N}}^{12}$ beta spectra. The ratio of the shape factors of the two spectra is expected to have an energy dependence of (1.10±0.17)% per MeV. If the CVC hypothesis is not invoked, this ratio is estimated to be an order of magnitude smaller. The two spectra have been measured with a magnetic spectrometer and found to give for this ratio the value (1.30±0.31)% per MeV. The branching fraction of the ${\mathrm{N}}^{12}$ decay to the 7.6-MeV state of ${\mathrm{C}}^{12}$ has been determined to be (3.0±0.5)%.