Direct Comparison of theB12andN12Beta Spectra

An experimental direct comparison of the shapes of the ${\mathrm{B}}^{12}$ and ${\mathrm{N}}^{13}$ beta decay spectra was made as a test of the validity of the conserved vector current theory (CVCT). The comparison was made at corresponding momentum points normalized to the end-point momenta of the spectra using a 5-channel 180° magnetic spectrometer. The comparative shape factor was measured to have an energy dependence of 1.62±0.28% per MeV, to be compared with a predicted value of 1.10±0.17% per MeV on the basis of the CVCT. The measured end-point energy of the ${\mathrm{N}}^{12}$ ground-state spectrum is 16.384±0.015 MeV. The comparison of the spectra also yielded a value of 2.2±0.6% for the branching fraction of the ${\mathrm{N}}^{12}$ decay to the 7.6-MeV level of ${\mathrm{C}}^{12}$, 0.85±0.6% for the fraction to the 10.1-MeV level, and an indication of a branch to one or more levels around 12 MeV.