The branching fractions of the ${\mathrm{B}}^{12}$ and ${\mathrm{N}}^{12}$ beta decays to the 4.43-MeV level of ${\mathrm{C}}^{12}$ were determined to be (1.3±0.1)% and (2.4±0.2)%, respectively, by means of $\beta \gamma$-coincidence measurements. The ${\mathrm{B}}^{12}$ and ${\mathrm{N}}^{12}$ half-lives were remeasured, using a beta-ray spectrometer to identify the emitted beta particles, and found to be 20.2±0.2 msec and 11.0±0.1 msec, respectively. The thin-target cross-section curve for the ${\mathrm{B}}^{10}({\mathrm{He}}^3,n){\mathrm{N}}^{12}$ reaction was measured over the energy range from 2.0 to 6.3 MeV. Using the above half-lives and branching ratios, the ratio of the ${\mathrm{N}}^{12}$ to ${\mathrm{B}}^{12}$ $\mathrm{ft}$ values for transitions to the 4.43-MeV level of ${\mathrm{C}}^{12}$ was calculated to be 0.92±0.06. With the use of published data and the above half-lives, the ratio of $\mathrm{ft}$ values of the ${\mathrm{N}}^{12}$ to ${\mathrm{B}}^{12}$ ground-state transitions was calculated to be 1.16±0.03. Thus, there appears to be a significant difference in the ${\mathrm{B}}^{12}$ and ${\mathrm{N}}^{12}$ ground-state transition $\mathrm{ft}$ values.

• Received 26 November 1962

DOI:https://doi.org/10.1103/PhysRev.130.292