Beta Decay ofB13

Abstract

The $\beta$ decay of ${\mathrm{B}}^{13}$ has been studied by means of plastic and NaI (Tl) scintillation detectors. The activity was formed in the ${\mathrm{B}}^{11}(t, p){\mathrm{B}}^{13}$ reaction using a mechanically chopped beam of 3.0-MeV tritons from a Van de Graaff accelerator. $\beta -\gamma$ coincidence measurements have confirmed a previously reported $\beta$ branch to the 3.68-MeV second excited state of ${\mathrm{C}}^{13}$. By means of a $\beta$-neutron time-of-flight method, $\beta$ branches to neutron-emitting levels of ${\mathrm{C}}^{13}$ at 7.5 and 8.86 MeV have been observed. The ${\mathrm{C}}^{13}$ levels in MeV, ${\mathrm{B}}^{13}$ $\beta$ branches to these levels in percent, and the corresponding $log\mathrm{ft}$ values are ground, 92.1±0.8, 4.02±0.02; 3.68, 7.6±0.8, 4.47±0.05; 7.5, 0.094±0.020, 5.37±0.09; and 8.86, 0.16±0.03, 4.63±0.08. The present results are consistent with recent assignments of $J^{\pi }={\frac{5}{2}}^{-} \mathrm{and} {\frac{1}{2}}^{-}$ to the 7.55- and 8.86-MeV ${\mathrm{C}}^{13}$ levels, respectively. It is suggested that the 7.55-MeV member of the 7.50-7.55-7.68 ${\mathrm{C}}^{13}$ triplet is the one populated in ${\mathrm{B}}^{13}$ $\beta$ decay. The experimental $log\mathrm{ft}$ values agree well with theoretical calculations by Cohen and Kurath.