Beta Decay ofNa25andAl29

Abstract

The $\beta$-decay modes of ${\mathrm{Na}}^{25}$ and ${\mathrm{Al}}^{29}$ have been studied by observation of delayed $\gamma$ rays with a Ge(Li) $\gamma$-ray spectrometer system. The nuclei were produced via the ${\mathrm{Na}}^{23}(t, p){\mathrm{Na}}^{25}$ and ${\mathrm{Al}}^{27}(t, p){\mathrm{Al}}^{29}$ reactions, respectively, at the incident triton energy of 2.7 MeV. For ${\mathrm{Na}}^{25}$, the previously reported $\beta$ branches to the 0.975- and 1.612-MeV states of ${\mathrm{Mg}}^{25}$ are confirmed, while new branches were found to the states at 1.965 and 2.801 MeV. Decay modes (labeled by the final-state energy in ${\mathrm{Mg}}^{25}$) and their relative intensities, assuming a ground-state branch of 65%, are 0.975 MeV, 25.5%; 1.612 MeV, 8.8%; 1.965 MeV, 0.4%; and 2.801 MeV, 0.3%. Accurate excitation energies in ${\mathrm{Mg}}^{25}$ deduced from the $\gamma$-ray pulse-height distribution are (in keV) 585.9±0.4, 975.3±0.3, 1612.0±0.4, 1965.2±0.9, and 2801.0±0.7. In the decay of ${\mathrm{Al}}^{29}$, previously reported branches to the 1.273- and 2.427-MeV states of ${\mathrm{Si}}^{29}$ were confirmed, while a new branch is reported to the state at 2.028 MeV. Assuming zero ground-state branching, the decay modes with their relative intensities are 1.273 MeV, 89.1%; 2.028 MeV, 4.1%; and 2.426 MeV, 6.8%. The state at 3.069 MeV is populated with an intensity ≤0.1%. Excitation energies in keV of 1273.2±0.8, 2028.2±0.8, and 2426.3±0.8 have been obtained for these states in ${\mathrm{Si}}^{29}$. For both nuclei, experimentally observed $log\mathrm{ft}$ values have been compared with nuclear-model predictions. An adequate description of the ${\mathrm{Na}}^{25}$ $\beta$ decay in terms of the Nilsson model is obtained by introducing mixing of the two $K=\frac{1}{2}$ bands based on neutrons in orbits 9 and 11 in ${\mathrm{Mg}}^{25}$. The $J^{\pi }={\frac{1}{2}}^{+}$ states of these bands are at 0.586 and 2.562 MeV, respectively. The collective-model interpretation of ${\mathrm{Si}}^{29}$ is strengthened by the observation of a $\beta$ branch to the 2.028 MeV level in ${\mathrm{Si}}^{29}$; quantitative agreement, however, with the ${\mathrm{Al}}^{29}$ $\beta$-decay modes could not readily be obtained using the Nilsson-model wave functions. Possible spherical-shell-model configurations to explain the decay modes are suggested. The $\beta$-decay strengths deduced from these observations for ${\mathrm{Na}}^{25}$ and ${\mathrm{Al}}^{29}$ are compared with analogous $\gamma$-ray strengths observed in ${\mathrm{Al}}^{25}$ and ${\mathrm{P}}^{29}$, respectively.