The first reported measurements of the $\beta$ decay, half-life, and mass of ${}_{}{}^{31}{}_{}{}^{}\mathrm{Al}$ are presented. The new activity was produced by the ${}_{}{}^{18}{}_{}{}^{}\mathrm{O}({}_{}{}^{18}{}_{}{}^{}\mathrm{O}, \alpha p){}_{}{}^{31}{}_{}{}^{}\mathrm{Al}$ reaction using 41-MeV incident ${}_{}{}^{18}{}_{}{}^{}\mathrm{O}$ ions, and was transferred pneumatically to a remotely located station where delayed $\gamma$ rays and $\beta$ rays were counted with Ge(Li) and NE 102 detectors, respectively. $\gamma$-ray energies (in keV) and relative intensities for the ${}_{}{}^{31}{}_{}{}^{}\mathrm{Si}$ daughter transitions are 621.81±0.30 (9.94±0.65), 752.23 ±0.30 (18.5±0.8), 1564.49±0.30 (17.3±1.6), 1694.73±0.30 (58.9±1.6), and 2316.64±0.40 (72.8±1.8). The ${}_{}{}^{31}{}_{}{}^{}\mathrm{Si}$ excitation energies (in keV) and relative $\beta$ branching intensities are 752.24±0.30 (<3.0), 1694.78±0.30 (49.0±1.7), and 2316.73±0.40 (100±2.5). A tentative $\beta$- ray transition to a state at $E_x=2787.7\mathrm{}\pm 0.8$ keV is also observed. Upper limits on the strength of some possible $\gamma$-ray transitions following $\beta$ decay to higher levels are given. ${}_{}{}^{31}{}_{}{}^{}\mathrm{Al}$ decays with a half-life of 644±25 msec. By measuring the energy spectra of $\beta$ rays populating the second and third excited states of ${}_{}{}^{31}{}_{}{}^{}\mathrm{Si}$ the mass excess of ${}_{}{}^{31}{}_{}{}^{}\mathrm{Al}$ has been measured to be -15008±100 keV. The mass excess of ${}_{}{}^{34}{}_{}{}^{}\mathrm{P}$ has been remeasured by a similar technique to be -24 550±90 keV, and this value has been used to revise previous predictions for the masses of ${}_{}{}^{33}{}_{}{}^{}\mathrm{Si}$ and ${}_{}{}^{35}{}_{}{}^{}\mathrm{P}$. The masses of several $T_z=\frac{5}{2}$ and $T_z=3\mathrm{}$ nuclides in the $2s-1d$ shell are compared with theoretical estimates. The $\beta$-decay measurements for ${}_{}{}^{31}{}_{}{}^{}\mathrm{Al}$ are shown to be in poor agreement with simple collective-model calculations, and in good agreement with recent detailed shell-model calculations.

• Received 2 February 1973

DOI:https://doi.org/10.1103/PhysRevC.7.2409