Isospin-Forbidden Particle Decay of the FirstT=32States inBe9andB9

The particle decays of the first $T=\frac{3}{2}$ state in ${}_{}{}^9{}_{}{}^{}\mathrm{Be}$ (${}_{}{}^9{}_{}{}^{}\mathrm{B}$) have been studied by the ${}_{}{}^7{}_{}{}^{}\mathrm{Li}({}_{}{}^3{}_{}{}^{}\mathrm{He},pn)$ reaction at $E\left({}_{}{}^3{}_{}{}^{}\mathrm{He}\right)=10.0\mathrm{}$ MeV (8.7 and 7.95 MeV). Protons (neutrons) emitted at 0° were detected in coincidence with decay neutrons (protons). Neutron energy was determined by the associated-particle time-of-flight technique. The ratios of partial neutron (proton) widths to the ground and first excited states of ${}_{}{}^8{}_{}{}^{}\mathrm{Be}$ to the ground-state radiative width ${\Gamma }_{{\gamma }_0}$ are found to be, respectively, 6.4 ± 2.0 and 20.4 ± 4.6 for the ${}_{}{}^9{}_{}{}^{}\mathrm{Be}$ analog state (<1.5 and 13.9 ± 2.1 for ${}_{}{}^9{}_{}{}^{}\mathrm{B}$). Taking ${\Gamma }_{{\gamma }_0}=10.5\mathrm{}\pm 1.5$ eV, these results give ${\Gamma }_{n_0}=67\mathrm{}\pm 26$ eV, ${\Gamma }_{n_1}=213\mathrm{}\pm 56$ eV, ${\Gamma }_{p_0}<18\mathrm{}$ eV, and ${\Gamma }_{p_1}=147\mathrm{}\pm 30$ eV for the mirror decays.