The mean lives of the 2.15- and 1.74-MeV levels of ${}_{}{}^{10}{}_{}{}^{}\mathrm{B}$ were measured by the Doppler-shift attenuation method. The reaction ${}_{}{}^{11}{}_{}{}^{}\mathrm{B}$ (${}_{}{}^3{}_{}{}^{}\mathrm{He}$,${}_{}{}^4{}_{}{}^{}\mathrm{He}$)${}_{}{}^{10}{}_{}{}^{}\mathrm{B}$ was used and the direction of the recoiling ${}_{}{}^{10}{}_{}{}^{}\mathrm{B}$ nuclei was selected by a coincidence technique. The mean lives were found to be (1.41±17) × ${10}^{12}$ sec for the 2.15-MeV level and (1.52±0.24) × ${10}^{-13\mathrm{}}$ sec for the 1.74-MeV level. The results are compared with estimates based on the independent-particle model. The mean lives of allowed $M1\mathrm{}$ transitions are found to agree with these estimates. Evidence was found for the inhibition of an $M1\mathrm{}$ transition for which $\Delta T=0\mathrm{}$ in a self-conjugate nucleus, and for enhancement of an $E2\mathrm{}$ transition.

• Received 19 April 1965

DOI:https://doi.org/10.1103/PhysRev.139.B1149