The mean lifetime of the first excited state of B12 at 0.95 MeV was measured by the Doppler shift attenuation technique using the ${\mathrm{B}}^{11}(d, p){\mathrm{B}}^{12}$ reaction to populate the state. The result is $\tau =(3.4\mathrm{}\pm 1)\times {10}^{-13\mathrm{}}$ sec. Sum rules are invoked to show that the deexcitation of this state to the $J^{\pi }=1^{+}$ ${\mathrm{B}}^{12}$ ground state cannot be predominantly $E2\mathrm{}$ so that the spin parity of the ${\mathrm{B}}^{12}$ first excited state cannot be $3^{+}$. The angular distribution of the ground-state decay from this level was measured at a deuteron energy of 0.8 MeV. The result rules out a zero-spin assignment. It is concluded that the available experimental evidence indicates $2^{+}$ for the spin parity of the ${\mathrm{B}}^{12}$ first excited state.

• Received 31 July 1963

DOI:https://doi.org/10.1103/PhysRev.132.2273