Lifetime of the First Excited State ofB11

Using the reaction ${\mathrm{B}}^{11}(p, p^{\prime }){\mathrm{B}}^{11*}$ as the source of Doppler-broadened 2.14-Mev gamma radiation, resonance fluorescence from the first excited state of ${\mathrm{B}}^{11}$ has been observed. For a bombarding energy of 3.0 Mev and a thick target of ${\mathrm{B}}_4$C, the shape of the cone containing gamma rays capable of exciting the 2.14-Mev level was determined. It agrees with the expectation based on the known excitation curve and the kinematics of the ${\mathrm{B}}^{11}(p, p^{\prime })$ reaction assuming an isotropic distribution of the ${\mathrm{B}}^{11*}$ recoils. From the experimental resonance scattering cross section a mean life of 4.8×${10}^{-15\mathrm{}}$ second was calculated for the first excited state of ${\mathrm{B}}^{11}$, assuming the spin of this level to be ½. A self-absorption experiment yielded ${\tau }_{\gamma }=4.3\mathrm{}\times {10}^{-15\mathrm{}}$ second when a value of 1250° was used for the Debye temperature of ${\mathrm{B}}_4$C. A final value of (4.7±0.6)×${10}^{-15\mathrm{}}$ second was adopted for the mean life of the 2.14-Mev first excited state of ${\mathrm{B}}^{11}$. This value of the lifetime further strengthens the argument for spin ½, but it does not allow any conclusion about the $M1\mathrm{}$ or $E1\mathrm{}$ character of the gamma-ray transition and thus about the parity of the 2.14-Mev level.