Doppler-broadened 4.43-Mev $\gamma$ radiation from the reaction ${\mathrm{N}}^{15}(p, \alpha ){\mathrm{C}}^{12*}$ has been used to measure the lifetimes of the 4.43-Mev level of ${\mathrm{C}}^{12}$ and the 4.46-Mev level of ${\mathrm{B}}^{11}$ by the nuclear resonance fluorescence technique.

For ${\mathrm{C}}^{12}$, the 90° and 126° scattering give mean lives of (${6.7}_{-0.4\mathrm{}}^{+1.5\mathrm{}}$)×${10}^{-14\mathrm{}}$ sec and (${5.5}_{-0.4\mathrm{}}^{+1.2\mathrm{}}$)×${10}^{-14\mathrm{}}$ sec, respectively. The self-absorption of the resonance radiation gives (6.6±1.3)×${10}^{-14\mathrm{}}$ sec. The adopted mean value is (6.5±1.2)×${10}^{-14\mathrm{}}$ sec.

For ${\mathrm{B}}^{11}$, the angular distribution of the resonance-scattered radiation is $1+(0.1\pm 0.1)P_2\left(cos\theta \right)$, and the cross section corresponds to $\tau =\left({{1.02}_{-0.07\mathrm{}}}^{+0.23\mathrm{}}\right)\times {10}^{-15\mathrm{}}$ sec if the spin of the excited states is $\frac{5}{2}$. Interpretation of the self-absorption of the resonance radiation is uncertain because of lack of information as to the Debye temperature of ${\mathrm{B}}_4$C. Values of 1250°K and 1860°K give $\tau =1.33\mathrm{}\times {10}^{-15\mathrm{}}$ sec and $\tau =1.14\mathrm{}\times {10}^{-15\mathrm{}}$ sec. The adopted mean value is $\tau =(1.17\mathrm{}\pm 0.17)\times {10}^{-15\mathrm{}}$ sec.

• Received 2 December 1957

DOI:https://doi.org/10.1103/PhysRev.110.154