The Doppler-shift-attenuation technique was used to investigate the lifetimes of excited states of ${\mathrm{Be}}^{10}$, ${\mathrm{B}}^{10}$, ${\mathrm{B}}^{11}$, ${\mathrm{C}}^{11}$, and ${\mathrm{C}}^{12}$. Gamma rays from nuclear levels populated by $d$, ${\mathrm{He}}^3$, and $\alpha$ bombardment of thick Be and Cu-Be targets were detected with a lithium-drifted germanium detector at both forward and backward directions relative to the incident beam direction. Lifetimes were then extracted by comparing the differences in the Doppler shift as measured with the two targets. Gamma-ray peaks from ${\mathrm{Be}}^9$+$d$ were observed for the ${\mathrm{Be}}^{10}$ 5.96 → 3.37, ${\mathrm{Be}}^{10}$ 3.37 → 0, ${\mathrm{B}}^{10}$ 3.59 → 0.72, and ${\mathrm{B}}^{10}$ 5.16 → 2.15 transitions and mean lifetimes of <0.8×${10}^{-13\mathrm{}}$ sec, (1.6±0.3)×${10}^{-13\mathrm{}}$ sec, (1.20±0.43)×${10}^{-13\mathrm{}}$ sec, and <0.8×${10}^{-13\mathrm{}}$ sec were obtained for the initial states-of these transitions. The Doppler shift of the ${\mathrm{C}}^{12}$ 4.43 → 0 transition was investigated using the ${\mathrm{Be}}^9(\alpha , n){\mathrm{C}}^{12}$ reaction. A mean lifetime of (${5.7}_{-1.7\mathrm{}}^{+2.3\mathrm{}}$) × ${10}^{-14\mathrm{}}$ sec was obtained for the ${\mathrm{C}}^{12}$ 4.43-MeV level. Levels in ${\mathrm{B}}^{11}$ and ${\mathrm{C}}^{11}$ were formed by the ${\mathrm{Be}}^9({\mathrm{He}}^3, p){\mathrm{B}}^{11}$ and ${\mathrm{Be}}^9({\mathrm{He}}^3, n){\mathrm{C}}^{11}$ reactions, and upper limits were set for the mean life-times of five levels in these nuclei. The strengths of known transitions from the ${\mathrm{B}}^{10}$ 2.15- and 3.59-MeV levels are discussed in terms of the independent-particle model. The agreement is found to be generally good for the 3.59-MeV level and poor for the 2.15-MeV level. Several experimental and theoretical points remain to be clarified.

• Received 11 April 1966

DOI:https://doi.org/10.1103/PhysRev.148.1072