Gamma-Ray Decay of the 7.66-Mev Level ofC12

Abstract

Proton-gamma-gamma triple coincidence measurements have been carried out on the ${\mathrm{B}}^{10}({\mathrm{He}}^3, p){\mathrm{C}}^{12}$ reaction at $E_{{\mathrm{He}}^3}=2.2\mathrm{}$ Mev. Protons were detected in a $\frac{3}{4}$-in. diameter CsI crystal subtending a solid angle of 27% of $4\pi$ at the target, the gamma-ray detectors were 5 in. × 5 in. NaI crystals, and the coincidence resolving time was 8×${10}^{-9\mathrm{}}$ sec. The spectrum of protons in triple coincidence with the two gamma-ray detectors, each channeled from 2.4 to 5.0 Mev, contains a line corresponding to the alpha-emitting 7.66-Mev 0+second-excited state of ${\mathrm{C}}^{12}$. This line is interpreted as resulting from the 3.23-4.43-Mev cascade gamma-ray decay of the 7.66-Mev level through the 4.43-Mev 2+ first-excited state. The ratio of the triples to singles counting rates of the 7.66-Mev proton line, when corrected by the appropriate factors for gamma-ray efficiency, leads to a 3.23-Mev gamma-ray branch of (3.3±0.9)×${10}^{-4\mathrm{}}$ per decay of the 7.66-Mev level. This branch, which compares with a previous theoretical estimate of ∼2×${10}^{-4\mathrm{}}$, is stronger than the direct ground-state transition by a factor of 50.