Nuclear Pair Emission from the 7.656-Mev Level inC12

The 7.656-Mev nuclear pair transition from the 0+ second excited state of ${\mathrm{C}}^{12}$ has been observed in the ${\mathrm{Be}}^9(\alpha , n){\mathrm{C}}^{12}$ reaction by means of an intermediate-image pair spectrometer. With a beam of 5.81-Mev alpha particles incident on a 0.7-Mev thick Be foil target the observed intensity ratio of the 7.656-Mev pair line to the 4.433-Mev pair line from the 2+ first excited state of ${\mathrm{C}}^{12}$ was (5±1.5)×${10}^{-4\mathrm{}}$. Approximately the same intensity ratio was found with both 5.38- and 5.81-Mev alpha particles incident on thick (6 mg/${\mathrm{cm}}^2$) Be targets. By applying the appropriate factors for the spectrometer efficiency and for the internal pair conversion coefficient of the 4.433-Mev transition the derived ratio of pair to total widths of the 7.656-Mev level is $\frac{{\Gamma }_{e\pm }}{\Gamma }=8.2\mathrm{}\times {10}^{-7\mathrm{}}\times R$ where $R=\frac{N_{4.433}}{N_{7.656}}$, the ratio of neutron populations in the ${\mathrm{Be}}^9(\alpha , n){\mathrm{C}}^{12}$ reaction. As a rough estimate $R$ is assumed to be ∼8 based on the only information available. This leads to $\frac{{\Gamma }_{e\pm }}{\Gamma }\sim 7\times {10}^{-6\mathrm{}}$ which is a factor of ∼15 smaller than estimates by Cook et al. in which the width ${\Gamma }_{\alpha }$ for the alpha-particle decay of the level was taken as $\frac{1}{10}$ of the Wigner limit. The most plausible explanation of the data is that ${\Gamma }_{\alpha }$ is close to the Wigner limit.