B12,C12, and the Red Giants

Abstract

Alpha-particle emission associated with the $\ensuremath{\beta}$ decay of ${\mathrm{B}}^{12}$ has been detected and the energy spectrum investigated. These studies show that (1.3\ifmmode\pm\else\textpm\fi{}0.4)% of all decays of ${\mathrm{B}}^{12}$ lead to the second excited state of ${\mathrm{C}}^{12}$ and that this state breaks up predominantly into three alpha particles with one alpha particle and the ground state of ${\mathrm{Be}}^{8}$ as an intermediate stage in the process. The most probable spin and parity assignments for the state appear to be $J={0}^{+}$, and analysis of the alpha-spectrum yields $Q({\mathrm{C}}^{{12}^{*}}\ensuremath{-}{\mathrm{Be}}^{8}\ensuremath{-}{\mathrm{He}}^{4})=278\ifmmode\pm\else\textpm\fi{}4$ kev, corresponding to an excitation energy in ${\mathrm{C}}^{12}$ of 7.653\ifmmode\pm\else\textpm\fi{}0.008 Mev. A new determination of the disintegration energy of ${\mathrm{Be}}^{8}$ yields $Q({\mathrm{Be}}^{8}\ensuremath{-}2{\mathrm{He}}^{4})=93.7\ifmmode\pm\else\textpm\fi{}0.9$ kev and hence $Q({\mathrm{C}}^{{12}^{*}}\ensuremath{-}3{\mathrm{He}}^{4})=372\ifmmode\pm\else\textpm\fi{}4$ kev. It is concluded, from the general principle of reversibility of nuclear reactions, that the second excited state of ${\mathrm{C}}^{12}$ as predicted by Hoyle is of a suitable character to act as a stellar thermal resonance in the Salpeter process, 2${\mathrm{He}}^{4}$\ensuremath{\rightleftharpoons}${\mathrm{Be}}^{8}$; ${\mathrm{Be}}^{8}(\ensuremath{\alpha}, \ensuremath{\gamma}){\mathrm{C}}^{12}$ under conditions expected in red giant stars.