Consequences of Charge Independence for Nuclear Reactions Involving Photons

Abstract

Some effects of the charge independence of nuclear forces on the emission and absorption of photons by light nuclei are investigated. It is found that the selection rules governing the change of isotopic spin $T$ in such transitions are of practical importance in nuclei with ${T}_{z}=0$, particularly the rule that $E1$ transitions without change of isotopic spin are forbidden. Two examples of forbidden $E1$-emission (in ${\mathrm{N}}^{14}$ and ${\mathrm{O}}^{16}$) are discussed, and detailed experimental reinvestigation is suggested in conection with each of them.The main consequence for photonuclear reactions of the aforementioned selection rule is that for each ${T}_{z}=0$ nucleus there is a threshold for (allowed) $E1$ absorption, corresponding to the first level with $J={1}^{\ensuremath{-}}$, $T=1$. For ($\ensuremath{\gamma}, \ensuremath{\alpha}$)- and ($\ensuremath{\gamma}, d$)-reactions, where no isotopic spin can be carried off by the particle, $E1$ absorption can become effective only when there is enough energy to leave the residual nucleus in a $T=1$ state. The roles of $E2$ and $M1$ absorption, with and without change of isotopic spin, are discussed, and appropriate sum rules are derived. The rules governing ($\ensuremath{\gamma}, \ensuremath{\alpha}$) processes are applied particularly to reactions in ${\mathrm{C}}^{12}$ and ${\mathrm{O}}^{16}$, where considerable experimental evidence is available and appears to support the theoretical conclusions.