Unnatural Parity States inO16. I

Abstract

A detailed study of the de-excitation of gamma emitting states in ${\mathrm{O}}^{16}$ at 8.87, 10.94, and 11.06 Mev has been carried out using the ${\mathrm{N}}^{14}({\mathrm{He}}^{3}, p\ensuremath{\gamma}){\mathrm{O}}^{16}$ reaction. Branching ratios and angular distribution measurements on both proton groups and gamma radiations strongly suggest assignments ${2}^{\ensuremath{-}}$, ${0}^{\ensuremath{-}}$, and ${3}^{+}$, respectively, although in the latter case the ${2}^{\ensuremath{-}}$ possibility is not excluded. The experimental results are compared with predictions of the alpha-particle and intermediate-coupling shell models. While considerable areas of agreement exist, serious discrepancies remain in both cases. The identification of the ${0}^{\ensuremath{-}}$, 10.94-Mev state precludes Dennison's preferred "identification (a)" in the alpha-particle model and fixes the exchange mixture in the shell model calculations; the ${\mathrm{O}}^{16}$ mixture closely resembles a Serber rather than the Rosenfeld mixture commonly in use in these shell model calculations. From the measured de-excitation branching ratio $\frac{{\ensuremath{\Gamma}}_{\ensuremath{\alpha}}}{{\ensuremath{\Gamma}}_{\ensuremath{\gamma}}}$ for the 10.94-Mev state and the systematics of gamma-radiation and alpha-particle transition widths, an upper limit of \ensuremath{\sim}2\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}9}$ is obtained for the relative intensity of an opposite-parity component in the wave function of this state.