Coulomb Excitation ofF19by Alpha Particles

Abstract

Gamma rays emitted in the excitation of ${\mathrm{F}}^{19}$ by $\alpha$ particles of 0.6 to 2.8 Mev have been studied. Resonances are found in the reaction ${\mathrm{F}}^{19}(\alpha , p){\mathrm{Ne}}^{22*}$ at $\alpha$ particle energies greater than 1.3 Mev and in the inelastic excitation of 109-kev and 196-kev levels in ${\mathrm{F}}^{19}$ at energies greater than 2.2 Mev. At bombarding energies below 2 Mev, the cross sections for inelastic excitation of ${\mathrm{F}}^{19}$ decrease much too slowly for compound nucleus formation and are identified as being due to Coulomb excitation. The observed cross sections in the region 0.6 Mev to 2 Mev agree well with the theory for Coulomb excitation. The electromagnetic transition probabilities for decay of these states deduced from the excitation cross sections are in good agreement with those found from direct measurement of the lifetimes by Thirion, Barnes, and Lauritsen. Together with the results of Peterson, Barnes, Fowler, and Lauritsen on the inelastic excitation of fluorine by protons, these experiments lead to spin and parity assignment of ${\mathrm{½}}^{-}$ for the 109 kev state and $\frac{5}{2^{+}}$ for the 196-kev state of ${\mathrm{F}}^{19}$. The observed angular distributions of the $\gamma$ rays from Coulomb excitation by $\alpha$ particles are also in accord with theory.