Gamma-Ray Transitions between the Low-Lying Levels ofF18

Abstract

Gamma-ray decay modes of the ${\mathrm{F}}^{18}$ levels below an excitation energy of 4 MeV have been investigated through the ${\mathrm{O}}^{16}({\mathrm{He}}^3,p\gamma ){\mathrm{F}}^{18}$ reaction using a 30-cc lithium-drifted germanium, Ge(Li), detector. Gamma-ray spectra were recorded at 90°, 7°, and 173° to the ${\mathrm{He}}^3$ beam using a thick quartz target and ${\mathrm{He}}^3$ bombarding energies of 3.45 and 6.0 MeV. Ge(Li)-NaI(Tl) coincidence spectra were also recorded at $E_{{\mathrm{He}}^3}=3.2\mathrm{}$ MeV. Sharper upper limits than obtained previously were set on the relative intensities of many unobserved transitions. A previously unreported transition from the $J=1\mathrm{} 3.13$-MeV level to the $J^{\pi }=0^{-} 1.08$-MeV level was observed with a branching ratio of (27±4)%. The mean lifetime of the 3.13-MeV level was inferred from a Doppler-shift measurement to be ($0.37_{-0.10\mathrm{}}^{}{}_{}{}^{+0.15\mathrm{}}$) × ${10}^{-12\mathrm{}}$ sec. It is conjectured that the 3.13-MeV level is the lowest $J^{\pi }=1^{-}$ level belonging predominantly to the $p^{-1\mathrm{}}{\mathrm{}(2s,1d)\mathrm{}}^3$ configuration.