Levels ofF18from theO16(He3,pγ)F18Reaction

Abstract

Angular correlations in the ${\mathrm{O}}^{16}({\mathrm{He}}^3,p\gamma ){\mathrm{F}}^{18}$ reaction have been studied through two-parameter analyses of proton—gamma-ray coincidences at ${\mathrm{He}}^3$ bombarding energies of 4.65, 5.40, and 6.40 MeV. Protons were detected by an annular solid-state detector centered at ${\theta }_p={180}^{\circ }$; gamma rays were detected with a NaI(Tl) spectrometer for five angles in the range $0^{\circ }\le {\theta }_{\gamma }\le {90}^{\circ }$. Branching ratios were determined for nine of the ten ${\mathrm{F}}^{18}$ levels of $3.4<\mathrm{}{E}_{\mathrm{ex}}<4.9\mathrm{}$ MeV; the exception is the 4.74-MeV level, which was not observed. From analysis of angular-correlation data the following spin-parity limitations are obtained for the triplet of levels at $E_{\mathrm{ex}}\sim 3.8$ MeV: 3.72-MeV level ($J=1\mathrm{}$); 3.79-MeV level ($J=1,2, or 3$); and 3.84-MeV level ($J^{\pi }=2^{\mathrm{}(+)\mathrm{}}$). The preference for an even-parity assignment for the 3.84-MeV level is quite strong. The decay modes of the 3.79-MeV level are consistent with the assumption that this is the $3^{-}$ member of the group of odd-parity levels which includes those at 1.08 MeV ($0^{-}$), 2.10 MeV ($2^{-}$), and 3.13 MeV (expected $1^{-}$). For those levels with $E_{\mathrm{ex}}>4\mathrm{}$ MeV, the correlation analysis provides restrictions on possible level spins and on multipole-mixing amplitudes for some of the principal deexcitation transitions. These results are consistent with other available information. The 4.65-MeV level of ${\mathrm{F}}^{18}$ is found to have $J>~3$, and decays by transitions to the 0.94-MeV level ($J^{\pi }=3^{+}$) and to the 1.13-MeV level [$J^{\pi }={\mathrm{}\left(5\right)\mathrm{}}^{+}$] with branching ratios of 15 and 85%, respectively. This appears to be the most likely candidate for the $J^{\pi }=4^{+}$, $T=1\mathrm{}$ analog of the 3.55-MeV $4^{+}$ state of ${\mathrm{O}}^{18}$. The ${\mathrm{F}}^{18}$ 4.40-MeV level is also observed to decay to the 1.13-MeV level, and is a possible but less likely candidate for the analog of this ${\mathrm{O}}^{18}$ level.