Studies ofF18from theO16(He3,pγ)F18Reaction

Abstract

The Doppler-shift attenuation method was used to investigate the lifetimes of various gamma-ray-emitting states of ${\mathrm{F}}^{18}$ populated via the ${\mathrm{O}}^{16}({\mathrm{He}}^{3},p\ensuremath{\gamma}){\mathrm{F}}^{18}$ reaction at a ${\mathrm{He}}^{3}$ bombarding energy of 3.4 MeV. Gamma-ray spectra were measured with a lithium-drifted germanium spectrometer for detection angles ${\ensuremath{\theta}}_{\ensuremath{\gamma}}={8}^{\ensuremath{\circ}}$ and ${\ensuremath{\gamma}}_{\ensuremath{\gamma}}={172}^{\ensuremath{\circ}}$ relative to the incident beam direction. The results determine the mean lifetimes (in picoseconds) for the levels of ${\mathrm{F}}^{18}$, identified by their excitation energies (in MeV) as follows: 1.70 (0.86 \ifmmode\pm\else\textpm\fi{}0.20); 2.10 (4.1\ifmmode\pm\else\textpm\fi{}1.6); 2.53 (0.67\ifmmode\pm\else\textpm\fi{}0.18); 3.06 (0.17); 3.35 (0.46\ifmmode\pm\else\textpm\fi{}0.10); and 3.84 (0.073). For each of the four longer-lived levels, the mean life was also determined from a theoretical fit to the experimental line shapes; these results have been incorporated in the values quoted for the 1.70-, 2.10-, 2.53-, and 3.35- MeV levels. The particle-gamma angular-correlation method of Litherland and Ferguson was used to gain information on the decay modes and spin assignments of the levels of ${\mathrm{F}}^{18}$ below an excitation energy of 3.4 MeV. Protons were detected with an annular counter at 180\ifmmode^\circ\else\textdegree\fi{} to the ${\mathrm{He}}^{3}$ beam and gamma rays were detected at several angles between 0\ifmmode^\circ\else\textdegree\fi{} and 90\ifmmode^\circ\else\textdegree\fi{} to the beam. Measurements were carried out for ${\mathrm{He}}^{3}$ bombarding energies of 3.44, 4.65, and 5.40 MeV. These results establish spin assignments of $J=2$ for both the 2.10- and 3.06-MeV levels of ${\mathrm{F}}^{18}$, and are consistent with or confirm previously established spin-parity assignments for the remaining levels as follows: 0.937 (${3}^{+}$), 1.70 (${1}^{+}$), 2.53 (${2}^{+}$), 3.13 (1), and 3.35 (2 or 3). It is further established that the 2.10- and 1.08-MeV levels have the same parity. The most probable spin of the 1.08-MeV level is found to be $J=0$ (although $J=1$ or 2 are not excluded) in which case the lifetime determined for the 2.10-MeV level establishes that the 2.10 \ensuremath{\rightarrow} 1.08 transition are an enhanced $E2$ of strength approximately 20 Weisskopf units. Additional information was obtained for the mixing ratios of various groundstate and cascade transitions in ${\mathrm{F}}^{18}$.