The Doppler-shift attenuation method was used to investigate the lifetimes of various gamma-ray-emitting states of ${\mathrm{F}}^{18}$ populated via the ${\mathrm{O}}^{16}({\mathrm{He}}^3,p\gamma ){\mathrm{F}}^{18}$ 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 ${\theta }_{\gamma }=8^{\circ }$ and ${\gamma }_{\gamma }={172}^{\circ }$ 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 ±0.20); 2.10 (4.1±1.6); 2.53 (0.67±0.18); 3.06 (<0.17); 3.35 (0.46±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° to the ${\mathrm{He}}^3$ beam and gamma rays were detected at several angles between 0° and 90° 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 $J=2\mathrm{}$ 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 $J=0\mathrm{}$ (although $J=1\mathrm{}$ or 2 are not excluded) in which case the lifetime determined for the 2.10-MeV level establishes that the 2.10 → 1.08 transition are an enhanced $E2\mathrm{}$ of strength approximately 20 Weisskopf units. Additional information was obtained for the mixing ratios of various groundstate and cascade transitions in ${\mathrm{F}}^{18}$.

• Received 16 June 1966

DOI:https://doi.org/10.1103/PhysRev.151.792