O18Excitation Energies and Lifetimes from Ge(Li) Measurements in theF19(t, αγ)O18Reaction

Abstract

Ge(Li) spectrometry has been used to study the $\gamma$-ray deexcitation of states of ${}_{}{}^{18}{}_{}{}^{}\mathrm{O}$ produced in the ${}_{}{}^{19}{}_{}{}^{}\mathrm{F}(t, \alpha ){}_{}{}^{18}{}_{}{}^{}\mathrm{O}_{}^{*}{}_{}{}^{}$ reaction at $E_t=2.6\mathrm{}$ MeV. Precision excitation energies were determined from both singles and coincidence measurements. Multiparameter data collection techniques were used to store $\gamma$-ray spectra measured at ${\theta }_{\gamma }=0, 45, \mathrm{and} 90°$ in coincidence with $\alpha$ groups populating various final states of ${}_{}{}^{18}{}_{}{}^{}\mathrm{O}$. $\alpha$ particles were detected in an annular detector at ${\overline{\theta }}_{\alpha }=180\mathrm{}°$, thus defining a narrow cone of forward-going ${}_{}{}^{18}{}_{}{}^{}\mathrm{O}$ recoils which stopped in the Ni target backings. The observed Doppler shifts were analyzed, via the Doppler-shift attenuation method, to determine the lifetimes of the initial $\gamma$-emitting states. Resulting excitation energies (in keV) and meanlives (in psec) for the first 15 excited states of ${}_{}{}^{18}{}_{}{}^{}\mathrm{O}$ are as follows: 1982.16 ± 0.20 ($\tau ={2.9}_{-0.6\mathrm{}}^{+0.9\mathrm{}}$), 3555.07 ± 0.45 (> 4.0), 3634.50 ± 0.40 (1.33 ± 0.20), 3920.6 ± 0.4 (0.024 ± 0.010), 4456.1 ± 0.5 (0.065 ± 0.015), 5098.5 ± 1.2 (0.062 ± 0.025), 5260.4 ± 1.2 (0.12 ± 0.03), 5336.4 ± 0.6 (0.20 ± 0.04), 5377.8 ± 1.2 ($\gamma$ transition were fitted, as a function of ${\theta }_{\gamma }$, in order to determine relative intensities and thus branching ratios. An analysis of the correlation data provides confirmation or support for previously reported conclusions on spin-parity assignments, and in addition restricts possible assignments for the 6351-keV state to $J=1\mathrm{} or 2$. These results are compared to previous data on ${}_{}{}^{18}{}_{}{}^{}\mathrm{O}$ and to theoretical considerations.