Study of SomeF20Excited States

Excited states of ${}_{}{}^{20}{}_{}{}^{}\mathrm{F}$ which deexcite via $\gamma$-ray cascades through the 823-keV state were studied using the ${}_{}{}^{18}{}_{}{}^{}\mathrm{O}({}_{}{}^3{}_{}{}^{}\mathrm{He},p)$ reaction at a beam energy of $E_{{}_{}{}^3{}_{}{}^{}\mathrm{He}}=2.88\mathrm{}$ MeV. Particle $\gamma$-ray angular correlations in a standard collinear geometry were obtained using NaI detectors for observing those $\gamma$ rays which were in coincidence with proton groups representing excited states up to 4.3 MeV excitation. The analysis of these data involved the simultaneous fitting of the correlations of all possible cascading $\gamma$ rays from the state under study and resulted in the measurement of spins, branching, and multipole mixing ratios for some excited states and their subsequent electromagnetic deexcitations. Coincident $\gamma$-ray spectra were also obtained with a Ge(Li) detector which provided additional information regarding the more complex decay schemes. Previously unreported lifetime information for the 1824- and 1972-keV states was obtained using the Doppler-shift-attenuation method and the ${}_{}{}^{18}{}_{}{}^{}\mathrm{O}(t,n\gamma ){}_{}{}^{20}{}_{}{}^{}\mathrm{F}$ reaction at a beam energy of $E_t\simeq 2.85$ MeV. These mean lifetimes are ≤ 65 fsec and ≥ 1.1 psec, respectively. The combined data for the 1824-keV state, as well as recently reported two-particle transfer data, suggest that the 823- and 1824-keV states are the $J^{\pi }=4^{+}$ and $5^{+}$ members, respectively, of the ground-state rotational band. Data obtained regarding the 1972-keV state suggest that this state has a $J^{\pi }=3^{-}$ assignment, while those for the 2195-keV state imply a $J^{\pi }=3^{+}$ assignment. The results of this experiment are compared with recently reported shell-model calculations.