Spectroscopy ofO18:C14(7Li,tγ)18O reaction; observation of selective enhancedE1 deexcitations of cluster states

The alpha-particle transfer reaction ${}_{}{}^{14}{}_{}{}^{}\mathrm{C}$ ${(}^7$Li,tγ${)}^{18}$O was used to study gamma deexcitation modes in ${}_{}{}^{18}{}_{}{}^{}\mathrm{O}$ at a bombarding energy ${\mathit{E}}_{\mathit{L}}$=15 MeV. In this measurement 4×${10}^6$ triton-gamma coincidence events were collected. All natural parity states up to the alpha-particle threshold at 6.227 MeV were studied. These data together with our previously published study of states above threshold, using the α${+}^{14}$C radiative capture reaction, provide relatively complete spectroscopic information on states in ${}_{}{}^{18}{}_{}{}^{}\mathrm{O}$ up to 8.3 MeV in excitation energy. Previously reported major gamma decay branching ratios were remeasured with higher accuracy and good agreement is found. Several new weak deexcitation modes are reported. Enhanced E1 decays, B(E1)≃${10}^{\mathrm{-}2}$ Weisskopf unit, were observed among the ${\mathit{J}}^{\mathrm{\pi }}$=$0_2^{+}$, $1_1^{\mathrm{-}}$, $1_2^{\mathrm{-}}$, $2_3^{+}$, and $3_3^{\mathrm{-}}$ states, representing some of the most enhanced E1’s found in even-even nuclei. A strong empirical correlation between enhanced E1 decays and cluster structure is established. These enhanced B(E1)’s suggest a selective deexcitation mode in which the states involved preferentially decay to cluster states. The cluster states are strongly populated in cluster transfer reactions and weakly populated in (t,p) reactions. Predictions of the vibron cluster model are compared with our data. The presence in our data of contaminant lines from ${}_{}{}^{14}{}_{}{}^{}\mathrm{C}$ ${(}^7$Li,α${)}^{17}$N allowed extraction of some information on ${}_{}{}^{17}{}_{}{}^{}\mathrm{N}$.