From a study of the ${}_{}{}^{19}{}_{}{}^{}\mathrm{F}$(n,γ) reaction with thermal neutrons incident on a Teflon target, 168 γ rays have been detected and incorporated into a level scheme of ${}_{}{}^{20}{}_{}{}^{}\mathrm{F}$ consisting of 35 previously known levels and a new one at 5939 keV. Two low-energy primary E1 transitions of energies 584 and 665 keV together account for more than half of the total capture cross section. They populate, respectively, states at 6018 and 5936 keV (both ${\mathit{J}}^{\mathrm{\pi }}$=$2^{\mathrm{-}}$). These states are also excited strongly in the ${}_{}{}^{19}{}_{}{}^{}\mathrm{F}$(d,p) reaction. From each of these states, 17 γ rays were observed to the lower-lying states. These γ rays constitute the largest number of branches reported from any nuclear bound state. A weak (6±1μb)γ ray of energy 4630.6±0.9 keV, placed as a transition between the neutron-capturing state (which is a $0^{+}$ and $1^{+}$ mixture) and the 1971-keV, ($3^{\mathrm{-}}$) state, might represent the first observation of a primary M2 transition in the (n,γ) reaction. The total thermal-neutron-capture cross section of ${}_{}{}^{19}{}_{}{}^{}\mathrm{F}$ was measured as 9.51±0.09 mb; and the neutron separation energy of ${}_{}{}^{20}{}_{}{}^{}\mathrm{F}$ as 6601.35±0.04 keV. Estimates of direct neutron capture have been made using physically realistic optical-model parameters. These model estimates are in reasonable agreement with the measured (partial) cross sections. While constructing the (n,γ) level scheme, the existing data on bound levels in ${}_{}{}^{20}{}_{}{}^{}\mathrm{F}$ were critically evaluated.

The lifetime values for many levels are poorly known. Therefore, the lifetimes for 25 levels were measured by the Doppler-shift-attenuation method using the inverse reaction ${}_{}{}^2{}_{}{}^{}\mathrm{H}$${(}^{19}$F, pγ) on implanted deuterium targets. The experimental level properties such as excitation energies, ${\mathit{J}}^{\mathrm{\pi }}$ assignments, branching ratios, and lifetimes have been compared with the results from a large-basis shell-model calculation. The agreement was found to be quite good, but this comparison points out also the need for acquiring new data to give more definitive ${\mathit{J}}^{\mathrm{\pi }}$ assignments. © 1996 The American Physical Society.

• Received 4 August 1995

DOI:https://doi.org/10.1103/PhysRevC.53.616