SPECTROSCOPY OF F-20 LEVELS

From a study of the F-19(n,gamma) reaction with thermal neutrons incid ent on a Teflon target, 168 gamma rays have been detected and incorpor ated into a level scheme of F-20 consisting of 35 previously known lev els and a new one at 5939 keV. Two low-energy primary El transitions o f energies 584 and 665 keV together account for more than half of the total capture cross section. They populate, respectively, states at 60 18 and 5936 keV (both J(pi)=2(-)). These states are also excited stron gly in the F-19(d,p) reaction. From each of these states, 17 gamma ray s were observed to the lower-lying states. These gamma rays constitute the largest number of branches reported from any nuclear bound state. A weak (6+/-1 mu b)gamma 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(-)) state, might represent the first observation of a primary M2 transition in the (n,gamma) reaction. The total thermal-neutron-capture cross section of F-19 was measured as 9. 51+/-0.09 mb; and the neutron separation energy of F-20 as 6601.35+/-0 .04 keV. Estimates of direct neutron capture have been made using phys ically realistic optical-model parameters. These model estimates are i n reasonable agreement with the measured (partial) cross sections. Whi le constructing the (n, gamma) level scheme, the existing data on boun d levels in F-20 were critically evaluated. The lifetime values for ma ny levels are poorly known. Therefore, the lifetimes for 25 levels wer e measured by the Doppler-shift-attenuation method using the inverse r eaction H-2(F-19, p gamma) on implanted deuterium targets. The experim ental level properties such as excitation energies, J(pi) assignments, branching ratios, and lifetimes have been compared with the results f rom a large-basis shell-model calculation. The agreement was found to be quite good, but this comparison points out also the need for acquir ing new data to give more definitive J(pi) assignments.