Energy Level Parameters from Nuclear Resonance Fluorescence at 7 Mev

The recoil-broadened photon spectrum from the reaction ${\mathrm{F}}^{19}(p, \alpha \gamma ){\mathrm{O}}^{16}$ has been used to measure the elastic photon scattering cross sections at 7 Mev of 31 elements. The observed angular distributions are consistent with dipole transitions. A plot of the cross sections versus mass number shows definite peaks around the closed shell regions near $Z=50\mathrm{}$, $N=82\mathrm{}$ (Sn, Te, and Ba), and $Z=82\mathrm{}$, $N=126\mathrm{}$ (Pb and Bi). For six medium and heavy elements self-absorption measurements were made which, when analyzed in terms of a number of nonoverlapping Breit-Wigner resonances, yield values of the average partial radiation widths to the ground states, the average total radiation widths, and the average level spacings for those elements. The radiation widths are significantly larger than those determined from slow-neutron scattering and capture experiments and, excepting Pb and Bi, the average level spacings are also appreciably greater than would be expected from the neutron data. The observed widths and spacings are in order of magnitude agreement with the recent interpretation of the modified single-particle calculation of Blatt and Weisskopf.