Spins and Widths of Energy Levels in the 5-9-MeV Region

Abstract

Neutron-capture $\gamma$ rays from the Texas A&M research reactor have been used to excite one nuclear energy level each in ${}_{}{}^{90}{}_{}{}^{}\mathrm{Zr}$, Cd, Sn, Hg, ${}_{}{}^{205}{}_{}{}^{}\mathrm{Tl}$, and ${}_{}{}^{208}{}_{}{}^{}\mathrm{Pb}$, and two in ${}_{}{}^{209}{}_{}{}^{}\mathrm{Bi}$, in the 5-9-MeV region. Angular-distribution measurements of the scattered $\gamma$ rays showed that the transitions were all consistent with dipole radiation. However, for various reasons, the data could also be reproduced in most cases by assuming a mixture of dipole and quadrupole radiation. Three further measurements were made for the ${}_{}{}^{90}{}_{}{}^{}\mathrm{Zr}$, the ${}_{}{}^{205}{}_{}{}^{}\mathrm{Tl}$, the ${}_{}{}^{208}{}_{}{}^{}\mathrm{Pb}$, and one ${}_{}{}^{209}{}_{}{}^{}\mathrm{Bi}$ excitation: (1) an absolute differential cross-section measurement on the resonance scattering, (2) a resonance absorption measurement, and (3) a differential cross-section measurement at liquid-nitrogen temperature. From these data, values were found for ${\Gamma }_0$ (the excited level width for an electromagnetic transition to the ground state), $\Gamma$ (the excited level total electromagnetic width), and $\epsilon$ (the energy displacement between the neutron-capture $\gamma$-ray energy and the resonance energy). The values of ${\Gamma }_0$ (between 0.14 and 1.0 eV) were found to be about 10 times larger than the values usually found for $\gamma$-ray widths in this energy region as determined from neutron-capture reactions.