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

Abstract

Neutron-capture $\ensuremath{\gamma}$ rays from the Texas A 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 $\ensuremath{\gamma}$ 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 ${\ensuremath{\Gamma}}_{0}$ (the excited level width for an electromagnetic transition to the ground state), $\ensuremath{\Gamma}$ (the excited level total electromagnetic width), and $\ensuremath{\epsilon}$ (the energy displacement between the neutron-capture $\ensuremath{\gamma}$-ray energy and the resonance energy). The values of ${\ensuremath{\Gamma}}_{0}$ (between 0.14 and 1.0 eV) were found to be about 10 times larger than the values usually found for $\ensuremath{\gamma}$-ray widths in this energy region as determined from neutron-capture reactions.