Radiative Widths, Spins, and Parities of Nuclear Levels in the 6-9-MeV Region Excited by Neutron CaptureγRays

Abstract

Monochromatic photons obtained from thermal-neutron capture in iron were used for exciting nuclear levels in the energy region between 6 and 9 MeV. Angular distribution of the elastically scattered radiation was carried out for the determination of the spins of the resonance levels. Total and ground-state radiation widths of ten resonance levels in various isotopes were measured using self-absorption, temperature variation, and absolute scattering cross-section measurements. The parity of six resonance levels was measured using a Compton polarimeter, and strong $M1$ transitions were found in $^{141}\mathrm{Pr}$ and $^{208}\mathrm{Pb}$; the character of four transitions was found to be $E1$. The observed total radiation widths ${\ensuremath{\Gamma}}_{\ensuremath{\gamma}}$ of these bound levels were found to have the same magnitude as those of unbound levels obtained in neutron resonances, which indicate that ${\ensuremath{\Gamma}}_{\ensuremath{\gamma}}$ is continuous across the ($\ensuremath{\gamma},n$) threshold.