Radiative widths, spins, and parities of nuclear levels in the 6-9-MeV region excited by neutron capture γ rays

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 ¹⁴¹Pr and ²⁰⁸Pb; the character of four transitions was found to be E1. The observed total radiation widths Γγ of these bound levels were found to have the same magnitude as those of unbound levels obtained in neutron resonances, which indicate that Γγ is continuous across the (γ,n) threshold.