Photo-induced population of the h11/2 isomeric states in (γ, n) reactions
The mechanism of excitation of isomeric h11/2 states in nuclei around the closed shell at N=82 have been studied at the High-Intensity Gamma Source (HIγS). We have taken advantage of the monoenergetic (ΔE/E=1.5%) and pulsed γ-ray-beam from HIγS to perform in-beam spectroscopy measurements with an improved level of precision and sensitivity. The giant dipole resonances at 15 MeV in the N=82 target isotones 138Ba, 140Ce, and 142Nd have been excited, and following neutron emission the γ-ray cascades leading to the isomeric h11/2 state and the ground state were observed in the N=81 isotones. For all three nuclei a very similar de-excitation scheme was found. The only level observed from which the isomeric state was populated was found to be Jπ = 7/2-. The ground state is principally populated from the states with spin and parities Jπ = 1/2+, 5/2+, and 7/2+. The structure of the N=81 isotones and the role of the gateway states in isomer population will be discussed. The results of the measurements will be compared with statistical model calculations. © 2006 American Institute of Physics.
