Thermal and resonance neutron capture studies in Se targets with A = 74, 76, 77, 78, 80

Abstract

Thermal and resonance neutron capture γ-ray measurements have been performed on a natural Se target. Accurate energies and absolute intensities of primary and secondary γ-rays for thermal and resonance capture were measured for 74, 76, 77, 78, 80 Se. Neutron separation energies derived from thermal measurements for the following are 8027.5 ± 0.4 keV for 75Se [ 74Se+n]; 7418.8 ± 0.2 keV for 77Se [ 76Se+n]; 10497.8 ± 0.3 keV for 78Se [ 77Se+n]; and 6701.1 ± 0.6 keV for 81Se [ 80Se + n]. Ten resonances were analyzed and their spins and parities were determined or confirmed as 1 2 +for the 27.1 eV and 271.5 eV resonances in 74Se, and for the 377.0 eV and 862 eV resonances in 76Se; 1 − for 112.0 eV, 0 − for 211.6 eV, 1 − for 340.8 eV and (1 −) for 864.0 eV resonances in 77Se; 1 2 + for the 383.0 eV resonance in 78Se and ( 3 2 +)for the 1970 eV resonance in 80Se. From the resonance data assignments of possible spins and parities of 20 levels in 75, 78, 79Se were made. E1 strength functions for thermal capture in 74, 76, 78Se and for the 27.1 eV resonance in 74Se were calculated for both single-particle and giant dipole resonance models and seem to conform to the systematics in the Se mass region. Indications of non-statistical effects in thermal and resonance neutron capture were seen from the anomalous strength of the 7734.0 keV transition to the 1 2 − 293.1 keV state in 75Se. This is supported by the single-particle nature of the 293.1 keV state as is evident from a large (d, p) l = 1 spectroscopic factor to this state. The valence model for direct neutron capture mechanism does not, however, account for the effect.