Strength functions in the β-decay of short-lived fission products

Abstract

Approximate β-strength functions have been derived from measurements of the β − feed to high-lying excited states in 40 neutron-rich nuclei. These nuclei, all produced in thermal fission, cover a broad range of nuclides from bromine to yttrium (84 ≦ A ≦ 95) and from silver to lanthanum (116 ≦ A ≦ 143). The β − feed was determined by non-ideal total γ-ray absorption. Above the cut-off energy expected from pairing, the strength functions in general increase smoothly with energy when no strong allowed single-particle strength is concentrated in the observed energy range. The increase is in these cases largely proportional to the nuclear level density. When allowed single-particle strength is likely to occur the strength functions show significant structure even at higher energies (≈5 MeV) and the average strength then corresponds to a higher value of log ƒt. These data are consistent with the theory of high-energy β-decay in terms of a giant resonance, but draw attention to the persistence of significant shell structure in the tail part of the resonance although the definite but smeared-out shell-model transitions are hindered as would be expected. Direct information on the β-strength is needed for the evaluation of delayed neutron yields. The data provide such information for the wellknown delayed neutron precursors 87Br and 137I.