Total absorption γ -ray spectroscopy of the β -delayed neutron emitters Br 87, Br 88, and Rb 94

Abstract

We investigate the decay of Br87,88 and Rb94 using total absorption γ-ray spectroscopy. These important fission products are β-delayed neutron emitters. Our data show considerable βγ intensity, so far unobserved in high-resolution γ-ray spectroscopy, from states at high excitation energy. We also find significant differences with the β intensity that can be deduced from existing measurements of the β spectrum. We evaluate the impact of the present data on reactor decay heat using summation calculations. Although the effect is relatively small it helps to reduce the discrepancy between calculations and integral measurements of the photon component for U235 fission at cooling times in the range 1-100 s. We also use summation calculations to evaluate the impact of present data on reactor antineutrino spectra. We find a significant effect at antineutrino energies in the range of 5 to 9 MeV. In addition, we observe an unexpected strong probability for γ emission from neutron unbound states populated in the daughter nucleus. The γ branching is compared to Hauser-Feshbach calculations, which allow one to explain the large value for bromine isotopes as due to nuclear structure. However the branching for Rb94, although much smaller, hints of the need to increase the radiative width Γγ by one order of magnitude. This increase in Γγ would lead to a similar increase in the calculated (n,γ) cross section for this very neutron-rich nucleus with a potential impact on r process abundance calculations.