Total Absorption Gamma-Ray Spectroscopy of 87Br, 88Br and 94Rb Beta-Delayed Neutron Emitters

TL;DR

This study uses total absorption gamma-ray spectroscopy to analyze beta-delayed neutron emitters 87Br, 88Br, and 94Rb, revealing new gamma-intensity data, impacting reactor decay heat and antineutrino spectra calculations, and providing insights into nuclear structure and r-process nucleosynthesis.

Contribution

It provides new gamma-ray intensity data for key fission products, improving reactor decay heat and antineutrino spectrum models, and offers nuclear structure insights for neutron-rich isotopes.

Findings

Detected significant gamma emission from high excitation states.

Improved decay heat calculations reduce discrepancies with measurements.

Found a large gamma branching in bromine isotopes explained by nuclear structure.

Abstract

We investigate the decay of 87Br, 88Br and 94Rb using total absorption gamma-ray spectroscopy. These important fission products are beta-delayed neutron emitters. Our data show considerable gamma-intensity, so far unobserved in high-resolution gamma-ray spectroscopy, from states at high excitation energy. We also find significant differences with the beta intensity that can be deduced from existing measurements of the beta 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 235U fission at cooling times in the range 1 to 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 gamma emission from neutron unbound states populated in the daughter nucleus. The gamma 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 94Rb, although much smaller, hints of the need to increase the radiative width by one order-of-magnitude. This leads to a similar increase in the calculated (n,gamma) cross section for this very neutron-rich nucleus with a potential impact on r-process abundance calculations.