Enhanced gamma-Ray Emission from Neutron Unbound States Populated in beta Decay

Tain, J. L. and Valencia, E. and Algora, Alejandro and Agramunt, J. and Rubio, B. and Podolyák, Zsolt (2015) Enhanced gamma-Ray Emission from Neutron Unbound States Populated in beta Decay. PHYSICAL REVIEW LETTERS, 115 (6). ISSN 0031-9007

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Abstract

Total absorption spectroscopy was used to investigate the β-decay intensity to states above the neutron separation energy followed by γ-ray emission in 87,88 Br and 94Rb. Accurate results were obtained thanks to a careful control of systematic errors. An unexpectedly large γ intensity was observed in all three cases extending well beyond the excitation energy region where neutron pen- etration is hindered by low neutron energy. The γ branching as a function of excitation energy was compared to Hauser-Feshbach model calculations. For 87Br and 88Br the γ branching reaches 57% and 20% respectively, and could be explained as a nuclear structure effect. Some of the states populated in the daughter can only decay through the emission of a large orbital angular momen- tum neutron with a strongly reduced barrier penetrability. In the case of neutron-rich 94Rb the observed 4.5% branching is much larger than the calculations performed with standard nuclear sta- tistical model parameters, even after proper correction for fluctuation effects on individual transition widths. The difference can be reconciled introducing an enhancement of one order-of-magnitude in the photon strength to neutron strength ratio. An increase in the photon strength function of such magnitude for very neutron-rich nuclei, if it proved to be correct, leads to a similar increase in the (n, γ) cross section that would have an impact on r process abundance calculations.

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