Enhanced Gamma-Ray Emission from Neutron Unbound States Populated in Beta Decay
Abstract
Total absorption spectroscopy was used to investigate the beta-decay intensity to states above the neutron separation energy followed by gamma-ray emission in 87,88Br and 94Rb. Accurate results were obtained thanks to a careful control of systematic errors. An unexpectedly large gamma intensity was observed in all three cases extending well beyond the excitation energy region where neutron penetration is hindered by low neutron energy. The gamma branching as a function of excitation energy was compared to Hauser-Feshbach model calculations. For 87Br and 88Br the gamma 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 momentum 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 statistical 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,gamma) cross section that would have an impact on r-process abundance calculations.
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@article{arxiv.1505.05490,
title = {Enhanced Gamma-Ray Emission from Neutron Unbound States Populated in Beta Decay},
author = {J. L. Tain and E. Valencia and A. Algora and J. Agramunt and B. Rubio and S. Rice and W. Gelletly and P. Regan and A. -A. Zakari-Issoufou and M. Fallot and A. Porta and J. Rissanen and T. Eronen and J. Aysto and L. Batist and M. Bowry and V. M. Bui and R. Caballero-Folch and D. Cano-Ott and V. -V. Elomaa and E. Estevez and G. F. Farrelly and A. R. Garcia and B. Gomez-Hornillos and V. Gorlychev and J. Hakala and M. D. Jordan and A. Jokinen and V. S. Kolhinen and F. G. Kondev and T. Martinez and E. Mendoza and I. Moore and H. Penttila and Zs. Podolyak and M. Reponen and V. Sonnenschein and A. A. Sonzogni},
journal= {arXiv preprint arXiv:1505.05490},
year = {2015}
}