  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.

Nuclear Experiment