Evidence of nonstatistical neutron emission following beta decay near doubly magic Sn-132

Models of the beta-delayed neutron emission (beta n) assume that neutrons are emitted statistically via an intermediate compound nucleus post beta decay. Evidence to the contrary was found in an In-134 beta-decay experiment carried out at ISOLDE CERN. Neutron emission probabilities from the unbound states in Sn-134 to known low-lying, single-particle states in Sn-133 were measured. The neutron energies were determined using the time-of-flight technique, and the subsequent decay of excited states in Sn-133 was studied using gamma-ray detectors. Individual beta n probabilities were determined by correlating the relative intensities and energies of neutrons and gamma rays. The experimental data disagree with the predictions of representative statistical models which are based upon the compound nucleus postulate. Our results suggest that violation of the compound nucleus assumption may occur in beta-delayed neutron emission. This impacts the neutron-emission probabilities and other properties of nuclei participating in the r-process. A model of neutron emission, which links the observed neutron emission probabilities to nuclear shell effects, is proposed.

Automated summary

Models of beta-delayed neutron emission assume statistical emission of neutrons through an intermediate compound nucleus, but contrary evidence was found in an In-134 beta-decay experiment at ISOLDE CERN. Neutron emission probabilities from unbound states in Sn-134 to known low-lying, single-particle states in Sn-133 were measured using time-of-flight technique and gamma-ray detectors. The experimental data disagree with predictions of statistical models based on the compound nucleus assumption, suggesting a violation of this assumption in beta-delayed neutron emission.