Isotope near tin-100 flouts conventional wisdom
Experiments performed with at the Department of Energy's Oak Ridge National Laboratory (ORNL) with tin-101, which has a single neutron orbiting tin-100's closed shell of 50 protons and 50 neutrons, indicate an unexpected reversal in the ordering of lowest states in the nucleus.

The finding appears to violate a standard scenario offered by the nuclear shell model that has been the cornerstone for understanding the atomic nucleus for more than half a century, according to Oak Ridge National Laboratory.

The experiment, performed at ORNL's Holifield Radioactive Ion Beam Facility, found that the ground states of orbiting neutrons unexpectedly swap when three neutrons are added to the closed-shell tin-100 nucleus.

The international team of experimentalists and theorists was led by Iain G. Darby, PhD of the University of Tennessee, Knoxville, Tenn., who is now in Belgium and Robert K. Grzywacz, PhD from the department of physics and astronomy, University of Tennessee.

The researchers theorize that the swapping of ground-state spins between tin-101 and tin-103 is due to the neutrons' unusually strong orbital dependence of the pairing interaction and the relatively small difference between orbital energy states in tin-101.

The experiment required the unique radioactive ion beam capabilities of the Holifield Facility at ORNL, Oak Ridge, Tenn. plus digital signal processing instrumentation developed at ORNL to measure the extremely fast alpha particle decays in the neutron-deficient and very unstable tin isotopes. Tin-101 was made in the decay of the extremely short-lived, lightest-known alpha emitter tellurium-105, said ORNL.