Nuclear shell structures - the order and separation of the quantum states of the individual protons and neutrons - provide one of our most important guides for understanding the stability of atomic nuclei. It has long been recognized that nuclei with "magic numbers" of protons and/or neutrons (corresponding to closed shells) are particularly stable. Whether the major shell closures and magic numbers change in very neutron-rich nuclei is a fundamental and presently open question.
A unique opportunity to study these shell effects is offered by the neutron-rich Silicon-42 nucleus, which has 28 neutrons, a magic number in stable nuclei, and 14 protons. This nucleus has a 12-neutron excess over the heaviest stable silicon nuclide and has only one neutron less than the heaviest silicon nuclide observed so far.
A Florida State University/Michigan State University/Berkeley/Surrey collaboration measured Silicon-42 and two neighboring nuclei - Phosphorous-43 and Sulfur-44 - using a novel experimental technique, one- and two-nucleon knockout from beams of exotic nuclei, at the National Superconducting Cyclotron Laboratory's Coupled Cyclotrons Facility. We found strong evidence for a well-developed proton sub-shell closure at Z=14, the near degeneracy of the s1/2 and d3/2 proton orbits in the vicinity of Silicon-42, and a nearly spherical shape for Silicon-42.
J. Fridmann et al., Nature 435 (2005) 922.
cottle at nucmar.physics.edu, Florida State University.