Magic Nucleus Silicon-42

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.

The Segmented Germanium Array (SeGA) photon spectrometer in front of the S800 spectrograph used in this experiment.

J. Fridmann et al., Nature 435 (2005) 922.

Paul Cottle
cottle at, Florida State University.