'Magic' nucleus 42Si

Nuclear shell structures-the distribution of the quantum states of individual protons and neutrons-provide one of our most important guides for understanding the stability of atomic nuclei. Nuclei with 'magic numbers' of protons and/or neutrons (corresponding to closed shells of strongly bound nucleons) are particularly stable(1,2). Whether the major shell closures and magic numbers change in very neutron-rich nuclei (potentially causing shape deformations) is a fundamental, and at present open, question(3,4). A unique opportunity to study these shell effects is offered by the Si-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 fewer than the heaviest silicon nuclide observed so far(5). Here we report measurements of Si-42 and two neighbouring nuclei using a technique involving one- and two-nucleon knockout from beams of exotic nuclei(6,7). We present strong evidence for a well-developed proton subshell closure at Z = 14 (14 protons), the near degeneracy of two different (s(1/2) and d(3/2)) proton orbits in the vicinity of Si-42, and a nearly spherical shape for Si-42.