‘Magic’ nucleus 42Si

Silicon: the question is 42 Dramatic advances are being made in nuclear structure physics because of the availability of facilities producing beams of exotic radioisotopes. Of these, silicon-42 is of particular interest for study of the effects of packing extra neutrons into a nucleus. Neutrons and protons in atomic nuclei are arranged in a series of energy levels or ‘shells’. When a shell has a full quota of particles, the nucleus is particularly stable: the ‘magic numbers’ for stability are 2, 8, 20, 28, 50, 82 and 126. Silicon-42 owes its existence to its 28 neutrons, but is still a short-lived atom. This short lifetime has been explained as a consequence of strong deformation but this is contradicted in a new study that suggests that silicon-42 is almost spherical. 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 42 Si 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 42 Si 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 42 Si, and a nearly spherical shape for 42 Si.