Fridmann J, Wiedenhöver I, Gade A, Baby L T, Bazin D, Brown B A, Campbell C M, Cook J M, Cottle P D, Diffenderfer E, Dinca D-C, Glasmacher T, Hansen P G, Kemper K W, Lecouey J L, Mueller W F, Olliver H, Rodriguez-Vieitez E, Terry J R, Tostevin J A, Yoneda K
Department of Physics, Florida State University, Tallahassee, Florida 32306-4350, USA.
Nature. 2005 Jun 16;435(7044):922-4. doi: 10.1038/nature03619.
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. 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. A unique opportunity to study these shell effects is offered by the 42Si 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. Here we report measurements of 42Si and two neighbouring nuclei using a technique involving one- and two-nucleon knockout from beams of exotic nuclei. 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 42Si, and a nearly spherical shape for 42Si.
核壳层结构——单个质子和中子的量子态分布——是我们理解原子核稳定性的最重要指导之一。具有质子和/或中子“幻数”（对应于强束缚核子的封闭壳层）的原子核特别稳定。在富含中子的原子核中，主要壳层闭合和幻数是否会发生变化（可能导致形状变形）是一个基本的、目前尚未解决的问题。42Si核提供了一个研究这些壳层效应的独特机会，它有28个中子（这是稳定原子核中的一个幻数）和14个质子。这个原子核比最重的稳定硅核素多12个中子，并且比目前观测到的最重硅核素只少一个中子。在这里，我们报告了使用一种涉及从奇异核束中敲除一个和两个核子的技术对42Si和两个相邻核的测量结果。我们提供了强有力的证据，证明在Z = 14（14个质子）处有一个发育良好的质子子壳层闭合，在42Si附近两个不同的（s(1/2)和d(3/2)）质子轨道近乎简并，以及42Si近乎球形的形状。
