The Evolution of Shell Spacing in Light Nuclei
It is known that the traditional magic numbers do not remain a robust feature in all nuclei. Described as large gaps between the underlying single-particle orbitals, the locations of the magic numbers have been observed to vary throughout the chart of nuclides due to movement of the orbital energies.
One of the most drastic regions for shell evolution is in the neutron p-sd orbitals (3 < N < 20). Here, single-particle energy gaps at N = 8 and 20 are reduced while there is an emergence of new shell gaps at N = 16 and Z = 14 in the neutron-rich nuclei. A great deal of recent effort, both experimentally and theoretically, has been invested in these nuclei to disentangle the dominant components of the nuclear force driving these new features.
In this talk, results from recent measurements on the single-particle structure of p-sd shell nuclei (A ~ 20) will be presented. Spectroscopic information key to understanding the underlying single-particle structure, including angular distributions, single-particle energy centroids, and two-body matrix elements, have been extracted from single-neutron adding (d,p) reactions carried out in inverse kinematics.
The HELIOS spectrometer, a world leading device for these types of reactions, was used to measure and identify outgoing protons. The light-mass neutron-rich radioactive beams were provided by the ATLAS in-flight facility at Argonne National Laboratory. I will discuss the impact of these results on our current understanding of shell spacing in light nuclei, including comparisons with modern theoretical calculations, and comment on the future direction of work along these lines.