For those of us who are not experts in Nuclear physics, the apparent paradigm is that the nucleus is composed of a number of protons and neutrons bound together.
However it seems that when there is an excess of neutrons (as in strange radioactive isotopes created in collisions at CERN), then neutrons tend to form a shell around the rest of the nucleus, or even to form a halo around the nucleus within which some neutrons orbit.
Such structures are difficult to explore because the nuclei are generally very short-lived. However a new experimental set-up is being devised whereby a large number of antiprotons are bottled, and then introduced to these exotic nuclei. Annihilation between matter and anti-matter is very fast, even compared to the decay rate of exotic radioactive nuclei, so by observing the comparative rates at which protons and neutrons are annihilated, some deductions can be made about the prevalence of neutrons and protons in the various levels or layers of the exotic nucleus.