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Nuclear skin

In nuclear physics, nuclear skin is the effect wherein nuclei possess an outer layer or "skin" of nucleons (either protons or neutrons) that exist around of an inner structure of mixed nucleons.^{[citation needed]}
Generally, the effects of nuclear skin can result in an unconventionally large nuclear radius, and an altered density distribution.^{[citation needed]}
Nuclear skin is technically characteristic in all nuclei that possess an inequal amount of protons and neutrons, however, the effect of nuclear skin is only noticeably pronounced in exotic conditions, such as in "neutron-rich" and "magic" nuclei.^{[citation needed]}

Description

In the nuclear shell model, nucleons are thought to compose an elaborate structure by way of the the Pauli exclusion principle. This structure varies depending on the number of protons and/or neutrons in the nucleus, but under conditions in which one nucleon is more abundant (typically neutrons), the excess nucleons form a nuclear skin.^{[citation needed]}
Notably, despite acting in every instance of inequal proton-neutron ratios, nuclear skin is considered to be negligible in most nuclides, except in exotic environments, such as in magic, neutron-rich nuclei.

The most significant effect of nucleon skin is on the structure of nuclei, such that its density distribution is no longer uniform.^{[citation needed]}

This unconventional density distribution can be thought of in two sections:

An inner structure of mixed nucleons that follows conventional nuclear density;
And an outer structure comprised of a specific type of nucleon; this outer layer is what is considered to be the nuclear skin.

This is a departure from conventional nuclei, the radius of which are often described using the formula $R=A^{1/3}R_{0}$ , where $A$ is the mass number and $R_{0}$ is 1.25 fm, with typical deviations of up to 0.2 fm from this value.^{[citation needed]} The nucleons of the inner structure are thought have a radius and density consistent with these conventions.^{[citation needed]}

History

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Experimental approaches

Some exotic phenomena in specific nuclei facilitates unusually pronounced neutron skin, allowing for a higher observable resolution of its cross section in an electrostatic lens, notably in that of a particle accelerator.^{[citation needed]}

Magic numbers

The presence of nuclear skin can be amplified in nuclei that possess magic numbers, where specific ("magic") numbers of nucleons fit into a shell structure that enable a tighter structure. The contraction of this closed shell structure allows for further discrimination between the "inner layer" nucleons and the nuclear skin.
For this reason, magic nuclei (particularly doubly magic nuclei) are often chosen for observation in experimental research of nuclear skin.^{[citation needed]}

Neutron-rich nuclei

In bound nuclei, due to the non-linear strength of the Coulomb interaction between electrons and protons at high atomic mass, as well as the charge-neutrality of neutrons, the table of nuclides contains more neutron-rich nuclei than proton-rich nuclei as atomic mass increases.^{[citation needed]}
Additionally, "neutron-rich" isotopes of nuclei that have a high mass number have a more pronounced outer layer of excess neutrons. This creates a more pronounced difference in density distribution between the excess neutrons in the neutron skin and the "inner layer" of the mixed-nucleon structure, which is preferable for experimental observations.^{[citation needed]} Consequently, the majority of study on the subject of nuclear skin has been observed on neutron skin, as bound, heavy, neutron-rich, doubly magic nuclides are common.^{[citation needed]}

Techniques

Coulombic fission can be induced through hadron scattering to create intended isotopes for observation of nuclear skin qualities, which can then be observed in a fragment separator.^{[citation needed]}

Additionally, electron bombardment takes advantage of the weak force interaction to observe the distribution of neutrons in nuclei through parity violation, which can provide insight into neutron skin structure.^[1]^[2]

Examples

Pb-208, an isotope of lead with 82 protons and 126 neutrons (doubly magic). Subject of research conducted at the Thomas Jefferson National Accelerator Facility by the "PREX" team.^[3]^[4]