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The sizes of proton and neutron orbits in calcium and scandium nuclei were measured from the momenta of knocked-out particles.
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Inside an atomic nucleus, protons and neutrons don't sit still like peas in a pod. Each particle occupies its own orbit — a region of space where it can be found. The larger the orbit, the more 'smeared out' the particle is. Scientists measured the sizes of such orbits in calcium and scandium nuclei: they knocked particles out of the nucleus and observed the momentum with which they flew out.
The method is similar to a game of billiards. If you strike a ball near the edge of the table, it will travel differently than a ball in the center. Same here: a particle that occupied a large orbit gets a different momentum when knocked out than a particle from a small orbit. By comparing the measured momenta with calculations, the scientists reconstructed the sizes of the orbits.
Neutron orbits with high angular momentum turned out to be 0.48–0.78 femtometers smaller than orbits with lower momentum. A femtometer is one millionth of a billionth of a meter. For comparison, the size of the calcium nucleus itself is about 3.5 femtometers.
The result for proton orbits was less definitive: there isn't enough data yet to say exactly how they change from nucleus to nucleus. But the method itself works: it allows us to peer inside the nucleus and see how particles are distributed within it. This is important for understanding how matter is structured at the deepest level.
The calcium-52 nucleus contains 20 protons and 32 neutrons: it is one of the heaviest stable calcium nuclei, and its shape is close to spherical.
In Peter Watts's novel 'Blindsight,' humans try to understand an alien form of intelligence; studying nuclear orbits is also an attempt to understand a structure that isn't directly visible but only manifests through indirect signals.
