If you hit an atom's nucleus hard enough, it will fall apart. But exactly how it falls apart tells us something about the internal structure of the nucleus and perhaps about the interior of neutron stars. One of the unexpected things we seem to be learning is that the way particles in the nucleus pair up allows them to reach higher energies than expected, and having excess neutrons only encourages this behavior.

To someone like me—I never took any courses on nuclear physics—the nucleus is a bit like visiting a familiar beach and discovering a colony of dragons. The nucleus consists of protons, which are positively charged. These should repel each other, but the nucleus doesn’t explode because of neutrons. Neutrons are, as the name suggests, neutral. However, they are the glue that binds the protons together.

This description makes the nucleus sound like a disorganized mess of protons and neutrons, but it isn’t. The nucleus has a structure remarkably similar to the electrons orbiting the nucleus.

Like electrons, the protons and neutrons stack in order of energy to fill shells. Two nucleons—a nucleon is a proton or a neutron—occupy the first shell. The next shell can hold six, and so on. Nucleon stacking broadly explains experimental results.

Coming out of their shell

What sort of experiments? Well, roughly speaking, we put atoms up against the wall and shoot them with high-energy electrons. Occasionally, an electron hits the nucleus and pops out a neutron or a proton. The energy and momentum of the nucleons that we've popped out are about right for the shells that are expected to be occupied.

Except there are always some nucleons that have higher than expected energy. Imagine that we we're dealing with something like carbon, which has six protons and six neutrons. We expect the first two shells to be filled and the remaining four nucleons to partially fill the third shell. From that, we would not expect to observe nucleons with higher energy or momentum than that associated with the third shell.

But some 20 percent of the nucleons have an energy and momentum higher than this.

These results might be partially explained by neutron and proton pairing. This pairing allows the two to have a momentum and energy much larger than they normally would, and they could then occupy a shell much higher than expected from shell theory. In fact, they form a kind of current as they move around the nucleus.

This idea works quite well for nuclei that have an equal number of protons and neutrons because every neutron has a proton to pair up with. Most atoms do not have an equal number of protons and neutrons, though. Instead, they have a larger number of neutrons. Lead, for instance, has 82 protons and something like 125 neutrons. How do protons and neutrons pair up in the presence of an excess of neutrons? It is pretty easy to imagine that the extra neutrons act a bit like a third wheel, preventing protons from pairing off with the neutron they find most attractive. But there were no experiments to test this.

Finding proton-neutron love in a sea of neutrons

Experiments from the continuous electron-beam accelerator facility and large acceptance spectrometer at the US' Jefferson Laboratory have now shown that neutrons actually play nice. The researchers shot high-energy electrons at carbon, aluminum, iron, and lead. These atoms increase the ratio of neutrons to protons from 1.0 (no excess neutrons) in carbon to 1.5 (lots and lots of excess neutrons) in lead. The researchers then compared the number of high-energy neutrons and protons emitted by each element.

The researchers found that as more neutrons were added to the nucleus, the fraction of high energy neutrons that popped out went down, while the fraction of high energy protons went up.

These results support the pairing idea. At equal numbers, the chance of emitting a paired or unpaired nucleon is even, so the high energy nucleons are as likely as low-energy nucleons. However, in heavy nuclei, the excess neutrons cannot be paired and cannot have high energy. Hence, the fraction of high-energy neutrons goes down.

In contrast, the fraction of high-energy protons goes up, which implies that more protons are paired up with available neutrons. If excess neutrons disrupted pairing, the fraction of high-energy protons would stay the same or go down.

What does this mean? It may mean that our description of neutron stars needs to be updated. Neutron stars contain a small fraction of protons and electrons. It's likely that these protons will pair with neutrons and move to relatively high-energy states—and, yes, generate a current in the neutron star. These high-energy particles will change the rate at which the star cools.

There are also a number of experiments that involve banging nuclei together in an effort to uncover new physics. Understanding the results from these experiments means getting the nucleon interactions as correct as possible. The scattering results uncovered by the researchers will need to be taken into account in those experiments as well.

Nature Physics, 2018, DOI: 10.1038/s41586-018-0400-z. (About DOIs).