Can a Nucleus Have Four Neutrons and Two Faces?

Two cool new results in nuclear physics that promise to extend our understanding of the nucleus have been in the news. The first relates to the possibility of a stable four neutron nucleus, and the second on the relative stability of some light nuclei with deformed shapes occasioned by energy excitation. By relative stability one means relative longevity of the deformed shape prior to return to the ground shape or state.

On the first Physics World reports

in 2016, physicists working at the RIKEN nuclear-physics lab in Japan found evidence for tetraneutron in a different experiment that involved firing neutron-rich helium-8 nuclei at a helium-4 target. While they did not see direct evidence for a tetraneutron, careful measurements of the two helium-four particles produced in the collision suggest that the other four neutrons involved in the collision emerge in a bound state

The interesting thing about that is that there is no account consistent with known nuclear theory that allows for a stable four neutron nucleus. So, we might say that a tetraneutron nucleus is what philosophers of science refer to as an anomaly.

However, Physics World reports on recent calculations suggesting that a four neutron nucleus is not stable enough to constitute a nucleus

A larger energy width corresponds to a short lifetime for the tetraneutron, and this has led the team to suggest that the four-neutron system may not stick around long enough to be considered to be a nucleus

This is certainly worth following up for all those interested in the theory of the nucleus.

The second news item, reported by Science Magazine, is no less interesting

Constructed of protons and neutrons, atomic nuclei are generally considered to be spherical structures. However, in reality, most atomic nuclei are structures that are deformed to a greater or lesser extent: flattened or elongated along one, two, or sometimes even all three axes. What’s more, just as a ball flattens more or less depending on the force exerted on it by a hand, so atomic nuclei can change their deformation depending on the amount of energy they possess, even when they are not spinning

The actinides have relatively stable deformed shapes, in the sense spoken of in the introduction above, when excited

In the last few decades, evidence has been gathered confirming that a relatively stable state with a deformed shape is present in the nuclei of a small number of elements. Measurements have shown that the nuclei of some actinides – elements with atomic numbers from 89 (actinium) to 103 (lawrencium) – are capable of maintaining their ‘second face’ even tens of millions of times longer than other nuclei

However, it has been shown for the first time that this can apply to a lighter more stable nucleus, in this case Nickel-66

In the experiment in Bucharest, a target of nickel-64 was fired with nuclei of oxygen-18. Relative to oxygen-16, which is the main (99.76%) isotope of atmospheric oxygen, these nuclei contain two additional neutrons. During the collisions, both the excess neutrons can be transferred to the nickel nuclei, resulting in the creation of nickel-66, the basic shape of which is almost an ideal sphere.

With properly selected collision energies, a small portion of the Ni-66 nuclei thus formed achieve a certain state with a deformed shape which, as measurements showed, proved to be slightly more stable than all other excited states associated with significant deformation

What is interesting here is that

The calculations necessary for the preparation of the experiment proved to be so complex that a computer infrastructure of about one million processors was required to perform them…

…The new experimental method, proposed by Prof. Silvia Leoni (UniMi), combined with the computationally extremely sophisticated Monte Carlo shell model developed by the Tokyo University theorists, enabled the design of appropriate, accurate measurements…

…the measured delay times of return to the basic state correspond to an acceptable extent with the values provided by the new theoretical model, which further enhances the attainment of the achievement. None of the earlier models of nuclear structure allowed for such detailed predictions

Nuclear physics, both theoretical and experimental, is very much still alive.