The unexpected neutron ballet of oxygen-28

The stability of an atomic nucleus can be measured by the energy required to dislodge its nucleons (protons and neutrons) from the outermost orbitals. In so-called magic nuclei, whose orbitals are fully populated, the energy required is generally greater. One of them, however, oxygen-28, seems to escape this rule. Scientists from the SAMURAI21-NeuLAND collaboration, in which Olivier Sorlin, researcher at GANIL, is taking part, have demonstrated, through the study of a very close nucleus, fluor-30, that this nucleus is not magic after all and that its nucleons are not held there more firmly. Instead, a regime of superfluidity sets in, with neutrons jumping effortlessly between entangled orbitals.

Ever since they took up the challenge of studying oxygen-28, the scientists from the SAMURAI collaboration in Japan, which includes physicists from IN2P3, have been going from one surprise to the next. The latest one is not the least. This nucleus, composed of 8 protons and 20 neutrons, is particularly interesting because, despite its intrinsic characteristics, which should make it a ‘magic’ nucleus, i.e. more stable than its neighbours, it turns out to be highly unstable: its lifetime does not exceed 10-20 seconds. Physicists therefore sought to better understand its intrinsic functioning in order to assess whether or not it should be considered as magic. And what they found is very interesting. The victory of instability is due to a rapprochement of the orbitals in which the nucleons are arranged, which allows the installation of a superfluidity regime. Superfluidity? To fully understand this phenomenon, let’s go back to the basics of nuclear physics.

In atomic nuclei, the rule is that when one of them has the right number of nucleons to fill its orbitals, it is more stable and is said to be magical. This effect will also be increased tenfold if this optimal filling concerns both protons and neutrons. This is known as a doubly magical nucleus. Oxygen-28, with its 8 protons and 20 neutrons, belongs to this super-category and as such should, in theory, have greater stability. But another equally implacable rule of nuclear physics works in the opposite direction. This time it’s the difference between the number of protons and the number of neutrons that counts. The most stable nuclei have a substantially identical quantity. But as the difference increases, the nuclei become less and less stable. With 20 neutrons and 8 protons, oxygen-28 has a very strong tendency towards instability. And in the end, there’s no contest: with its tiny lifespan, it’s clear that the rules of instability reign supreme. The question was why.

To probe the magicity of oxygen-28, the scientists will seek to compare the energy needed to snatch a neutron from oxygen-28 with that needed to do the same with oxygen-29, which has one more neutron. The idea was that the magical oxygen-28 should retain its neutrons more firmly than oxygen-29. However, the ultra-short lifetimes of these two isotopes and, above all, the decay mode of oxygen-29 make this test extremely difficult. The scientists from the SAMURAI21-NeuLAND collaboration therefore proposed to get round the problem by studying nuclei that are very similar in terms of the number of nucleons, such as fluorine-30 (9 protons, 21 neutrons) and fluorine-29 (9 protons, 20 neutrons). The proximity between the two elements means that the results for fluorine isotopes can be extrapolated to those for oxygen. The measurements obtained are indisputable: the energy required to eject a neutron from each of the two fluorine isotopes is comparable, proving that the ‘magic’ stabilising configuration of 20 neutrons no longer exists.

« We believe that in this configuration, involving very loosely bound and very unstable nuclei, the very distinct orbitals that are normally found give way to a tangle of orbitals between which neutrons can circulate freely, explains Olivier Sorlin, researcher at GANIL who took part in the SAMURAI21-NeuLAND study. This new regime that is establishing itself is probably that of superfluidity, where neutrons pair with each other and jump from one orbit to another in an undifferentiated manner. In this context, the rules of magic, determined by the completeness or incompleteness of certain orbitals, no longer apply. This is why oxygen-28 is not magic. »

Graph showing the evolution of the separation energy (Sn) of the last neutron of Fluorine isotopes. The oscillations in the curve are due to the fact that, since neutrons evolve in pairs, this energy is systematically higher in isotopes containing an even number of neutrons than in those containing an odd number: pulling out an isolated neutron requires less energy. Between fluorine-29 and fluorine-30, these oscillations remain constant. This proves that the neutron orbitals are indeed entangled. The presence of the magic number N=20 would have led to a drop in Sn between fluorine-29 and fluorine-30.

But the surprises don’t stop there. While this is the first time that a superfluidity regime has been observed for exotic nuclei, the scientists were also able to make another disturbing observation. « Until now, we thought that neutron pairing in the context of superfluidity took place in the atomic nucleus only over long distances, when the two neutrons in a pair were in distant regions of the nucleus, continues Olivier Sorlin. However, the theoretical models that reproduce the experimental results for fluorine and oxygen nuclei suggest that the neutron pairs are much closer. If this result were to be confirmed by the new, more specific experiments planned by the SAMURAI collaboration, it would reshuffle the cards in terms of superfluidity”. We understand, with this surprise appearance of superfluidity instead of magic, the SAMURAI collaboration has opened the way to an exciting new quest.

Contact: Olivier Sorlin