Doubly magic oxygen isotope provides new clues to the strong nuclear force

A first observation of the volatile atomic nucleus oxygen-28 sheds new light on the strong nuclear force that is central to all visible matter in the universe. Chalmers researchers have contributed to the groundbreaking discovery, presented in Nature, with both extensive theoretical calculations and development of the experimental method and complex set of instruments used in the experiment.

The strong force holds neutrons and protons together in an atomic nucleus. It is one of the four fundamental forces of nature and acts only at a very short range. Despite its importance for the stability of atomic nuclei, there remains much more to discover about how it works. A better understanding of the strong force enables us to describe everything from the inner secrets of neutron stars to the life cycles of stars and the formation of different elements.

Researchers have long wanted to study the rare isotope oxygen-28 to learn more about the ability of the force to bind neutrons and protons together. The isotope is particularly interesting because of its composition of 20 neutrons and eight protons. This particular combination is called 'doubly magic' by physicists because it makes possible an extra stable structure with unique properties. However, the question is whether a doubly magic structure is sufficient to bind such a neutron-rich system together. The answer has now turned out to be 'no'.

The first observation of oxygen-28

An international collaboration led by the Tokyo Institute of Technology has now achieved the first observation of oxygen-28 in a large-scale experiment at the Radioactive Ion Beam Factory (RIBF) accelerator facility of the Japanese research centre RIKEN. The results show that oxygen-28 is an unbound atomic nucleus. It is revolutionary because the dividing line between bound and unbound systems is now established. This brings us one step closer to modelling how the strong force works.

Researchers at the Department of Physics at Chalmers have participated in this groundbreaking discovery by performing theoretical calculations and developing the technology underlying the experiment. Professors Thomas Nilsson and Christian Forssén are both co-authors of the scientific article in the journal Nature presenting the oxygen-28 observation. They explain why the observation is important: 'In short, we can say that we have now reached another phase in the study of the strong force and expanded what is possible,' says Thomas Nilsson.

'The oxygen-28 isotope in particular has long been a major issue in the scientific community because of its extreme ratio of neutrons to protons,' adds Christian Forssén. 'It has been one of the very few doubly magic systems that have been within reach to both observe experimentally and calculate theoretically. Now, technological breakthroughs have finally made this possible.'

Measured in a neutron detector

The oxygen-28 nucleus was created by 'kicking out' one or more nucleons from a radioactive beam to make an unbound system, a method that Thomas Nilsson and others in the department have played a leading role in developing for nearly three decades. It turned out that the system only exists for an astonishingly short time span, 10-21 seconds, and could therefore only be observed via its spontaneous decay when four neutrons left the nucleus. These were measured using an advanced neutron detector, designed according to a concept largely developed by the Chalmers group. 'Just the fact that we have now observed four neutrons simultaneously is a feat in itself - it is extremely difficult to detect even one,' says Thomas Nilsson.

Successful theoretical simulations

Once the measurements from the experiment had been collected, extensive analysis of the data was required, which was carried out by the Tokyo Institute of Technology, as well as theoretical simulations. This theoretical study was planned and conducted by Chalmers researchers together with colleagues from the Oak Ridge National Laboratory and Durham University. The calculations provide strong consistency with the experimental results within estimated error margins and have thus resulted in a better theoretical understanding of the strong force and the structure of oxygen-28.

'Oxygen-28 is an incredibly challenging test for our theoretical description of atomic nuclei and the strong force. Modern theoretical nuclear physics aims to construct a model that can describe all atomic nuclei from fundamental principles. The observation that oxygen-28 is unbound adds an important piece of the jigsaw to this work,' says Christian Forssén.

More on the experiment and its results

An isotope is a variant of an element that contains a specific number of neutrons and protons. The rarest ones can only be created for extremely short periods of time by astrophysical processes or in accelerator experiments.
A successful experiment with the RIBF particle accelerator at RIKEN in Japan has revealed for the first time the extremely neutron-rich and very short-lived oxygen isotope oxygen-28, here abbreviated to 28O, which consists of 20 neutrons and eight protons. This was done by allowing a (radioactive) particle beam of fluorine-29 to hit a beam target of liquid hydrogen and detecting the reactions whereby 28O was produced by a proton being knocked out of a fluorine-29 nucleus in the beam. Reactions leading to the formation of oxygen-27 were also detected.
The exotic oxygen isotope 28O was found to exist only for a very short time and then spontaneously decayed to oxygen-24 plus four neutrons. The isotope is thus not particle-bound. All these end products could be detected in the experiment (despite the major technical challenge of detecting multiple neutrons) so that the properties of the short-lived 28O isotope could be measured. This included measuring its life-time, which turned out to be an incredibly short 10-21 seconds.
The NeuLAND neutron detector has been designed for use at the FAIR (Facility for Antiproton and Ion Research) research facility under construction in Darmstadt, Germany, with significant Swedish involvement. The experiment is one of several carried out when NeuLAND was on loan to RIKEN for a multi-year experimental campaign, where, as an example, a correlated four-neutron system was observed using similar methods (Observation of a correlated free four-neutron system, Nature, June 2022).

» Original publication 1

» Original publication 2 (June 2022)

Source: Chalmers University of Technology