Neutron-rich nuclei reveal first observation of rare isotope

The assumption has always aligned with the theory that isotopes are short-lived resonances that decay through spontaneous neutron emission. 

However, this study changes the perspective and finds that neutron-rich isotopes, especially the light ones with neutron-to-proton ratios significantly different from that of stable nuclei, provide stringent tests of modern nuclear structure theories, scientists relayed. 

Nakamura told Nature that the strongest force in the Universe is the one that holds together the protons and neutrons in an atom’s nucleus. Therefore, scientists need to better understand this strong nuclear force to unlock how elements are forged, the physics of neutron stars, and more.

In simple terms, researchers tested theories about how atomic nuclei are held together when pushed to extreme conditions. They adopted a popular technique that loaded lightweight nuclei – oxygen with neutrons. 

While current theories indicate that protons and neutrons are inherently stable, the new study stated that they are stable because protons and neutrons fill up ‘shells’ in the nucleus. 

“When a shell is filled with just the right number of protons or neutrons, it becomes massively difficult to add or take away particles,” Nature added. “These are ‘magic’ numbers and have been thought to include 2, 8, 20, 28, 50, 82, and 126 particles. If a nucleus has a magic number of neutrons and protons, it becomes ‘doubly magic’ — and therefore even more stable.”

When the eight protons are combined with eight neutrons, forming an abundant form of oxygen, eight neutrons infused with 20 neutrons end up creating Oxygen-28, which was undetectable until recently. 

Decay in oxygen isotopes observed via new methods

The detection happened because of an intense streaming of radioactive isotopes produced by the Riken RI Beam Factory in Wako, Japan. 

According to Nature, the researchers shot a beam of calcium-48 isotopes at a beryllium target, which created a fluorine-29 isotope. They then fired 29F into a thick barrier of liquid hydrogen, knocking a proton out of the nucleus and generating O28.

Oxygen-28 was too short-lived, and thus further detailed analysis wasn’t performed. However, the team detected its disintegration yields, including oxygen-24 plus four neutrons, a measurement that seemed impossible only a few years ago.

Dr Kondo explained: “This result suggests that the ‘island of inversion,’ whereby the energy gap between neutron orbitals weakens or vanishes, extends beyond the fluorine isotopes 28F and 29F into the oxygen isotopes.”

The scientists compared the model’s decay energies in both O27 and O28 in a large-scale shell model calculation and a statistical approach also based on effective field theories of quantum chromodynamics, according to a statement by the researchers. 

Dr. Kondo added, “Specifically, the statistical coupled-cluster calculations suggested that the energies of 27O and 28O can provide valuable constraints for the interactions considered in such ab initio approaches.”

The new findings enhance our understanding of atomic nuclear structure, allowing scientists to explore subjects such as multi-neutron correlations, and further studies can also be analyzed using the multi-neutron-decay spectroscopy technique operated here.

The study was published on August 30 in the journal – Nature.

Study abstract:

Subjecting a physical system to extreme conditions is one of the means often used to obtain a better understanding and deeper insight into its organization and structure. In the case of the atomic nucleus, one such approach is to investigate isotopes that have very different neutron-to-proton (N/Z) ratios than in stable nuclei. Light, neutron-rich isotopes exhibit the most asymmetric N/Z ratios and those lying beyond the limits of binding, which undergo spontaneous neutron emission and exist only as very short-lived resonances (about 10−21 s), provide the most stringent tests of modern nuclear-structure theories. Here we report on the first observation of 28O and 27O through their decay into 24O and four and three neutrons, respectively. The 28O nucleus is of particular interest as, with the Z = 8 and N = 20 magic numbers1,2, it is expected in the standard shell-model picture of nuclear structure to be one of a relatively small number of so-called ‘doubly magic’ nuclei. Both 27O and 28O were found to exist as narrow, low-lying resonances and their decay energies are compared here to the results of sophisticated theoretical modelling, including a large-scale shell-model calculation and a newly developed statistical approach. In both cases, the underlying nuclear interactions were derived from effective field theories of quantum chromodynamics. Finally, it is shown that the cross-section for the production of 28O from a 29F beam is consistent with it not exhibiting a closed N = 20 shell structure.