A rare isotope of oxygen has finally been discovered, and it’s defying expectations

The superconducting toroidal cyclotron at the Radioisotope Beam Factory in Wako, Saitama, Japan.

The Riken RI Beam Factory in Waco, Japan, generates streams of radioisotopes with the help of this superconducting ring cyclotron.Credit: Nishina Center for Accelerator-Based Science

By combining a powerful set of tools with some experimental experience, physicists have discovered for the first time oxygen-28 — an isotope of oxygen that has 12 extra neutrons packed into its nucleus. Scientists have long speculated that this isotope is extraordinarily stable. But the initial notes of 28The kernel indicates that this is not the case: it disintegrates quickly after creation, and informs the team of this nature today1. If the results can be replicated, physicists may need to update theories about how atomic nuclei are structured.

The most powerful force in the universe is the one that holds protons and neutrons together in the nucleus of an atom. To uncover how elements form, the physics of neutron stars, and more, scientists need a better understanding of this powerful nuclear force, says Takashi Nakamura, a physicist at the Tokyo Institute of Technology. He and other researchers are testing theories about how atomic nuclei are held together by pushing them to extremes. One common method is to load lightweight nuclei, such as oxygen, with excess neutrons and see what happens.

Current theories state that atomic nuclei containing certain numbers of protons and neutrons are inherently stable. This is because protons and neutrons fill the “shells” in the nucleus. When the shell is filled with the right number of protons or neutrons, it becomes very difficult to add or remove particles. These are “magic” numbers, and are thought to include 2, 8, 20, 28, 50, 82 and 126 particles. If the nucleus contains a magic number of both neutrons and protons, it becomes “double magic” – and therefore more stable.

the most abundant form of oxygen, 16O, is double magic, because it has eight protons and eight neutrons. Oxygen-28, which has 8 protons and 20 neutrons, has long been expected to have a magic double as well. But physicists had not been able to detect it before.

ghost hunters

surveillance 28O requires many experimental works. Key to the whole process were the intense streams of radioactive isotopes produced by the Riken RI Beam Plant in Wako, Japan. Scientists fired a beam of calcium-48 isotope at a beryllium target, creating the isotope fluorine-29. The nucleus of this isotope contains one more proton than 28Oh but the same number of neutrons. Scientists broke next 29F to a thick barrier of liquid hydrogen, causing a proton to be ejected from the nucleus and generated 28Hey.

Group photo of the team setting up the oxygen-28 experiment at the RIKEN radioisotope beam plant.

A large international team of researchers used existing instruments at the Riken RI Beam Factory to detect oxygen-28.Credit: Yosuke Kondo

This rare form of oxygen was too short-lived to be observed directly. Instead, the team detected its decay products: oxygen-24 plus four neutrons, a measurement that seemed impossible just a few years ago.

Nakamura says up to two neutrons have been measured at the same time, but this is the first time scientists have detected four neutrons at once. “They’re like ghosts,” he says of neutrinos. With no electric charge, neutrons cannot grapple in the same way that protons can (24O, with its eight positively charged protons, can be introduced into a detector using a magnet). To monitor individual neutrons, the team used a powerful purpose-built detector on loan from the GSI Helmholtz Center for Heavy Ion Research in Darmstadt, Germany, as well as Riken instruments. In this specialized detector, incoming neutrons are detected when they strike protons. Nakamura says the study’s lead author, Tokyo Institute of Technology physicist Yosuke Kondo, used simulations to help verify these difficult measurements.

“They really did their homework,” says Robert Janssens, a physicist at the University of North Carolina at Chapel Hill. “They did all the checks you can do. It’s a tour de force.”

Atomic boundaries

Although the team was unable to get an accurate measurement of the age 28Oh, Nakamura says the counterpart didn’t act as if it were a double magic, but rather collapsed as soon as it came into existence.

“I was surprised,” he says. “Personally, I thought it was double magic. But that’s what nature says.”

This isn’t the first indication that nuclear physicists’ list of magic numbers is not universally applicable, says Rituparna Kanungu, a physicist at Saint Mary’s University in Halifax, Canada. She was part of a team of scientists who demonstrated this in 2009 24O – contrary to the nuclear rulebook – has a core that acts as if it were double magic2. Its eight protons and 16 neutrons are tightly bound together, giving it a relatively long lifetime – it’s about 61 milliseconds for half the time. 24Oh to disappear through radioactive decay. This means that on some cores, if they are strongly correlated, 16 can be a magic number as well.

“The magic numbers are not fixed,” says Janssens.

Right now, the confusing qualities of 28It raises a whole host of questions about the forces that hold the nuclei together. Physicists daydream about possible next steps. Nakamura wants to know if oxygen-30 can be detected. Because the stability of different isotopes is a relative measure, it might be useful to compare 28Ah with that heavier neighbor yet to be seen.

“It’s very simple and very complex,” says Janssens. “We don’t know at the moment how many protons and neutrons you can pack together in a nucleus” and keep them bound together, he adds. In other words, what is the limit?

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