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Uncommon oxygen isotope detected finally — and it defies expectations

Uncommon oxygen isotope detected finally — and it defies expectations

2023-08-31 12:14:51

The Superconducting Ring Cyclotron in the Radioactive Isotope Beam Factory in Wakō, Saitama, Japan.

The Riken RI Beam Manufacturing facility in Wako, Japan, creates streams of radioactive isotopes with assist from this superconducting ring cyclotron.Credit score: Nishina Middle for Accelerator-Based mostly Science

By combining a strong set of devices with some experimental savvy, physicists have for the primary time detected oxygen-28 — an isotope of oxygen that has 12 further neutrons packed into its nucleus. Scientists have lengthy predicted that this isotope is unusually secure. However preliminary observations of the 28O nucleus counsel that this isn’t the case: it disintegrates quickly after creation, a group studies in Nature right this moment1. If the outcomes could be replicated, physicists may have to replace theories of how atomic nuclei are structured.

The strongest pressure within the Universe is the one which holds collectively the protons and neutrons in an atom’s nucleus. To unlock how components are cast, the physics of neutron stars and extra, scientists want to raised perceive this sturdy nuclear pressure, says Takashi Nakamura, a physicist on the Tokyo Institute of Know-how. He and different researchers are testing theories about how atomic nuclei are held collectively by pushing them to extremes. One in style approach is to load light-weight nuclei, resembling oxygen, with extra neutrons and see what occurs.

Present theories state that atomic nuclei with sure numbers of protons and neutrons are inherently secure. It’s because protons and neutrons replenish ‘shells’ within the nucleus. When a shell is crammed with simply the precise variety of protons or neutrons, it turns into massively tough so as to add or take away particles. These are ‘magic’ numbers, and have been thought to incorporate 2, 8, 20, 28, 50, 82 and 126 particles. If a nucleus has a magic variety of each neutrons and protons, it turns into ‘doubly magic’ — and subsequently much more secure.

Probably the most considerable type of oxygen, 16O, is doubly magic, due to its eight protons and eight neutrons. Oxygen-28, with 8 protons and 20 neutrons, has lengthy been predicted to be doubly magic, too. However physicists haven’t been capable of detect it earlier than.

Ghost hunters

Observing 28O required a number of experimental feats. Key to the entire operation had been the extraordinary streams of radioactive isotopes produced by the Riken RI Beam Manufacturing facility in Wako, Japan. The scientists fired a beam of calcium-48 isotopes at a beryllium goal, which created a fluorine-29 isotope. The nucleus of this isotope has yet one more proton than does 28O however the identical variety of neutrons. The scientists subsequent smashed 29F right into a thick barrier of liquid hydrogen, knocking a proton out of the nucleus and producing 28O.

The a group photo of the team and the Oxygen-28 experiment setup at the RIKEN Radioactive Isotope Beam Factory.

A big worldwide group of researchers used devices on the Riken RI Beam Manufacturing facility to detect oxygen-28.Credit score: Yosuke Kondo

This uncommon type of oxygen was too short-lived to be noticed instantly. As a substitute, the group detected its decay merchandise: oxygen-24 plus 4 neutrons, a measurement that appeared not possible just a few years in the past.

Measuring as much as two neutrons on the identical time has been executed, however that is the primary time scientists have detected 4 concurrently, Nakamura says. “They’re like ghosts,” he says of neutrons. With no electrical cost, neutrons can’t be wrangled in the identical approach that protons can (24O, with its eight positively charged protons, may very well be ushered right into a detector with magnets). To watch particular person neutrons, the group used a strong detector constructed for that objective, on mortgage from the GSI Helmholtz Centre for Heavy Ion Analysis in Darmstadt, Germany, along with Riken’s devices. On this specialised detector, incoming neutrons are revealed once they knock protons round. Nakamura says that the examine’s lead creator, Tokyo Institute of Know-how physicist Yosuke Kondo, used simulations to assist to confirm these tough measurements.

“They’ve actually executed their homework,” says Robert Janssens, a physicist on the College of North Carolina at Chapel Hill. “They did all of the checks you could possibly do. It’s a tour de pressure.”

Atomic limits

Though the group wasn’t capable of get an actual measurement of the lifetime of 28O, Nakamura says that the isotope didn’t behave as if it had been doubly magic — it fell aside nearly as quickly because it got here into existence.

“I used to be shocked,” he says. “Personally, I assumed it was doubly magic. However that is what nature says.”

This isn’t the primary trace that nuclear physicists’ checklist of magic numbers is just not universally relevant, says Rituparna Kanungo, a physicist at Saint Mary’s College in Halifax, Canada. She was a part of a group of scientists that confirmed in 2009 that 24O — opposite to the nuclear rulebook — has a nucleus that behaves as if it’s doubly magic2. Its 8 protons and 16 neutrons are strongly sure to at least one one other, giving it a comparatively lengthy lifetime — it takes about 61 milliseconds for half of the 24O to vanish by way of radioactive decay. Which means in some nuclei, if they’re strongly sure, 16 may very well be a magic quantity, too.

“Magic numbers are usually not immutable,” Janssens says.

For now, the confounding qualities of 28O increase an entire host of questions in regards to the forces that maintain nuclei collectively. Physicists are daydreaming about potential subsequent steps. Nakamura desires to see whether or not it’s potential to detect oxygen-30. As a result of the steadiness of various isotopes is a relative measurement, it will be useful to check 28O with this heavier, yet-unseen, close to neighbour.

“It’s so easy and so sophisticated,” Janssens says. “We don’t know for the time being what number of protons and neutrons you possibly can put collectively in a nucleus” and have them keep sure collectively, he provides. “In different phrases, what’s the restrict?”

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