The atom that defies theory

2023-12-01 20:36:51

Magic exists. At least, according to theoretical research in nuclear physics, which more than 70 years ago identified “magic numbers”: 2, 8, 20, 28, 50, 82 and 126. These would represent the ideal number of neutrons or protons to ensure increased stability of isotopes of different chemical elements. It still had to be verified in the laboratory!

In the magazine Nature, an international team reported for the first time the formation of oxygen-28 (28O) thanks to collisions carried out in cyclotrons, in the facilities of the RIKEN research institute, in Japan. It was a series of highly energetic bombings which gave birth to this 28O. Calcium-48 was subjected to fragmentation which gave rise to various atoms, including fluorine-29. When projected onto a liquid hydrogen target, a proton dissociated, resulting in the creation of 28O.

And following decades of research, a twist: the element obtained does not behave as the theory predicted! The instability of 28O destabilizes scientists.

As a reminder, there are different versions of the chemical elements of the periodic table. Take oxygen, for example: all its forms contain the same number of protons (8), but the number of neutrons can vary – we then speak of isotopes. Some isotopes are stable, others decay more or less quickly: it all depends on the balance between the number of protons and neutrons. The most common oxygen, the one we breathe, is 16O. What is particularly remarkable regarding it is that it is “double magic” with its eight protons and eight neutrons – a very rare phenomenon.

Its counterpart 28O is also doubly magical with its 8 protons and 20 neutrons, 12 more than 16O. However, to the surprise of the scientific community, “28O spits out four neutrons almost instantly following its formation in the cyclotron,” says Rituparna Kanungo, professor of physics at Saint Mary’s University, still surprised.

This scientist associated with the Canadian cyclotron TRIUMF did not participate in the study, but she is greatly interested in it since she has also published work suggesting that the theory of magic numbers might not quite hold up. In 2009, she observed that the stability of 24O was greater than predicted, with a radioactive half-life of 61 milliseconds, despite a number of neutrons that was not at all “magical”.

His work and new research on 28O highlight the complexity of nature’s most powerful force: the force that holds protons and neutrons in a nucleus.

How does this advance science? Rituparna Kanungo reminds us that the Universe is a natural cyclotron. Nuclear fusion in the heart of the Sun continually produces isotopes, the most stable being found on surrounding planets. Understanding isotopes – from their formation to their disintegration – thus opens windows on the past and present of the Universe, in particular on the atoms which make up the Earth.

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