Valence proton could play a key role in oxygen neutron drip line anomaly – Physics World

Physics

Unexpected result: the SHARAQ detector at the Radioactive Isotope Beam Factory (Courtesy: RIKEN)

Adding a single proton to a doubly magic isotope of oxygen is enough to significantly alter its properties, an international team of physicists has discovered. Led by Tsz Leung Tang at the University of Tokyo, the researchers made the unexpected discovery after removing a proton from a neutron-rich isotope of fluorine. Their work could lead to a better understanding of the complex interactions that take place between protons and neutrons within atomic nuclei.

Basic information about how protons and neutrons interact within a nucleus can be gleaned from a nuclide chart, which plots the numbers of protons in an isotope against the number of neutrons. The “neutron drip line” in such a plot shows the maximum number of neutrons an isotope of each element can contain.

One particularly striking feature of this boundary is the sharp jump in neutrons between neighbouring oxygen and fluorine, which has one more proton than oxygen. An oxygen nucleus (containing eight protons) can contain up to 16 neutrons, however fluorine can contain as many as 22 neutrons. The reasons behind this jump are poorly understood, but researchers believe it is related to oxygen-24’s “doubly magic” nucleus, which contains extremely stable filled “shells” of protons and neutrons.

Valence and core

To explore the jump in more detail, Tang’s team prepared a beam of the isotope fluorine-25 at the Radioactive Isotope Beam Factory near Tokyo – which is run jointly by Japan’s national research institute RIKEN and the University of Tokyo. Fluorine-25 contains one more proton than oxygen-24 and can be thought of as an oxygen-24 core plus a single valence proton.

This latest research involved colliding fluorine-25 nuclei with a target to remove a proton. Using the SHARAQ detector, Tang and colleagues measured correlations between the motions of the collision products and found that around 65% of the resulting oxygen-24 nuclei were in an excited state. This is contrary to current theory, which predicts that the oxygen-24 core of fluorine-25 should exist in its lowest energy state.

This suggests that the addition of a single valence proton to oxygen-24 has a profound effect on the doubly magic core. Indeed, Tang’s team concluded fluorine-25’s excited core is likely responsible for the neutron drip line’s dramatic jump – although the reasons why such significant changes can be driven by a single proton remain a mystery.

The team now aims to uncover the physical mechanisms in future experiments. If successful, future experiments could lead to significant improvements to our understanding of the processes that occur inside atomic nuclei – and also provide new insights into the mysterious properties of neutron-rich astronomical features, including supernovae and neutron stars.

The research is described in Physical Review Letters.

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