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A team of physicists has used European Research Council funding to reveal how Oxygen-16’s nuclear shape changes depending on its state. Their findings could help expose how oxygen is produced.

Oxygen-16 is produced by a series of reactions inside red giant stars. Carbon and oxygen are formed when helium burns inside red giant stars. Carbon-12 forms when three helium-4 nuclei combine in a specific way (called the triple alpha process), whilst oxygen-16 is the combination of a carbon-12 and another helium-4 nucleus.

Although physicists knew what oxygen-16 was made of, they were still puzzled by the fact that both the ground and first excited states of the element had zero spin and positive parity. A similar situation occurs in carbon-12 with the ground state and second zero-spin state known as the Hoyle state. At room temperature, only the ground state of oxygen-16 is seen due to the very cold temperature compared to nuclear energies. Yet the excited states of oxygen-16 become important for the helium-burning reactions inside stars.

Physicist Dean Lee of North Carolina State University, USA said: “It’s expected that oxygen-16 would have zero spin and positive parity as its ground state. What is unexpected is that the first excited state also has these qualities. It made us wonder what the real difference is between the states, which required looking at the structure of the eight protons and eight neutrons in oxygen-16. We had addressed a similar puzzle for the ground state and Hoyle state of carbon-12.”

Lee and his colleagues had previously developed a new method for describing all the possible ways that protons and neutrons can bind with one another inside nuclei such as carbon-12 and the Hoyle state. They used an approach called ‘effective field theory’ formulated on a complex numerical lattice that allows the researchers to run simulations that show how particles interact, and so reveal the structure of the nuclei.

“These lattice simulations give us our first look at the structure of low-energy states of oxygen-16,” Lee said.

The team found their lattice revealed that although both the ground and first excited states of oxygen-16 ‘look’ the same in terms of spin and parity, they are in fact quite different structurally. In the ground state, the protons and neutrons are arranged in a tetrahedral configuration of four alpha clusters containing two protons and two neutrons each. For the first excited state, the alpha clusters are arranged in a square.