Alpha clusters found on neutron-rich surfaces of nuclei


Alpha spotter: the study was done using the Grand Raiden spectrometer at Osaka’s Research Centre for Nuclear Physics. (Courtesy: Thomas Aumann/TU Darmstadt)

“Alpha clusters” resembling helium-4 nuclei have been spotted on the neutron-rich surfaces of heavy atomic nuclei by an international team of physicists led by Junki Tanaka at TU Darmstadt in Germany and Yang Zaihong at Japan’s Osaka University. The physicists used a high-energy proton beam to knock the clusters off the surfaces of several of tin isotopes and their findings could provide a better understanding of radioactive decay in heavy nuclei and give us important insights into the compositions of neutron stars.

Heavy atomic nuclei tend to contain significantly more neutrons than protons and as a result nuclear physicists believe that these nuclei have neutron-rich “skins” on their surfaces. An understanding of these skins could provide important guidance to astrophysicists developing models of neutron stars – objects about 20 km in diameter with densities on par with nuclei. While neutron stars are made mostly of neutrons, about 5% of their mass comprises protons.

One of the challenges of calculating the properties of neutron stars is understanding how the protons interact with the neutrons. Although neutron stars are much bigger than nuclei, both objects should obey the same physical laws governing how neutrons and protons interact. As a result, studying the neutron-rich skins of nuclei could shed light on the equation of state of neutron stars – a physical model that links the radius of a neutron star with its mass.

Quantum tunnelling

One way that protons and neutrons could interact in a nucleus is to bind together to form an alpha particle (essentially a helium-4 nucleus). In 1928 George Gamow showed that alpha particles can quantum mechanically tunnel out of a nucleus to become free particles – explaining a common radioactive process called alpha decay. While alpha decay is a much-studied effect, it has never been shown conclusively that alpha particles exist in nuclei.

Darmstadt team member Stefan Typel has calculated that alpha particles should form in the neutron-rich skins of heavy nuclei. Now, his prediction has been confirmed by his colleagues who used the 392 MeV proton beam at the Research Centre for Nuclear Physics (RCNP) at Osaka University to knock away alpha clusters from the surfaces of a variety of tin isotopes, containing between 62 and 74 neutrons.

The team found that probability of alpha clusters being knocked away from the isotopes decreases gradually as their neutron numbers increase. Conversely, the thickness of the neutron skin is expected to increase with neutron number– confirming an interplay between alpha clusters and the thickness of neutron skins.

The team’s discovery could lead to a greatly improved theoretical toolset for understanding the properties of neutron stars using data from nuclear physics experiments. It also provides evidence for the existence of alpha particles in nuclei, more than 90 years after Gamow’s prediction.

The research is described in Science.

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