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Exotic nuclei shed light on the elements

Published Jan 04, 2011

A research team led by Bo Cederwall, professor of experimental nuclear physics at KTH, has identified the basic structure of the extremely neutron-poor atomic nucleus 92Pd. The discovery, which is now presented in the science journal Nature, is providing new insights into basic nuclear physics but may also have implications for our understanding of the evolution of stars and how some of our basic elements have been created.

Bo Cederwall
Bo Cederwall, professor of experimental nuclear physics at KTH.

In more detail, it is about the structure of the nucleus 92Pd, with an equal amount of neutrons and protons, which differs markedly from what has so far been observed in all the known atomic nuclei. In this exotic atomic nucleus strongly coupled neutron proton pairs have a decisive influence on the structure of the normal state of the nucleus and low-lying excited states. In other atomic nuclei, neutron-neutron pairs and proton-proton pairs dominate instead, which provide properties similar to superfluid helium and superconducting solids.

“These exotic atomic nuclei in their basic state have a completely different structure compared to ordinary atomic nuclei. The discovery provides a better understanding of the complex phenomenon that atomic nuclei are,” says Bo Cederwall.

He says that this particular research project has been going on for nearly a decade, but the breakthrough only came recently. It was not easy to discover the atomic nucleus 92Pd in the nuclear collisions that scientists use to study the most unstable nuclei.

“It’s like finding the famous needle in the haystack, only a few nuclei in a million are 92Pd, such as we want to study. It is a golden opportunity when they turn up,” says Bo Cederwall.

As help, the researchers have made use of very sophisticated equipment, which has been based on the accelerator facility GANIL (Grand Accélérateur National d’Ions Lourdes) in France. The detector system that has been used to identify the structure of 92Pd has been built up by the KTH research group among others.

“Highly sophisticated equipment is required to create and study atomic nuclei under extreme conditions in the laboratory. And extreme conditions are needed to test the theoretical models of the structure of atomic nuclei in a decisive way.

But it is believed that these exotic nuclei can also be created in nature - in certain types of violent stellar explosions. Theoretical models show that they can occur in binary systems of super-dense neutron stars and stars such as red giants,” says Bo Cederwall.

The work has been carried out by scientists at the Nuclear Physics Group at KTH in collaboration with colleagues from several other universities and research institutions.

For more information, contact Bo Cederwall on 08 - 55 37 82 03 or

Peter Larsson