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Nuclear Reactions in Exploding Stars

Schematic of a neutron capture reaction by a nickel-68 nucleus to become nickel-69 emitting gamma radiation. Figure credit: Erin O’Donnell, Michigan State University

Article appeared in JPhys+ blog

The field of Nuclear Astrophysics is an exciting field that brings together scientists from many different communities: Stellar Observers, astrophysics modelers, meteorite experts, nuclear experimentalists and theorists, atomic physicists...and the list goes on. We all come together to answer some of the most fundamental questions about the Universe, like “How do stars evolve and explode?”, and “How are the elements we see around us created?”

In a recent Journal of Physics G article (link), a collaboration between US and Norway presented a new measurement of an astrophysical nuclear reaction that, together with other reactions, drives the synthesis of heavy elements with atomic numbers around 30. During the reaction of interest a nickel-68 nucleus will capture a neutron from the stellar environment and increase its mass by one unit, becoming nickel-69. The original nickel-68 nucleus has a half-life of about 30 seconds, while the captured neutron’s half life is about 10 minutes. These rather short half lives create a significant experimental challenge because accelerator based experiments require one of the two participants in a reaction to live long enough to be used as a “target”, while the other is the “beam”. What can you do when faced with an important reaction that is almost impossible to measure? You look for creative “indirect” approaches to attack the problem.

An indirect technique for studying neutron capture reactions, the so called “β-Oslo” method, was recently developed by the same collaboration. The technique is used to extract some of the most fundamental properties of the participating nuclei, and use them to better understand neutron capture reactions. For many exotic nuclei, this technique provides the only constraint for these important reactions. The Journal of Physic G (link) publication presents the first application of the β-Oslo technique for the case of nickel-68 and helps to reduce the uncertainties associated with this reaction in heavy-element nucleosynthesis.

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