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Detailed fusion: The visualization of clustering in nuclear collisions


Colliding nuclei can fuse and form a new nucleus. Sometimes this fusion is temporary – the newly formed nucleus is in an excited state and decays after a short period of time into a stable state or falls apart. Such a short-lived nucleus is called a pre-compound. In a publication of the journal Physical Review C the two scientists Dr. Bastian Schütrumpf from GSI Helmholtzzentrum für Schwerionenforschung and Dr. Witold Nazarewicz from the Michigan State University, USA, take a closer look at the inner structure of such pre-compounds.

Atomic nuclei consist of two building blocks, the protons and neutrons. Schütrumpf and Nazarewicz developed a model to predict the behavior of these building blocks and also to visualize it. The new technique demonstrates that groups of protons and neutrons form temporary clusters which correspond to smaller, stable nuclei within the larger nucleus produced by the collision. These clusters are variable and can change between different states.

The researchers analyzed reactions triggered by collisions of oxygen, calcium, and carbon, which can – depending on the collision energy – result in either fusion or fission. The calculations reveal clusters corresponding to the nuclei helium-4, carbon-12, magnesium-24, and argon-36. For example, two oxygen-16 nuclei colliding at an energy of 20 mega electron-volt form a pre-compound in which two deformed carbon-12 clusters oscillate against two helium-4 nuclei (alpha particles).

“Such fleeting nuclear states often appear in stars or other space phenomena. This is why they are of great interest to the scientists and are frequently examined in collision reactions. To understand their structure is fundamental for deciphering them,” explains Dr. Bastian Schütrumpf, who works as a post-doc in the GSI research department “Theory”. “Previous theory and experiments have suggested the existence of clusters in the pre-compound. Existing models, however, couldn’t reveal their detailed nature.” To address this problem, Schütrumpf and Nazarewicz used a mathematical tool originally developed to describe electron arrangements within atoms and molecules and applied it to the nucleons.

In the future, the scientists want to improve and enhance their calculations. Thus, the model could tackle even more complex asymmetrical reactions with different nuclei. While the current application focusses on low-energy reactions, clustering of neutrons and protons is a ubiquitous phenomenon which impacts also high-energy collisions as they will occur e.g. at FAIR.

Further information:

Oxygen collision
Visualization of clustering in the collision of two oxygen nuclei.
Picture: B. Schütrumpf/W. Nazarewicz