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Long Beamtime Experiments for Nuclear Astrophysics

Final Report Summary - LOBENA (Long Beamtime Experiments for Nuclear Astrophysics)

Reaction rates of nuclear reactions are needed to understand many astrophysical objects and phenomena. Some of the reactions cannot be directly measured because the cross sections are unpractically small or very short-lived species are involved. In such cases reaction rates must be indirectly estimated. The goal of this project has been to determine properties of very short-lived nuclear resonances in 8Be, 12C, and 16O needed to indirectly determine the rates of astrophysical reaction rates. Specifically, a main goal has been to develop a new method to selectively populate resonances of interest using electromagnetic deexcitation. In addition to this, the proven method of populating resonances using weak interaction decays of unstable isotopes. To achieve these goals an efficient array of charged particle detectors and associated electronics was constructed, and a comprehensive software library containing the required tools to analyse such data has been developed and made available to the community.

A comprehensive set of measurement have been completed using both the electromagnetic and weak interactions to populate or search for nuclear states of astrophysical importance. Gamma-decaying resonances in 8Be, 12C and 16O were populated using reactions of protons impinging on either lithium, boron or nitrogen targets. Detailed scans were used to search for particularly favorable reaction energies, and for those energies long measurements were executed in order to search for the desired transitions.

The method of using electromagnetic transitions to selectively populate and identity very short-lived nuclear states was demonstrated to be viable, but highly challenging. The challenges mainly come from the fact that the transitions of interest are not the dominating mode of decay, which leads to a need to use both long measurement times and relatively intense beams of protons with associated induced backgrounds in the detectors. Despite these challenges important results have been achieved. The measurements focused on 8Be have provided a qualitatively much cleaner into this important system, presently under interpretation by state-of-the art nuclear theory. In 12C evidence for new short-lived nuclear states have been provided, while for 16O we were not able to identify transitions of interest.

The experiments using the previously proved method of weak interaction decays were completed with focus on both 12C using decay or 12N, and on 16O using the decay of 16N. The latter has let to the most accurate determination of the decay and accordingly a better indirect determination of the very important nuclear reaction of capture of helium on carbon to produce oxygen. The work on 12C has been harder to interpret because the features observed in the experimental spectra are relatively featureless and therefor somewhat ambiguous to interpret. This work is still in progress.