Nuclear scientists are detectives looking for clues in experiment and theory that will led to understanding the laws of nature and their consequences. A particular problem relates to how the elements were made during the evolution of the universe. Part of the investigation involves understanding properties of nuclei that control the rates for nuclear reactions in the hot astrophysical environment of novae. A few of these reactions turn out to be critical in determining the final abundance of elements up to calcium. One of these is rate for proton capture onto 30P to make 31S.

The figure shows the calculated states for 31S in comparison with experiment. The decay properties of the states in 31S just above the proton separation energy Sp determine the rate. This figure is like a nuclear finger print. Each nucleus is different. Experiment tries to clarify the finger print, and theory tries to match it with a model. In collaboration with Werner Richter at iThemba laboratory in South Africa, it was found that negative parity states dominate the proton capture rate [1]. The theoretical error in the energies of about 200 keV is too large to be able to calculate the rate to the precision needed. Thus theory and experiment have to be matched in order to make use of the 10 keV or less error in experiment.

This detective story involves a collaboration with Michael Bennett and Chris Wrede to establish how to match the experimental and theoretical finger prints for states above 6.1 MeV. This is made difficult because many of the states in this energy region do not have definitive angular momentum assignments. Also, it is predicted that there are several states not yet observed. The clues are provided by how the states in 31S are populated and decay in various reactions including beta decay of 31Cl into 31S.

The figure shows the energies for positive parity states (red lines ending in a circle) and negative parity states (blue lines ending in a cross) in 31S The length of each line is proportional to the total angular momentum J of the state. Some of these lines are labeled by the value of J-pi. The proton separation energy is shown by the horizontal black line. The black dots on the left-hand side are states where the J-pi values are not certain.


[1] Shell-model studies of the astrophysical rapid-proton-capture reaction 30P(p,gamma)31S, B. A. Brown, W. A. Richter and C. Wrede, Phys. Rev. 89, 062801(R) (2014). [link to paper].