Almost all atomic nuclei heavier than iron are produced by neutron capture reactions, which lead to unstable nuclei that subsequently beta decay, producing heavier elements in the process. We distinguish between the slow neutron capture (s-)process, which occurs over long time scales of thousands of years in certain types of stars, and the rapid neutron capture (r-)process, which occurs within seconds in violent processes such as neutron star collisions. The precise path of these processes depends on the number of available neutrons in the astrophysical environment, the cross section (probability) for neutron capture reactions, and the half-life of the unstable nuclei. We speak of branch points if the probabilities for neutron capture and beta decay are similar. Precise knowledge of these probabilities is important to understand the abundance of chemical elements in the universe.
The nucleus 204Tl with a half-life of 3.8 years is considered a branch point in the s-process. Knowledge of the neutron capture probability for 204Tl is needed to understand the abundance of stable 204Pb and, after one more neutron capture, radioactive 205Pb with a half-life of 17 million years. This long-lived isotope of lead plays an important role in determining the age of meteorites. The production of 204Pb gives us also information about the temperature and neutron density in stars where the s-process takes place. It is very difficult to measure the probability for neutron capture on 204Tl directly with neutrons because of its short half-life. In this project, you will measure the neutron capture probability for 204Tl indirectly by studying the properties of 205Tl using the nuclear reaction \(^{208}{\rm Pb}+p \enspace{\rightarrow}\enspace ^{205}{\rm Tl}+\alpha \) and using the experimental results as input for numerical calculations.
We will perform a dedicated experiment for this project at the Oslo Cyclotron Laboratory using OSCAR – the most efficient detector of its kind in the world. You will take an active part in this experiment and be responsible for the data analysis. Sorting through a large amount of data and extracting relevant quantities for 205Tl requires good programming skills. You will then use the experimental results as input for state-of-the-art computational tools to calculate the neutron capture cross section for 204Tl. This project combines experimental, theoretical, and computational methods.