Kaitlin Cook

I study nuclear reactions that occur at collision energies near the Coulomb barrier. This barrier is created by the sum of the repulsive Coulomb and attractive nuclear potentials, below which nuclear fusion can only happen due to quantum mechanical effects. At collision energies that are similar to the height of this barrier, the outcomes of nuclear reactions are extremely sensitive probes of the interplay between nuclear structure and reaction dynamics. This sensitivity means that by understanding near-barrier nuclear reactions, we can gain a deeper understanding of the nuclear many-body problem. The most famous example of this is in below-barrier fusion reactions, where coupling to internal degrees of freedom of the colliding nuclei can enhance fusion cross-sections by a factor of ~100!

Since nuclei are completely invisible (less than 10-14 meters across), and a collision of two nuclei takes only ~10-21 seconds, designing experiments that help us understand the huge variety of phenomena that occur is a significant intellectual challenge. Besides being fascinating (and fun), our knowledge of nuclear reactions has important consequences. Examples include understanding the origins of the elements, choosing the right reaction for making the next superheavy elements, and in uses of nuclear reactions for society (e.g. hadron therapy).

My main research interest lies in understanding nuclear reactions of nuclei that have low energy thresholds to removing some number of protons and neutrons. We say that these nuclei are weakly-bound, and their reaction outcomes are very different compared to those of regular nuclei. One important fact is that they are liable to break up during reactions. By performing clever experiments that measure the energy and angular correlations of charged particles in coincidence we can infer a lot of information about when, where, and how breakup occurs. As more exotic weakly-bound isotopes become accessible at new accelerator facilities, like FRIB, it is becoming critical to understand the role of weak-binding and associated cluster structures in reactions.

In addition to studying weakly-bound nuclei, I am interested in processes that prevent superheavy element production. The evaporation residue cross-sections in reactions forming the heaviest superheavy elements are extremely small. This is primarily because of fission of the compound nucleus, and (more significantly) separation of nuclei before they can fully equilibrate (quasifission). In quasifission processes, we have found that small changes in nuclear properties of the colliding nuclei (e.g. the mass asymmetry) have a huge effect on the time-scale and probability of quasifission. It is therefore crucial to understand the effect of the nuclear structure of colliding nuclei on quasifission outcomes. We can do this by measuring the correlated distributions of fragment masses, angles, and kinetic energies.

The larger “palette” of nuclei that will be available with FRIB at near-barrier energies means that we are entering an exciting new era for near-barrier reaction studies where we can better control for particular structures or properties. The main tool for these studies are large-acceptance position-sensitive charged-particle detectors, which allow us to measure energy and angular correlations of charged particles produced in nuclear reactions. Having a large acceptance means that we can make use of beams with small intensities.

As members of a new group, students will contribute to the development of new detector systems and methods to measure breakup, fusion and fission with beams from ReA6. Students will also run and analyze experiments held at FRIB and complementary stable beam experiments at The Australian National University, as well as collaborate with reaction theorists.