Nuclear Physics & Accelerator Applications

Atomic nuclei are completely invisible, being less than 10^{-14} metres across, and a collision of two nuclei takes only 10^{-20} seconds. In such seemingly infinitesimal and transient events, a tremendously wide range of phenomena occur. Understanding them fully represents a fascinating intellectual challenge, which is also relevant to many other fields of science.

The Nuclear Reaction Dynamics group has developed expertise in the design and development of unique, efficient particle detection systems. These are used in fundamental research into the important processes of nuclear fusion, where two nuclei merge into one, and nuclear fission, where one nucleus splits into two.

To fuse, nuclei have to tunnel through the fusion barrier, which is created by the sum of the long-range repulsive electrostatic and short-range attractive nuclear forces. The excitation of other nuclear degrees of freedom (rotation, vibration) during the collision results in a distribution of fusion barrier heights.

These distributions can be determined from extremely precise fusion probability measurements, through a simple and elegant mathematical transformation. They can be thought of as extremely high speed (10 -21 second) photographs, giving unique insights into nuclear interactions. The wide range of nuclei available as projectiles and targets allows different nuclear properties to be investigated.

Fusion reactions create hot nuclei, which can be spinning so fast that the centrifugal force tears them apart, and they fission. Measurements of the fission probability tell us about the delicate balance of forces holding nuclei together. Sophisticated experiments can measure the number of nucleons evaporated from the hot nuclei before they fission. These have shown that it takes a surprisingly long time (several 10 -20 seconds) before nuclei can split into two, showing that nuclei have a very high viscosity.

The detailed investigation of the mass-asymmetry dependence of such fusion and fission phenomena is an important part of the groupâ€™s research program, with the ultimate goal of developing a unified picture of fusion and fission dynamics.

SOLITAIRE is an innovative detector system for making precise measurements of the probabilities of fusion of atomic nuclei.