Construction and installation of the CUBE fission detector
Position data from one of the CUBE detectors
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 group’s research program, with the ultimate goal of developing a unified picture of fusion and fission dynamics.
SOLITAIRE, which has recently been installed in the Department of Nuclear Physics, is an innovative detector system for making precise measurements of the probabilities of fusion of atomic nuclei. The main features of the system are: a 6.5 T superconducting solenoid for separation of fusion products, an iron shield to minimize the magnetic field at the target and detector positions, and two position sensitive multi-wire proportional counters for detection of fusion products.
Fusion of atomic nuclei, initiated by energetic beams of nuclei from our accelerator, produces new unstable nuclei. These fused nuclei must be physically separated from the beam particles, so that the detectors used to identify the fusion products are not swamped by the extremely high rate of direct beam (up to 1000 billion particles/second) and scattered beam particles (1 million/second). Spatial separation between the fused products and elastically scattered particles is achieved using the 6.5 Tesla superconducting solenoid. The solenoid acts as a lens, generating an image of the target downstream, but with a different focal length for beam particles and fusion products (evaporation residues), as shown in the figure below. The beam particles are stopped, whilst the evaporation residues, focused to a point further along the beam axis, enter a position sensitive multiwire proportional counter. They are identified by their time-of-flight with respect to the beam pulse, and their energy-loss signal in the detector.
SOLITAIRE will be used to carry out sensitive studies of the influence of nuclear rotations and vibrations on the fusion process, which can give thousand-fold enhancements of fusion probabilities. This will impact on understanding fusion reactions leading to the formation of new heavy elements. Other applications of SOLITAIRE will be in gamma-ray spectroscopy, studying the structure of new nuclei, and in advanced materials characterization.