PSI - Group

Nuclear Physics
Research Programme


A comprehensive experimental investigation and theoretical modelling
of light-ion induced reaction mechanisms in medium mass targets
from the Coulomb barrier to the fission threshold



The experimental methods
involve both the spectroscopy of the residues created and the light particles emitted in the reaction.
The experiments are carried out at iThemba LABS by the Milano-WITS-iThemba-Stellenbosch collaboration.
    1. Stacked Foil Spectroscopy
    2. Residue Momenta by Doppler Gamma - Ray Spectroscopy
    3. Light particle emission spectroscopy
The reaction mechanisms of
  • complete fusion, as well as
  • incomplete fusion following the breakup of the projectile, particularly into alpha like fragments or the
  • transfer of a single nucleon 
were found to be significant processes.

A comprehensive theoretical model
based on tracking the time evolution of the composite system formed after the reaction mechanisms mentioned above has been continuously developed by comparisons to the experiments.The time evolution of the composite system is followed using a time differential method adapted from the Boltzmann Master Equation theory and coupled to a Monte Carlo approach, implemented at each time step. The theory can produce probability distributions for all relevant observables of the de-excitation by emission of light particles, from the fast stage stage through to the stage of the equilibrated residue.

Results
  • The role of pre-equilibrium emission has been found to be more significant than expected.
  • The entrance channel energy dependence of the mean field is significant.
  • Very fast and efficient de-excitation mechanisms have been discovered.
    • Fast alpha re-emission after a few interactions in the peripheral region of the target
    • The emission of intermediate mass fragments via a coalescence mechanism. During the fast stage of the interaction, a group of nucleons may be identified as being correlated in momentum space. The probability for these correlated nucleons to be emitted as a medium mass fragment can be evaluated based partly on the cross section for the inverse process.
  • The excitation of the target by the projectile or the fusing fragment has been evidenced, and represents a friction mechanism. This effect accounts for the softening of the breakup fragment spectra.
  • A new method of probing nuclear reaction mechanisms based on 8Be spectroscopy has been developed. This allows for an experimental distinction to be made between intial state and final state interactions.
  • There is a high yield of fragments similar in mass to alpha-like particles. It was shown these are not produced in final state reactions. Instead, one needs to develop the understanding of the break-up and co-alescence processes.
View the publications and outputs to 2004