Abstract:
For many years, particularly in the study of light particle induced reactions, the experimental evidence and theoretical interpretation of the data has been dominated by two processes. These are, fast direct reactions, where the projectile either feels the mean field or simply interacts with a few of the constituents of the target and the slower compound nucleus reactions where the projectile fuses with the target forming an excited compound nucleus. These two processes were initially considered to be unrelated, however later developments proved otherwise. In fact the mean field interaction or interaction of the incident particle with nucleons of the target may create a doorway state with a few excited particles and holes. This doorway state may not immediately decay into the continuum but may rather decay to a hallway state by means of intranucleon interactions. This hallway state may in turn decay to a more complex hallway state or into the continuum by emitting a fast particle or cluster of nucleons. This process eventually leads to the formation of an excited compound nucleus where the remaining excitation energy is randomly distributed among the many particles and holes, none of which possess sufficient energy to escape promptly to the continuum. In order to test whether the concepts derived from the study of light particle in duced reactions can be extended to heavy ion induced reactions, we have studied the interaction of a simple heavy ion 12C, incident on the medium mass, mono-isotopic 103rh. We have adopted and developed the relatively simple yet powerful activation technique to obtain the comprehensive data set necessary to rigorously test the theoretical interpretation of these reactions as developed by a group at the University of Milan. In particular, this technique allows the measurement of absolute cross sections of a large number (> 50 in our case) of residues with an error of 20% or less. New gamma spectroscopy laboratories were built at both the irradiation facility and home institute in order to process the activated foils for the lengthy off-line data collection. New and modified experimental and computational techniques were developed and perfected for the irradiations as well as data acquisition and reduction of the enormous amount of raw data. Variations on the technique enabled us to collect excitation functions over a broad energy range, from the Coulomb barrier up to 400 MeV in appproximately 40 steps, as well as a number of angular distributions and recoil energy range distributions at a few energies of interest. To obtain a theoretical description for this data which is also of a general nature, only assumptions derived from a large number of heavy ion interactions have been used. One of the main assumptions made is the separation in time of the mean field and two body interactions. The mean field dominates as the projectile and target approach one another while the two body nucleon-nucleon interactions are responsible for the subsequent evolution of the excited intermediate system. Another important assumption made is that to a first approximation, only a few basic processes need be considered to accurately reproduce the reaction cross section. These processes arc, the complete fusion of the 12C, the partial fusion of an a particle or 8Be and transfer of a single nucleon from the projectile to the target. The magnitudes of these contributing reaction mechanisms are determined by extending the concept of an entrance channel, critical angular momentum to all of these cases, considering the centrifugal potential, a coulomb potential of partly overlapping charge distributions and a Saxon-Woods nuclear potential. The de-excitation of the intermediate excited nucleus is described using the statistical approach of the Boltzmann Master Equation Theory, modified to include amongst others, emission of particles and clusters of particles into the continuum. This is employed in conjunction with a Monte Carlo calculation to evaluate the probability of events which lead to these emissions. The most striking results of these experiments and the subsequent theoretical interpretation is that, following the incomplete fusion of an a particle or two loosely bound a's in the form of a 8Be the re-emission of a fusing a particle with a large fraction of its initial energy, is very important. In fact the probability of this phenomenon is even greater than that observed in α particle induced reactions since the incomplete fusion processes of our experiments, occur in a low density, peripheral region of the target nucleus. Consequently, as these incomplete fusion processes become more important with increasing projectile energy, we find with increasing 12C energy an increasingly high number of low energy equilibrated nuclei with mass and charge close to that of the target. Their further decay via evaporation leads to the dominant formation of near target nuclei.