Date of Award


Document Type


Degree Name

Doctor of Philosophy (PhD)

First Advisor

Michael L. Cherry


Nuclear emulsion chambers with lead targets have been exposed to a 158 GeV per nucleon beam of $\sp{208}$pb nuclei, by far the heaviest ion accelerated to such a high energy to date. These interactions frequently produce more than 1000 charged particles. In order to measure multiplicities and secondary particle trajectories in these extremely large events, an automatic CCD-based microscope system has been developed to analyze images, count secondary tracks, measure their trajectories, and estimate their charge. Based on the analysis of 40 high-multiplicity Pb-Pb events measured using this system, we assess the degree to which these interactions can be described as a superposition of individual nucleon-nucleon interactions. The measured pseudorapidity distributions agree very well with the superposition-based FRITIOF parton model, although the actual multiplicities are somewhat lower than the calculated ones. The Pb-Pb pseudorapidity distributions are compared to those from emulsion exposures to other beams. At energies of 158-200 GeV per nucleon, the target and projectile tails of the pseudorapidity distributions are consistent with limiting fragmentation of the target and beam nucleons, supporting the wounded nucleon description of superposition. The variations in the central pseudorapidity regions can also be described as the sum of production from wounded target and projectile nucleons. In agreement with previous studies, we find that multiplicities of heavy nucleus interactions increase more rapidly with energy than in nucleon-nucleon interactions. However, the produced multiplicity per wounded nucleon in central Pb-Pb interactions is no greater than in central interactions of protons, oxygen, or sulfur on silver-bromine targets. This suggests that the increase in multiplicity may not be the effect of re-interaction of particles in large nuclei, as previously conjectured.