The Rossi-α method determines the prompt neutron decay constant in a nuclear fissioning system at or near delayed critical. Knowledge of the prompt neutron decay constant is important for a critical system as it is a major contributor to the dynamic system behavior. The classical method for the Rossi experiment used gated circuitry to track the time when a neutron was incident upon the detector.The downside of this method is that the circuitry was complex and only one single fission chain could be measured at a time. The modern method allows many chains to be measured simultaneously by a pulse time tagging system such as the LANL custom designed List-mode module. This thesis examines the implementation of the modern Rossi-α method on the all highly enriched uranium, HEU, Zeus experiment. Measurements are taken at several subcritical configurations, at critical in the presence of a source, and at one supercritical point. During the experiment, the List-mode module generates time tags of incoming neutron pulses.After the experiment, this list of neutron pulses is complied using custom software into a histogram.This histogram is fit using off the shelf graphing software to determine the value of α. The subcritical measurements of α are used to extrapolate α at delayed critical.The extrapolation determined the value of α at delayed critical to be α = 89910 s-1.This value is compared to the measured value of α at delayed critical which is determined to be α = 90408.4 s-1. These values differ by 0.55% which is remarkably good agreement.This thesis also examines the expected value of α using a Monte Carlo transport code, MCNP.MCNP determined the value of α at delayed critical to be 100048 ± 0.584 s-1.This result differs by 11.3% from the extrapolated value of α determined experimentally.When compared to systems with similar neutron spectra, the measured value of α fits well in comparison to historical measurements.
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