1. A fission detection system for characterizing an unknown radioactive source emitting grouped neutrons through active interrogation, comprising: neutron detector means for counting the neutrons emitted by the unknown source; multiplicity counter means for detecting the presence of grouped neutrons emitted by the unknown source; environmental detector means for measuring a count of environmental neutrons created by background environmental sources; a known source of neutrons placed at a defined distance from the unknown source, wherein the defined distance is sufficient to increase the fission strength of the unknown source above a ten to one ratio relative to the background environmental sources; analysis means for calculating a difference in neutron counts among the unknown source, the environmental sources, and a Poisson distribution based on the same mean count rate of the multiplicity counter; graphing means for plotting the Poisson distribution of neutrons on a graph, superimposing a plot of the distribution of environmental neutrons over the Poisson distribution on the graph, and superimposing a plot of the neutrons from the unknown source over the distribution of environmental neutrons and the Poisson distribution on the graph to display an indication of excess correlation due to fission through a difference in an upper portion of histogram shapes between the plot of the neutrons from the unknown source compared to the plot of the Poisson distribution, wherein the upper portion of the histogram shapes comprises a portion of a respective histogram shape above the mean count rate; and a response system functionally coupled to the analysis means and configured to trigger an alarm if the difference in the upper portion of the histogram shapes exceeds a defined threshold.