1. A method of monitoring and characterizing an unknown radioactive source emitting grouped neutrons, comprising: counting the neutrons emitted by the unknown source; detecting the grouped neutrons; measuring a count of environmental neutrons created by background environmental sources; plotting a Poisson distribution of neutrons on a graph; superimposing a plot of the distribution of environmental neutrons over the Poisson distribution on the graph; superimposing a plot of the grouped neutrons over the distribution of environmental neutrons and the Poisson distribution on the graph; and indicating the presence of a difference between the plot of the grouped neutrons over the distribution of environmental neutrons and the Poisson distribution to infer the existence of fissioning material associated with the unknown radioactive source.