1. A method of determining whether a radiation source has the characteristic of fission, comprising: counting neutrons emitted from the radiation source; defining a measurement time period; determining the number of times that a plurality of neutrons was simultaneously emitted from the radiation source within each measurement time period after the measurement time period is repeated a defined number of times to derive a neutron count measurement that indicates a number of times a simultaneously emitted group of neutrons was observed per measurement time period; producing a measured histogram corresponding to the neutron count measurement; deriving a Poisson count distribution for a hypothetical Poisson neutron source, wherein the Poisson count distribution represents a count distribution histogram for a random source; overlaying the measured histogram and the Poisson count distribution histogram with the mean count of the measured histogram in the same position as the mean count of the Poisson distribution histogram; and determining whether the number of times a particular number of simultaneously emitted group of neutrons was observed in the measured histogram is at least ten times greater than the Poisson count distribution for the group of neutrons at a portion of the measured histogram above the mean count value, and characterizing the source as fissile if so.