Abstract: Embodiments for providing rapid characterization of an unknown nuclear source are described. A sequential Bayesian particle filter is used to analyze in real time the neutron inter-arrival times to estimate the multiplication of the source, the mass of the spontaneously fissioning isotope, the neutron detection efficiency, and the neutron lifetime. A method defines an array of trial solutions, each specifying a combination of fissile parameters characterizing the source; determines a first time interval between a first neutron and a second neutron detected by the neutron detector; calculates a probability distribution of an array of the parameter values; determines additional time intervals between each subsequent successive pairs of neutrons detected by the neutron detector; and refines the probability distribution based on the additional time intervals using a recursive Bayesian process to estimate the most probable combination of parameter values.