Multiplicity correlation between neutrons and gamma-rays emitted from SNM and non-SNM sources

Keywords: Special nuclear materials Cosmic-ray radiation Monte Carlo simulation Joint and marginal probability density functions Multiplicity correlation a b s t r a c t The challenge in detection and identification of Special Nuclear Materials (SNM) is to discriminate between the time-correlated neutrons and gamma-rays emitted from SNM and those originating from non-correlated or differently-correlated environmental non-SNM sources. Time-correlated neutron and gamma-ray bursts can be generated by penetrating components of cosmic radiation. The characteristic features or attributes of correlated signatures can be revealed by analyzing the joint probability density functions (JPDFs) of various parameters of neutrons and gamma-rays. Monte Carlo simulations of SNM and cosmic-ray (non-SNM) sources of neutrons and gamma-rays are performed. For both SNM and non-SNM sources, energy-lifetime JPDF of neutrons, energy-lifetime JPDF of gamma-rays, and JPDFs of neutron-gamma-ray multiplicity are evaluated. Mean values, standard deviations, covariance and correlation are estimated. It is found that the number (multiplicity) of neutrons and gamma-rays emitted from an SNM source is moderately correlated ($0.48). The multiplicity of neutrons and gamma-rays generated by cosmic-ray showers at sea level is only weakly correlated ($À0.046). The exploitation of neutron-gamma-ray multiplicity correlation in detectors can provide a tool to discriminate non-SNM sources. The threat of proliferation activities and unauthorized movement of SNM cannot be underestimated [1]. The detection and identification of SNM is enormously difficult due to the complex physics laws, where background radiation, distance and time factors can severely attenuate gamma-ray or neutron signals [2]. The time course of neutrons and gamma-rays contains information that can be used to determine properties of SNM sources. For nuclear materials that support fission chains, a random event such as spontaneous fission is followed by a correlated number of neutrons and gamma-rays emitted by the fission chain [3]. These particles are emitted simultaneously and are therefore correlated in time. Recognition of these multiple particles provides a method to detect SNM. The SNM signatures can be very robust and distinct compared to natural background radiation and time-correlated cosmic-ray (non-SNM) sources [4]. Their understanding is essential for detection and identifying the presence of SNM. Many neutron detectors heavily rely on neutron multiplicity and time-correlation techniques [5]. Both passive and active interrogation approaches exist for searching of radiological threats. Naturally emitted neutrons and gamma-rays by SNM form the basis for all passive detection methods [6,7]. Active interrogation methods probe SNM by various types of radiation yielding very unique and often strong …