SGaRD (Spectroscopy, Gamma rays, Rapid, Deterministic) code is used for the fast calculation of the gamma-ray spectrum, produced by a spherical shielded source and measured by a detector. The photon source lines originate from the radioactive decay of the unstable isotopes. The leakage spectrum is separated in two parts: the uncollided component is transported by ray tracing, and the scattered component is calculated using a multigroup discrete ordinates method. The pulse height spectrum is then simulated by folding the leakage spectrum with the detector response function, which is precalculated for each considered detector type. An application to the simulation of the gamma spectrum produced by a natural uranium ball coated with plexiglass and measured using a NaI detector is presented. The SGaRD code is also used to infer the dimensions of a one-dimensional model of a shielded gamma ray source. The method is based on the simulation of the uncollided leakage current of discrete gamma lines that are produced by nuclear decay. The material thicknesses are computed with SGaRD using a fast ray-tracing algorithm embedded in a nonlinear multidimensional iterative optimization procedure that minimizes the error metric between calculated and measured signatures.

Abstract. SGRD (Spectroscopy, Gamma rays, Rapid, Deterministic) code is used for fast calculation of the gamma ray spectrum produced by a spherical shielded source and measured by a detector. The photon source lines originate from the radioactive decay of the unstable isotopes. The emission rate and spectrum of these primary sources are calculated using the DARWIN code. The leakage spectrum is separated in two parts, the uncollided component is transported by ray-tracing and the scattered component is calculated using a multigroup discrete ordinates method. The pulsed height spectrum is then simulated by folding the leakage spectrum with the detector response functions which are pre-calculated using MCNP5 code for each considered detector type. An application to the simulation of the gamma spectrum produced by a natural uranium ball coated with plexiglass and measured using a NaI detector is presented.

Detection and mass estimation of fissile material are important tasks. The aim of this paper is to show how one can try to solve such an inverse problem with an iterative forward approach.