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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.

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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.

Fissile matter detection and characterisation are crucial issues; especially in nuclear safety, safeguards, matter comptability, reactivity measurements. In this context, we want to identify a source of fissile matter knowing external measures such as instants of detection of neutrons during an interval of measure. Thus we observe the neutrons detection times emitted by the fissile matter and going through the detector, then we compute the moments of the empirical distribution of the number of neutrons detected during a time gate T. In order to identify the source we have to get the following parameters: the multiplication factor k of the system, the intensity of the source S, the fission efficiency ε F .Given the parameters of the source there are some models that allow us to predict the moments of counted number of neutrons during a time gate T. We consider a point model stating monokinetic neutrons are moving in an infinite, isotropic and homogeneous medium. The method makes it possible to compute the first moments of the count number distribution.Then, given the moments of counted number of neutrons during a time gate T we want to get the parameters of the fissile source. In order to achieve this goal, we will use the following method • Bayesian approach in order the get the distribution of parameters. The a posteriori distribution is non-trivial, samples can be achieved with Markov Chain Monte-Carlo methods with covariance matrix adaptation (MCMC with CMA). UNCECOMP 2021 4 th ECCOMAS Thematic Conference on Uncertainty Quantification in Computational Sciences and Engineering M. Papadrakakis, V. Papadopoulos, G. Stefanou (eds.