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The mass thickness and atomic number of materials shielding radioactive sources emitting multiple resolvable gamma-ray energies can be characterized by measuring the attenuation and Compton scatter of emitted gamma rays in recorded spectra against estimated values for a suite of materials and thicknesses. Compton imaging using a maximum-likelihood expectation maximization-based reconstruction can be used to separate angular spectra allowing simultaneous characterization of multiple shielded sources. Using the described algorithm on experimental 133Ba data, we demonstrate estimation of mass thickness and atomic number for iron, tin, and lead shields with another bare source in the field of view with average standard error of 0.6 g/cm2 and 1.5, respectively, while an aluminum shield is reconstructed with ambiguous atomic number but correct thickness. [ABSTRACT FROM AUTHOR]

Recoil of constituent nuclei from neutron elastic scatter in pixelated, 3-D CdZnTe gamma-ray detectors is detectable given current low energy thresholds. Fast neutrons are attenuated by CdZnTe detectors via outscatter and measured gradients in neutron interaction rates across detector pixels that enables 1-D fast neutron source localization through a maximum likelihood estimator. Experimental results using an MP320 deuterium–deuterium neutron generator with the four detector crystal Orion prototype successfully localize four different source locations across a 1-D field of view to within absolute measurement errors between 2.5° and 14.0°. [ABSTRACT FROM AUTHOR]

To accurately characterize shielded special nuclear materials (SNM) using passive gamma-ray spectroscopy measurement techniques, the effective atomic number and the thickness of shielding materials must be measured. Intervening materials between the source and detector may affect the estimated source isotopics (uranium enrichment and plutonium grade) for techniques which rely on raw count rates or photopeak ratios of gamma-ray lines separated in energy. Furthermore, knowledge of the surrounding materials can provide insight regarding the configuration of a device containing SNM. The described method was developed using spectra recorded using high energy resolution CdZnTe detectors, but can be expanded to any gamma-ray spectrometers with energy resolution of better than 1% FWHM at 662 keV. The effective atomic number, Z, and mass thickness of the intervening shielding material are identified by comparing the relative attenuation of different gamma-ray lines and estimating the proportion of Compton scattering interactions to photoelectric absorptions within the shield. While characteristic \text K_\alpha x-rays can be used to identify shielding materials made of high Z elements, this method can be applied to all shielding materials. This algorithm has adequately estimated the effective atomic number for shields made of iron, aluminum, and polyethylene surrounding uranium samples using experimental data. The mass thicknesses of shielding materials have been estimated with a standard error of less than 1.3 g/cm2 for iron shields up to 2.5 cm thick. The effective atomic number was accurately estimated to 26 ± 5 for all iron thicknesses. [ABSTRACT FROM PUBLISHER]