Importance of Voxel Size in Defect Localization Using Gamma-Ray Scattering

The voxel, defined as the volume of the intersection between incident (primary) and scattered beams, plays an important role in the localization of defects in samples having several interests. In this work, the gamma rays emitted from a ¹³⁷Cs radioactive source (having the strength of 222 GBq) are scattered from various regions of a wood sample. The scattered gamma flux is detected by an NaI(Tl) scintillation detector placed at 110 deg to the primary gamma-ray beam. Defect (decay) in the wood is simulated by drilling two collinear cylindrical flaws (having diameters of 0.8 and 1.2 cm) in the wood sample and then filling it with a mixture of sawdust and glue. Three sets of collimators with diameters of 6, 7, and 8 mm for the source and detector are used to vary the voxel size (volume). It has been found that better contrast (29.43% for a 1.2-cm defect and 16.37% for an 0.8-cm defect) is achieved for the smallest voxel (16.13 cm³) in comparison to the other two voxels (25.65 and 38.36 cm³). Further, better contrast for the smallest voxel is confirmed by comparing gray images obtained using MATLAB for all three voxel sizes at different scan positions. It has been concluded that for a given experimental setup, the accuracy of defect (decay) detection demands reduced voxel size.