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Compton backscattering of gamma rays is a nondestructive technique that can be used for material characterization and detection of defects and inclusions in materials. The methodology allows one-side inspection of large structures, is relatively inexpensive and can be portable. The concept is based on detection of backscattered radiation produced from a collimated beam aimed at the sample. The energy spectrum of backscattered photons can be used to determine local density perturbations. In this work we carried out a geometry and collimation study of the Compton backscatter device that consists of a Ø2 mm collimated 241 Am (100 mCi) gamma source and a high resolution CdTe semiconductor detector. Acrylic and plaster blocks with cylindrical holes and steel inclusions were used as samples. Collimators in the detector with Ø7 mm, Ø5 mm, Ø3 mm and Ø 1.5 mm were tested in two different scattering angles: 135° and 150°. The results showed that the backscatter geometry of 135° with the Ø7 mm collimator resulted in higher contrast in the intensity versus position profile without loss of spatial resolution. The tests on plaster blocks with steel rods inclusions suggest that, for a low energy and activity gamma source, the effects of beam attenuation are more decisive to the scattered intensity than increasing of material density. With the density contrast analysis, size and depth of steel rods can be determined.

A Geometry and Collimation Study of a Compton Backscatter Device for Inclusions Detection in Materials