How are gamma rays used to find hidden defects in metal castings?

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radioactivity Figure 1: Radioactive decay of beryllium-7 to lithium-7 by electron capture (EC; see text).

Gamma rays are used in nondestructive testing to identify internal and external flaws in metal castings. The process involves directing a beam of gamma-ray radiation through the metal object. As the radiation passes through the material, it impinges on a photographic film placed on the opposite side. Because defects such as cracks, voids, or inclusions have different densities than the surrounding metal, they absorb or scatter the radiation differently, creating a shadow image on the film that reveals the location and nature of the internal flaws.1