Symposium on Advanced Sensors and Modeling Techniques for Nuclear Reactor Safety

Backscatter Scanning for Detection of Hidden Defects

Radiation has been used to nondestructively examine samples for over a century. Neutral particles such as ionizing photons and neutrons, in particular, have wavelengths that allow the internal structure of cm-scale objects to be observed through processes such as radiography and tomography. A related technology, backscatter scanning, is shown to be useful as a crude imaging modality that is appropriate for certain applications. This presentation considers the problem of detecting defects below the surfaces of engineered structures. Engineered structures are designed and built to exacting standards, unlike biological systems which vary one from another. A possible way of constructing images from backscatter responses obtained as a probe scans, in discrete steps, over the surface of an engineered structure is presented. The technique involves template-matching and use of a figure-of-merit, and its standard deviation, calculated from a set of responses obtained over a “window” that is allowed to roll over the surface being investigated. Two applications of this backscatter scanning technique are considered: hidden corrosion within a lap joint between two layers and cracks that emanate from fastener holes. Possible reactor safety applications also are discussed.