Dynamic Radiological Testing Methods

The design and performance of dynamic flaw detection systems using x and gamma radiation and scintillation detection are discussed. The basic methods discussed are transmission and backscatter techniques. In the latter method, the information is carried by degraded photons resulting from Compton interaction in the test medium. In the transmission method, the information is contained in the unscattered beam of gamma photons that traverse the test medium. Sensitive radiation detectors, suitably collimated for the resolution required, are used to sense the pertinent radiation. Variations in the number of photons detected per unit time, not attributable to known dimensional changes in the test medium, are indicative of anomalies in the specimen. During inspection, the test medium is either in continuous motion or stepped in some systematic way to provide the coverage desired. Optimum performance can be achieved in a given situation by proper selection of source, collimation, detector, scan speed, and time constant. A simple expression relating these parameters is developed. Results obtained with experimental systems are in agreement with the theoretically predicted performance.