Concepts for a sub-millimetre resolving gamma ray CT for nondestructive testing applications

Recently we have developed a high resolution gamma ray CT system for process diagnostics. Typical application areas are nuclear fuel element bundles, chemical reactors, and hydrodynamic machines. In order to use gamma ray CT for non-destructive testing application, such as crack detection in metal parts, an increase in spatial resolution is required. Therefore, we tested two different approaches to increase resolution. The first technique is spatial collimation, where scanning is performed with an additional tungsten collimator in front of the detector arc. The collimator comprises slits of 0.33 mm width to narrow the sampling radiation beams. Thereby, the collimator also provides additional protection against false detection of scattered gamma photons. For a full sampling of the projection space the collimated detector needs to be laterally moved in six steps in addition to the rotary scanning motion of the tomography system. This increases measurement time proportionally and further requires an additional precise translation stage. As an alternative we tested a so called wobbling scheme that has frequently been used with earlier emission and transmission type CT scanners to increase resolution. With the wobbling scheme the detector has to be displaced laterally as with the collimation technique but the collimator is no longer needed. Since the collimator is a heavy and expensive part this approach is attractive. However, to produce the same sharp projection information as with the collimated detector it is necessary to deconvolve the measurement data with the detector’s response function. In our article and presentation we will discuss the methods in details and compare results that have been obtained from practical measurements on phantoms and industrial objects.