Sensitive inspection of void defects using synchrotron refraction imaging with quantitative modelling of contrast enhancement

Abstract

Few techniques are available to assess air inclusions in opaque plastic materials. This study showed a remarkable contrast gain for small air voids by synchrotron radiation imaging with refraction mode: comparing an absorption image, a 60-fold gain in contrast was observed for a 66 µm void. Minimally, a 24 µm inclusion was visualised using a detector with 6 µm × 6 µm resolution. The mechanism of contrast gain for small inclusions on refraction images cannot be explained by X-ray absorption, and X-ray refraction was proposed as its cause. In order to analyse the object-size dependency of contrast, the signal intensity profile due to refracted X-rays at the boundary was fitted by an analytical serpentine curve, which is one of the cubic curves. Calculation showed that signal intensity at the centre of an object increased with the decrease of an object size by the overlapping of X-rays refracted to the inside, and that the increase was steep for objects of 100 µm and smaller. As a conclusion, X-ray refraction produced an extremely high contrast gain for small air voids in synchrotron radiation imaging with refraction mode, and consequently, this imaging will be a sensitive technique for non-destructive detection and mapping of minute air inclusions.