ORIENTATION CONTRAST IMAGING OF MICROSTRUCTURES IN ROCKS USING FORESCATTER DETECTORS IN THE SCANNING ELECTRON-MICROSCOPE

We have developed a system using 'forescatter detectors' for backscatt ered imaging of specimen surfaces inclined at 50-80 degrees to the inc ident beam (inclined-scanning) in the SEM. These detectors comprise se mi conductor chips placed below the tilted specimen. Forescatter detec tors provide an orientation contrast (OC) image to complement quantita tive crystallographic data from electron backscatter patterns (EBSP). Specimens were imaged using two detector geometries and these images w ere compared to those collected with the specimen surface normal to th e incident beam (normal-scanning) using conventional backscattered ele ctron detector geometries and also to an automated technique, orientat ion imaging microscopy (OIM). When normal-scanning, the component of t he BSE signal relating to the mean atomic number (z) of the material i s an order of magnitude greater than any OC component, making OC imagi ng in polyphase specimens almost impossible. Images formed in inclined -scanning, using forescatter detectors, have OC and z-contrast signals of similar magnitude, allowing OC imaging in polyphase specimens. OC imaging is purely qualitative, and by repeatedly imaging the same area using different specimen-beam geometries, we found that a single imag e picks out less than 60% of the total microstructural information and as many as 6 combined images are required to give the full data set. The OIM technique is limited by the EBSP resolution (1-2 degrees) and subsequently misses a lot of microstructural information. The use of f orescatter detectors is the most practical means of imaging OC in tilt ed specimens, but it is also a powerful tool in its own right for imag ing microstructures in polyphase specimens, an essential asset for geo logical work.