SUBSURFACE GAMMA RAY BACKSCATTER SCANNING MICROSCOPY

CONTACT MICROSCOPY AND TRANSMISSION RADIOGRAPHY USING INTENSE SOURCES OF SOFT X-RAYS HAVE BEEN ADVANCING WITH THE DEVELOPMENT OF GLANCING ANGLE REFLECTORS AND MULTILAYER MONOCHROMATOR OPTICAL DEVICES. SIMILAR USES FOR MORE PENETRATING X- AND GAMMA RADIATION MUST CONTINUE TO RELY ON COLLIMATOR SCHEMES BECAUSE OF THE UNLIKELIHOOD OF DEVELOPINGLENSES OR MIRRORS FOR HIGHER ENERGY PHOTONS. RESOLUTION OF RELATIVELY DEEP SUBSURFACE STRUCTURES MAY BE MADE MORE FEASIBLE BY DETECTING GAMMA PHOTONS SCATTERED BACK FROM A RESTRICTED VOLUME INSTEAD OF TRANSMITTED THROUGH THE SPECIMEN. BACKSCATTERING IS MORE SENSITIVE TO BOTH LOCAL DENSITY AND AVERAGE ATOMIC NUMBER. THIS HAS ALREADY ENABLEDA NONDESTRUCTIVE TECHNIQUE FOR FINDING MACROSCOPIC VOIDS, CRACKS AND INCLUSIONS IN COMPOSITE MATERIALS. PHASE I EFFORTS WILL DETERMINE FEASIBILITY OF EXTENDING THIS GAMMA BACKSCATTER SCANNING TO MICROSCOPIC RESOLUTION. FINE RESOLUTION WILL DEPEND ON STACKS OF CONVERGING PLANAR VANES OR SIMILAR STRUCTURES TO FOCUS THE VIEW TO A SMALL (PIXEL) REGION. IMAGE CONSTRUCTION WILL PROCEED BY SCANNING AT VARIOUS DEPTHS BELOW THE SURFACE WITH EFFECTS DUE TO SURFACEFEATURES OR CHANGING ATTENUATION ENROUTE ELIMINATED BY A DUAL DETECTOR CONFIGURATION. ADVANTAGE OVER EXISTING NEAR-FIELD OR CONTACT SYSTEMS IS THREE DIMENSIONALITY AND DEEP SUBSURFACE IMAGING.