  The present paper discusses a Non Destructive Evaluation (NDE) technique
involving Compton back-scattering. Two 15 cm x 15 cm x 15 cm cubical concrete
blocks were scanned for detection of voids. The setup used a PC controlled
gamma scattering scanning system. A 137Cs radioactive source of strength 153.92
GBq with lead shielding and a collimated and shielded 50% efficiency coaxial
HPGe detector providing high resolution energy dispersive analysis of the
scattered spectrum, were mounted on source and detector sub-assemblies
respectively. In one of the concrete blocks air cavities were created by
insertion of two hollow cylindrical plastic voids each of volume 71.6 cm3. Both
the concrete blocks, one normal and another with air cavities were scanned by
lateral and depth-wise motion in steps of 2.5 cm. The results show that the
scattering method is highly sensitive to changes in electronic and physical
densities of the volume element (VOXEL) under study. The voids have been
detected with a statistical accuracy of better than 0.1% and their positions
have been determined with good spatial resolution. A reconstruction algorithm
is developed for characterization of the block with voids. This algorithm can
be generally applied to such back-scattering experiments in order to determine
anomalies in materials.
