Lateral migration radiography application to land mine detection, confirmation and classification

Lateral migration radiography (LMR) employs scattered photons to acquire detailed images of covered objects. Images of plastic encased real mines buried in soil using LMR have shown dramatic differences compared to images generated using simulated mines. The major characteristic that enables the discernibility of land mines to the degree of actual type identification is the presence of voids (air volumes) required for the operation of the fuse assembly or for blast direction control. Air volumes greatly modify the detected field of both once and multiple-scattered photons. The LMR system consists of an x-ray generator and two uncollimated detectors positioned to detect once-scattered photons and two collimated detectors designed to detect primarily multiple-scattered photons. The x-ray generator is located in the gap between symmetrically arranged detectors; the collimated x-ray beam typically has a spot size of 1.5 x 1.5 cm with perpendicular incidence on the soil surface. The optimal x-ray spectra for land mine detection with the LMR system range from 130 to 180 kVp with mean x-ray energies of from around 40 to 60 keV. Air volumes modify both exit paths and the position of first-scatter events; they also modify the migration paths of multiple-scattered photons, thus producing different images in the two detector types. The burial mode (below surface or laid on the surface) of the land mine can also be discerned by LMR due to a shadowing effect seen for surfaced-laid land mines. The presence of even a minute amount of metal in the land mine also aids in discerning the mine, because metal produces a signal decrease in both types of detectors. Monte Carlo calculations are performed with the MCNP code to obtain an understanding of the details of the photon lateral migration process. Images generated from these Monte Carlo calculations are in agreement with the experimental measurements. The real mine images confirm that LMR is capable not only of mine detection, but also of mine identification. (C) 2000 Society of Photo-Optical Instrumentation Engineers. [S0091-3286(00)02709-4].