One-sided imaging of large, dense objects using 511-keV photons from induced-pair production

This work investigates the use of annihilation photons from gamma-ray-induced electron-positron pair production to inspect objects when only one side is accessible. The Z²-dependence of the pair-production cross section and the high penetration of 511-keV photons allow this method to differentiate high-Z materials in low-Z matrices. The experimental results for the dependence of the backstreaming photon yield on Z shows a factor of 20 increase for elements over the range 4<Z<82. Results for point-by-point “images” obtained in line scans of representative geometries are also shown. Comparisons of experimental data and simulation studies based on the EGS4 Monte Carlo code agree to within 5%