Experimental validation of Geant4 simulation model for backscatter X-ray security scanner

Customs services perform X-ray security screening to prevent illicit articles from entering countries. Primary contraband products illegally distributed include unspecified types of narcotics, explosives, and seeds. The challenge in detecting contraband items using only a conventional transmission X-ray inspection system stems from their tendency to be concealed within thin layers in luggage, in addition to their low atomic number and density. Meanwhile, for such materials, backscatter X-ray imaging, based on Compton scattering, has been shown to provide superior image contrast when compared with transmission imaging. Due to these advantages, backscatter imaging has been extensively adopted for the detection of contraband in baggage, vehicles, and port containers. In the present study, the reliability of the Geant4 (GEometry ANd Tracking4) simulation model for a recently developed backscatter X-ray security scanner was experimentally validated. To this end, a comparative analysis was performed for X-ray source spectra and backscatter image profiles obtained through both experiments and simulation models. The tube voltages were 50, 80, and 100 kV, with a tube current of 1 mA. To obtain the backscatter X-ray images, the object was scanned by a collimated pencil-beam while moving on a conveyor belt. In the simulation, the X-ray source term was obtained by means of monoenergetic electron-beam bombardment onto the target. Then, the scanning pencil-beam and moving object were simulated using 4-D simulations. The percent differences between the simulated and measured source spectra and the image profiles were all found to be within 5%. Consequently, it was determined that the simulation model for the backscatter X-ray security scanner can be relied upon.