Calculation of Effective Atomic Number and Normal Density Using a Source Weighting Method in a Dual Energy X-ray Inspection System

Abstract

Among various radiation sources, the conventional X-ray source is widely used in industrial and medical applications. In spite of its technical advantages, however, analyzing the attenuation factor at a specific energy band is technically difficult because of a continuous energy feature due to the bremsstrahlung effect. In order to obtain the physical properties of an inspection object using an X-ray source, the energy-resolving X-ray method reflecting the characteristic of continuous energy is a very useful tool. In this paper, the effective atomic number (Z_(eff)) and normal density (ρ) obtained by the source weighting method on a dual energy X-ray inspection system are presented and discussed. Benchmark calculations by simulation experiments were performed using four different materials, i.e., polyethylene, acetal, urethane, and TNT, and to verify the methods, attenuation experiments using four polymerized compounds were carried out. The effective attenuation coefficients (μ_1, μ_2) of the experiment object at the source energies E_1 and E_2 were calculated using measured transmitted spectra. Using the ratio of μ_1 to μ_2, the effective atomic number and the normal density were then calculated. As a calculated result, the effective atomic numbers of each material was within 7 % relative difference with referencevalues, and the normal densities were within 4 % relative difference with the reference values except for polyethylene with an 8 % relative difference. This observation led us to the conclusion that the source weighting method is valid for calculating effective attenuation coefficient, effective atomic number, and normal densities in the X-ray inspection system.