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. If the physical properties of an inspected object are to be obtained by 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 (p) obtained by using the source weighting method on a dual energy X-ray inspection system are presented and discussed. Benchmark calculations by using simulation experiments were performed using four different materials, i.e., polyethylene, acetal, urethane, and TNT, and to verify the methods, we carried out attenuation experiments using four polymerized compounds. The effective attenuation coefficients (mu(1), mu(2)) of the experiment object at the source energies E(1) and E(2) were calculated using measured transmitted spectra. Using the ratio of mu(1) to mu(2), we then calculated the effective atomic number and the normal density. As a calculated result, the effective atomic numbers of each material were within 7% of the reference value, and the normal densities were within 4% of the reference values except for polyethylene with an 8% relative difference. This observation leads us to the conclusion that the source weighting method is valid for calculating the effective attenuation coefficient, effective atomic number, and normal densities in the X-ray inspection system.