Development of X-ray scanner using 450-kVp X-ray

The objective of this paper is to develop an X-ray scanner that consists of a 450-kVp X-ray generator, a linear detector array, an electric circuit for data acquisition, and image processing software. This work is composed of three main parts. First, Monte Carlo simulations and experiments have been performed to determine the scintillator geometry and its surface condition so that the light output from it can be maximized. According to the simulations and experiments, the CdWO₄ (CWO) of 1.7 mm×3.0 mm×10 mm with ground surface, which is treated with grinding material of 28-μm grain size, gives the best light yield. Secondly, a PIN photodiode for this system is fabricated and its characteristic parameters are measured to compare with those of two commercial products, one from Detection Technology and the other from Hamamatsu. From experimental results, at zero bias where the system is operated, all three diodes have nearly identical leakage currents of a few pA, and junction capacitances of this study and DT are about 40 pF/mm² and that of Hamamatsu about 9 pF/mm². At 490 nm, the peak wavelength of the CWO light, the spectral responses of this work, DT, and Hamamatsu are 0.4, 0.3, and 0.2, respectively. It appears that the photodiode of this work gives excellent light sensitivity in the range of 400 to 900 nm. Thirdly, the spatial resolution of the system including our developed data acquisition system (DAS) shows 0.29 lp/mm at MTF of 10%, and this system satisfies the resolution of 4.75-mm objects that the U.S. Customs Service has suggested for cargo container inspection systems.