MEASUREMENT OF DIAGNOSTIC X-RAY SPECTRA USING CdZnTe DETECTOR

INTRODUCTION We directly measured diagnostic X-ray photon spectra produced by a single-phase 2-pulse diagnostic X-ray unit driven at 50 to 100 kV and a few mA(1), a mammographic X-ray unit at 25 to 32 kV and 3 times of 80 mAs(2) and an X-ray computed tomography (CT) scanner at 120 kV and 3 mA(3) have been analyzed. Since detected spectra were distorted by the response of CdZnTe(CZT) detector and did not present the true photon spectra, the correction by the stripping procedurec(4) was applied. The response function of detector used in this procedure has been determined by an evaluation of interactions (K-escape, coherent scattering, Compton scattering) and incomplete charge collection calculated using the Monte Carlo method(4-7). Castro et al.(7) have attempted to perform the direct measurement of diagnostic X-ray spectra using a low-efficiency CdTe detector. They analyzed photon spectra produced by a portable diagnostic X-ray spectra unit from 45 to 100kV. They have used only an average value of Kα(25 keV) radiation on account of the little difference between Cd(23 keV) and Te(27 keV) absorption coefficients and have not considered Kβradiations of Cd and Te. They have used the constant value for the incomplete charge collection and have not considered Compton scattering. Therefore their correction of spectra have been bad accuracy. But we have used Kα1,Kα2,Kβ1,Kβ2 radiations of Cd,Zn and Te respectively and have used the weight function for the incomplete charge collection and have considered Compton scattering. The Monte Carlo simulations were continued with changing the important factors[mean path length of hole λh, dead layer of CZT crystal (3×3×2 mm) and weight factor Wq ] of incomplete charge collection until the fittest response function was found out. Therefore our correction of spectra have improved the accuracy. Corrected photon spectra were compared with the diagnostic X-ray spectral data published by Bureau of Radiological Health (BRH) measured using a Ge detector. Exposures were calculated from the corrected photon spectra and compared with exposures measured using an ionization chamber.