Conversion gain fluctuation in electron-hole pair creation: Swank factor of CdZnTe

Cadmium zinc telluride (CdZnTe) is currently of interest as a photoconductor in direct conversion digital X-ray medical imaging. A critical performance parameter of an X-ray detector is the Swank factor for degradation of the signal to noise ratio, or zero spatial frequency detective quantum efficiency, due to conversion gain fluctuation in electron-hole pair creation. Based on the measured electron-hole pair creation energy and energy band gap, the Swank factor is evaluated by the Markov process. At incident X-ray energy below the K-edge of Cd, the Swank factor is nearly 1.0. At higher energy above the K-edge of Cd, however, the Swank factor is largely affected by the escape of Cd/Te characteristic X-rays, If the photon energy is below the K-edge of Cd, the fluctuation in EHP creation has almost no influence on detector performance.