Impact of Lubberts Effect on Amorphous Selenium Indirect Conversion Avalanche Detector for Medical X-Ray Imaging

The exponential X-ray absorption makes the indirect conversion X-ray image sensors vulnerable to the Lubberts effect, which in turn makes the sensor more sensitive to the electronic noise. A cascaded linear-system model is proposed to find the required electric field to overcome the effect of electronic noise and depth dependent X-ray absorption (Lubberts effect) in amorphous selenium indirect conversion avalanche detectors. The model also includes scattering due to K-fluorescence reabsorption. The effect of depth dependent X-ray absorption is more pronounced in thicker detectors. It is observed that, at the Nyquist frequency (fN) of 2.5 mm
^-1
, the presampling modulation transfer function of CsI deteriorates from 0.75 to 0.1 due to Lubberts effect in a CsI layer having thickness of 0.6 mm. The detective quantum efficiency (DQE) at fN (2.5 mm
^-1
) drops from 0.037 to 0.01 at a field of 60 V/μm due to Lubberts effect. The Lubberts fraction decreases with increasing the field thereafter. The avalanche gain enhances the signal strength and improves the frequency dependent DQE(f) by overcoming the Lubberts effect and as well as the effect of the electronic noise. An avalanche gain of 45 is sufficient to overcome the effect of the electronic noise.