An apparatus for detecting X-rays and converting the detected X-ray intensities into digital signals is disclosed. The apparatus places Analog to Digital Conversion (ADC) chips directly under a scintillator array along the X-ray beam direction and uses a shield that is placed between a photodiode substrate and an Analog to Digital Conversion (ADC) chip to block X-rays from directly reaching the dies of the ADC chips, which are sensitive to X-rays. Also an X-ray CT system utilizing the disclosed apparatus for detecting X-rays is provided.