Abstract: An X-ray detecting system includes a number of X-ray detectors mounted on the periphery of a vertical ring rotatable about a patient to be scanned diametrically opposite from an X-ray tube, slit collimator and chopper wheel mounted on the ring. Each detector includes a scintillator crystal typically of NaI, a photomultiplier tube that receives light from the crystal proportional to the incident X-rays, an amplifier energized by the output of the photomultiplier tube that provides a proportional signal to an LED that energizes both the photomultiplier tube (PMT) to provide light feedback and a photodiode that provides an electrical output signal so that the total current at the PMT anode resulting from X-ray light and LED light is constant. Circuitry is preferably included so that the LED supplies only the chopped part of the X-ray equivalent light signal with practically no d-c component.