An X ray detector with single photon measurement capabilities includes a layer of photosensitive material and an N×M array of photo-detector diodes in the layer of photosensitive material. The photo-detector diodes have a bias potential interface and a diode output interface. An N×M array of high gain, low noise readout unit cells are assigned to the photo-detector diodes. Each readout unit cell has an input interface connecting the diode output interface to a high-gain charge-to-voltage amplifier with integration capacitors. The high-gain charge-to-voltage amplifier can switch between different gains. A comparator and a digital block monitors the charge of the integration capacitance and switches the gain depending from the monitored charge of the integration capacitance. The pixel detector provides the possibility of in pixel intermediate evaluation of an automatic gain switching circuit state to increase the dynamic range of the detector in case of quasi continuous incoming particle flux.