A single photon counting pixel detector chip has a negligible dead time and consequentially high frame rates. The detector chip contains: a) a layer of photosensitive material; b) an N×M array of photo-detector diodes arranged in the layer of photosensitive material; and c) a N×M array of readout unit cells. The readout unit cell contains an input interface connected to a diode output interface, a high-gain charge to voltage amplifying device and a pixel counter being connected to an output of the high-gain voltage amplifying device. The pixel counter is split into a first number of nibble counters. The basic counter cell contains a counting element, a switch, a temporary storage element and an output stage. Additionally, the detector chip has a side shift register to read out the nibble counters row-wise with a predetermined number of nibble row selections.