Imaging with the invisible light

We describe a UV photodetector with single photon (electron) counting and imaging capability. It is based on a CsI photocathode, a GEM charge multiplier and a self-triggering CMOS analog pixel chip with 105 k pixels at 50 μm pitch. The single photoelectron produced by the absorption of a UV photon is drifted to and multiplied inside a single GEM hole. The coordinates of the GEM avalanche are reconstructed with high accuracy (4 μm rms) by the pixel chip. As a result the map of the GEM holes, arranged on a triangular pattern at 50 μm pitch, is finely imaged.