A gamma-ray detector comprising: a housing having a window through which gamma rays can enter; the housing comprising; a two-dimensional array of electrically conducting carbon nanotubes, which act as a cathode, on a substrate; a metal plate electrode, which acts as an anode, positioned opposite the substrate so that there is a gap containing a gas between the electrode and the nanotubes; and a voltage source connected to the substrate and the electrode such that when in use the voltage between the substrate and the electrode is maintained below the arc-threshold voltage so that no electric current can flow through the gap between the nanotubes and the electrode; wherein the electrode and the substrate are arranged such that when a gamma ray enters the housing and hits one of the nanotubes a spark discharge is triggered between the nanotubes and the plate electrode creating an electrical current that can be measured.