Copper diffusion in mercuric iodide was studied by low‐temperature photoluminescence (PL) spectroscopy and Auger electron spectroscopy. A broad radiative emission band at a wavelength of about 6720 A˚ in the PL spectra was found to be related to Cu incorporation in the crystal. PL spectra obtained from surface doping experiments indicate that Cu is a rapid diffuser in HgI2bulk material. Auger electron spectroscopy performed as a function of depth from the crystal surface confirms the rapid bulk diffusion process of Cu in HgI2. Fabrication of HgI2nuclear detectors with Cu electrodes indicates that Cu is not acceptable as an electrode material, which is consistent with the fact that it diffuses easily into the bulk crystal and introduces new radiative recombination centers.