Photodisintegration (also called phototransmutation) is a physical process in which an extremely high energy gamma ray is absorbed by an atomic nucleus and causes it to enter an excited state, which immediately decays by emitting a subatomic particle. A single proton, neutron or alpha particle is effectively knocked out of the nucleus by the incoming gamma ray. Photodisintegration is endothermic (energy absorbing) for atomic nuclei lighter than iron and sometimes exothermic (energy releasing) for atomic nuclei heavier than iron. Photodisintegration is responsible for the nucleosynthesis of at least some heavy, proton rich elements via p-process which takes place in supernovae.