Indirect energy deposit
Charged particles - whether alpha, beta, or fission products - continuously deposit their energy in the mediums they cross by pulling electrons away from the surrounding atoms (left). The energy exchange takes place directly. In the case of gamma photons, the process takes place over two stages. No energy is lost until the photon interacts with an electron through a photoelectric or here Compton effect (right). This interaction causes this electron to move, ionize atoms and deposit energy; this indirect motion is the way neutral particles lose their initial energy.