A variety of energy emissions occur as a result of primary beam interaction with the specimen surface. Secondary electrons, x-rays, visible photons, near IR photons, and Auger electrons are emitted during inelastic scattering of electrons. Backscattered electrons (BSE) are emitted during elastic scattering of primary electrons. Backscattered electrons are those electrons which pass through the electron cloud of an atom and change direction without much energy loss. BSEs may diffuse into the sample or may escape from the sample surface. In practice, the primary electron beam penetrates deeply into low Z (atomic number) materials and produces few BSEs while high Z materials retard primary beam penetration and emit large numbers of BSEs. According to Murata et al., the higher the atomic number, the smaller the mean free path between electron scattering events (i.e. 528 Å for Al vs. 50 Å for Au at 30 KeV) and the higher the probability of scattering.