The rapid technical development of FIM (Focused Ion Beam) technology has spawned an increase in spatial resolution capability in scanning ion microscopy (SIM) technology. Furthermore, FIM has been used for preparation of thin specimens in transmission electron microscopy and micro-fabrication of electronic devices in the semiconductor industry. Recently, a scanning ion microscope with a helium field ion source has been developed. Thus, the contrast formation of emission electron images in scanning ion microscopy has been the object of study for analyzing images of materials specimens, similar to the theory behind scanning electron microscope (SEM) contrast formation. Furthermore, whether the electron emission yield γ induced by ion impact is periodic or non-periodic as a function of Z2 (the atomic number of the target) has not been well studied in the low energy region from several keV to the several tens of keV values used in SIM.