penetrating power

"Penetrating power" is defined as the power (length) of an electron beam transmitted for a substance. An electron beam at 100 kV (accelerating voltage) has a transmissivity of 100 nm. As the accelerating voltage is higher, the transmissivity increases. However, this is saturated due to the relativistic effect; At 1000 kV, the transmissivity is about 3.3 times larger than that at 100 kV. Penetrating power is expressed by the reciprocal of the absorption coefficient. In the case of a TEM, electrons that do not pass through the objective aperture are regarded to be absorbed. Thus, elastic scattering at high angles is regarded as absorption. Among inelastic scattering, plasmon scattering exhibits large energy change (～15 eV) but takes place at small angles less than about 10^-3 rad, thus passing through the objective aperture. The Plasmon scattering does not contribute to absorption. On the other hand, thermal diffuse scattering exhibits small energy change (～0.1 eV), but the scattering occurs at high angles; thus this is regarded as absorption. Inner-shell electron excitation exhibits energy changes of larger than 10 eV, but its scattering cross section is small (large mean free path). Thus, its contribution to absorption is small. The mean free path of inelastic scattering is about several 100 nm and this is about 10 times larger than that of elastic scattering.

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