透過能

penetrating power

電子線が物質を透過する能力のこと。100kVの電子線は約100nmの透過能がある。加速電圧が高くなると透過能は増すが、相対論効果によって飽和する。1000kVでは100kVの3.3倍くらいの透過能になる。透過能は吸収係数の逆数で表わされる。透過電子顕微鏡の場合、対物絞りに入らない電子は吸収されたとみなされるので、高角に散乱される弾性散乱も吸収とみなされる。非弾性散乱のうち、プラズモン散乱は、エネルギー変化は大きいが(～15eV)、10^-3radくらいに散乱され絞りの中に入ることが多いので、吸収とはみなさない。熱散漫散乱は、エネルギー変化は小さいが(0.1eV)、高角に散乱されるので吸収となる。非弾性散乱の平均自由行程は数100nmで、弾性散乱の平均自由行程の10倍程度である。内殻電子励起は、エネルギー変化は10eV以上と大きいが、散乱断面積が小さく(平均自由行程が大きい)、吸収への寄与は小さい。

"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.