This book (Practical Electron Microscopy and Database) is a reference for TEM and SEM students, operators, engineers, technicians, managers, and researchers.
 

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During interaction of incident electrons with the specimen in EMs, one important energy-loss process is atomic ionization, where electrons of atoms are knocked out from the inner (or called core) shells (i.e. K, L, M, etc.) in the specimen. In this case, the shell electrons are moved outside the attractive field of the nucleus, and thus the atom is ionized. This process requires that the inner electron receives an amount of energy equal to or greater than the critical ionization energy, E_c. The critical ionization energy is a function of the specific atom and the specific electron shell and thus, is uniquely determined.