critical excitation energy, E q in in situ microanalysis

https://dx.doi.org/10.1351/goldbook.C01394
Minimum energy to excite a specific analytical signal. The term is most frequently used in connection with the generation of X-ray spectra, designating the ionization (binding) energy of the orbital on which the transition ends.
Source:
PAC, 1983, 55, 2023 (Nomenclature, symbols and units recommended for in situ microanalysis (Provisional)) on page 2027