Almost any high-energy photon will act as a source for the production of characteristic x-rays provided that it is sufficiently energetic to eject an electron from the appropriate atomic level of the element to be excited. The minimum energy required is simply the binding energy t/J of the electron in the appropriate shell. Binding energies for the different shells and subshells are often denoted by subscripts such as t;K. t/JL, t/JLoiih etc. Frequently, the binding energy is identified as the absorption edge energy Eat. since it corresponds to the energy at which the mass absorption coefficient for the element changes abruptly. It is also referred to as the critical excitation potential since t/J in kiloelectronvolts (keV) corresponds to the minimum voltage in kilovolts (kV) on the x-ray tube required to excite the characteristic x-rays. In Table 3.1, critical excitation potentials are listed for exciting the Ka, La, and Ma lines of selected elements covering the range from atomic numbers 9 to 90. Since x-ray spectrometers generally cover the range from 0.3 to 18 A or 0.7 to 40 keV, excitation energies from 1 keV to beyond 50 keV are normally required.