Difração de raio x
M, etc., levels is not shown, it illustrates the main principles. The arrows show the transitions of the atom, and their directions are therefore just the opposite of the arrows in Fig. 1-7, which shows the transitions of the electron. Thus, if a K electron is removed from an atom (whether by an incident electron or x-ray), the atom is raised to the K state. If an electron then moves from the L to the K level to fill the vacancy, the atom undergoes a transition from the K to the L state. This transition is accompanied by the emission of Ka characteristic radiation and the arrow indicating
Kot emission is accordingly drawn from the K state to the L state.
Figure 1-9 shows clearly how the wavelengths of characteristic emission lines can be calculated, since the difference in energy between two states will equal hv, where v is the frequency of the radiation emitted when the
1-5] ABSORPTION 15 atom goes from one state to the other. Consider the Kai characteristic line, for example. The "L level" of an atom is actually a group of three closely spaced levels (Li, Ln, and LIU), and the emission of the Kai line is due to a K > Lm transition. The frequency VKai of this line is therefore given by the equations hi>K Therefore
= WK =
i. he
'
e\K
12,400
he
.
*
(1-16)
where VK is the K excitation voltage (in practical units) and \K is the K absorption edge wavelength (in angstroms).
Figure 1-10 summarizes some of the relations developed above. This curve gives the short-wavelength limit of the continuous spectrum as a function of applied voltage.
Because of the similarity between
Eqs. (1-4) and (1-16), the same curve also enables us to determine the critical excitation voltage from the wavelength of an absorption edge.
FIG. 1-10. Relation between the voltage applied to an x-ray tube and the short-wavelength limit of the continuous spectrum, and between the critical