Difração de raio x

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Although this diagram is simplified, in that the substructure of the L,
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

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