X-Ray Spectra


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X-Ray Spectra

 

the emission and absorption spectra of X rays, that is, of electromagnetic radiation with wavelengths from 104 to 103 angstroms (A). The spectra of X rays produced, for example, in an X-ray tube can be investigated by means of a spectrometer with a crystal analyzer or diffraction grating or by means of an apparatus that does not contain a crystal or grating but consists of a pulse-height analyzer and a detector, such as a scintillation, gas proportional, or semiconductor counter. X-ray photographic film and various ionizing-radiation detectors are used to detect X-ray spectra.

The emission spectrum of an X-ray tube consists of a continuous spectrum on which the characteristic X-ray spectrum is superimposed. The continuous spectrum is produced by the deceleration of the charged particles bombarding the target—that is, it is the spectrum of bremsstrahlung. The intensity of the bremsstrahlung increases rapidly as the mass of the bombarding particles decreases; the intensity reaches a significant value in the case of excitation by electrons. The spectrum of the bremsstrahlung is continuous because a particle can lose any part of its energy in producing bremsstrahlung. The spectrum is continuously distributed over all wavelengths λ down to a minimum wavelength, called the short-wavelength cutoff, λ0 = hc/eV, where h is Planck’s constant, c is the speed of light, e is the charge of the bombarding particle, and V is the potential difference traversed by the particle. As the energy of the particles increases, the intensity I of the bremsstrahlung increases, and the cutoff wavelength λ0 becomes shorter (Figure 1). As the atomic number Z of the target atoms increases, I also increases.

Figure 1. Distribution of bremsstrahlung intensity / with wavelength λ for different X-ray tube voltages V

Characteristic spectra are emitted by those atoms of a target for which an electron is removed from one of the inner shells, such as the K-, L-, or M-shell, when a high-energy charged particle or a photon of primary X-radiation collides with the atom. The state of an atom having a vacancy in an inner shell (the initial state) is unstable. An electron from one of the outer shells can fill this vacancy; when it does so, the atom undergoes a transition to a final state of lower energy, where there is a vacancy in an outer shell. The atom can emit the excess energy in the form of a photon of characteristic radiation. Since the energies E1 of the initial state and E2 of the final state of the atom are quantized, a line of the X-ray spectrum of frequency v = (E1—E2)/h results. The radiative quantum transitions of atoms from an initial K-state form the K-series; such transitions produce the hardest X-radiation, that is, the X-radiation with the shor

Figure 2. Diagram of the K-level, L-level, and M-level of an atom and of the principal lines of the K-series and the L-series

test wavelengths. The L-series, M-series, and N-series are formed in a similar way (Figure 2). The position of the lines of the characteristic spectrum depends on the atomic number of the element forming the target (seeMOSELEVS LAW).

Each series of characteristic X-ray spectra is excited by bombarding particles having a certain excitation potential V, where q indicates the series being excited. As V increases further, the intensity I of the lines of the spectrum at first increases in proportion to (V - Vq)2 and then slows; when V ≈ 11 νq’ the intensity begins decreasing.

The relative intensities of lines of the same series are determined by the probabilities of the quantum transitions and, consequently, by the corresponding selection rules. The brightest lines are those of electric dipole radiation. Weak lines of electric quadrupole, electric octupole, magnetic dipole, and magnetic quadrupole radiation can also be detected in characteristic spectra.

X-ray absorption spectra are obtained by passing primary X-radiation with a continuous spectrum through a thin absorber. When this is done, the distribution of intensity over the spectrum changes. There are observed absorption discontinuities, or jumps. The X-ray absorption spectrum consists of the “absorption edges” occuring at the wavelengths corresponding to the discontinuities. For each level, X-ray absorption spectra have a minimum critical frequency vq (hvq = eVq) at which the first absorption jump is observed (Figure 3).

Figure 3. Dependence of bremsstrahlung intensity / on frequency ν near vq: (1) without passing through an absorber, (2) after passing through an absorber

X-rays have found application in X-ray spectroscopy, X-ray spectral analysis, and X-ray diffraction analysis.

M. A. BLOKHIN

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