Bremsstrahlung

When an energetic electron beam impinges upon a (high-Z) material, X-rays in a broad wavelength band are emitted. This radiation is called Bremsstrahlung as it is released during the sudden deceleration of the primary electrons, as a result of their interaction with the electrons of the lattice atoms in the target. At each colli- 

24/V (kV) where E is the maximum energy of the impinging electrons and V the potential used to accelerate them. The continuum distribution reaches a maximum at 1.5 2 X so that an increase in the accelerating potential V causes a shift of the continuum towards shorter wavelengths. In Fig. 11.6 bremsstrahlung spectra emitted by X-ray tubes operated at different accelerating potentials are shown. 

FIGURE 1.4 The speed v of the charged particle is greater than the speed dn of light in the material n is the refractive index. 

When interpreting spectra, it is worth remembering that a 511 keV photon can also be expected whenever a radionuclide emits positrons as part of its decay process. Common examples of such nuclides are Na, Zn and Cu. The interpretation of the presence of a 511 keV peak is not, therefore, as obvious as it might appear. There are three possible explanations, which are not mutually exclusive  

for reasons of sensitivity, it is essential to count sources close to the detector and absorbers offer little relief, then there is little to be done except, perhaps, the under-used last resort - radiochemical separation of the nuclide of interest. Systems using magnets to divert the (3 particle away from the detector have been demonstrated but appear to offer only small improvements in precision and again demand a substantial source-detector distance to be effective. 

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As early as the 1930s X-ray absorption experiments were being carried out using a continuum source of X-rays (the bremsstrahlung mentioned in Section 8.1.1.1), a dispersive... 

A typical x-ray photoelectron spectmm consists of a plot of the iatensity of photoelectrons as a function of electron E or Ej A sample is shown ia Figure 8 for Ag (21). In this spectmm, discrete photoelectron responses from the cote and valence electron energy levels of the Ag atoms ate observed. These electrons ate superimposed on a significant background from the Bremsstrahlung radiation inherent ia n onm on ochrom a tic x-ray sources (see below) which produces an increa sing number of photoelectrons as decreases. Also observed ia the spectmm ate lines due to x-ray excited Auger electrons. 

Fig. 20. Primary x-ray line and Bremsstrahlung background excited by bombardment with 15 keV electrons, (a) Linear scale plot, (b) Logarithmic scale...

There are two processes where nuclear and atomic contributions are iaterrelated. These are the emission of electrons from the atomic shells as an alternative to the emission of a photon and the emission of bremsstrahlung photons ia the P decay process. 

The particle notation is j3 for electrons from j3 -decay, e for internal-conversion electrons, and IB for photons from internal bremsstrahlung. Ref 15. 

A consequence of absorption of X rays is the inner shell ionization of the absorbing atoms and the subsequent generation of characteristic X rays from the absorbing atoms, called secondary fluorescence, which raises the generated intensity over that produced by the direct action of the beam electrons. Secondary fluorescence can be induced by both characteristic and bremsstrahlung X rays. Both effects are compo-sitionally dependent. 

Since no background correction can be made, dot maps of minor and trace constituents are subject to possible artifacts caused by the dependence of the bremsstrahlung on composition, particularly with EDS X-ray measurement. 

Fig. 2.3. Schematic diagram of X-ray monochromatization to remove satellites, eliminate bremsstrahlung background and separate the Al Ko i,2 doublet. Courtesy of Kratos Analytical.

In an electron-excited X-ray spectrum the discrete X-ray lines are superimposed on a continuous background this is the well-known bremsstrahlung continuum ranging from 0 to the primary energy Eq of the electrons. The reason for this continuum is that because of the fundamental laws of electrodynamics, electrons emit X-rays when they are decelerated in the Coulomb field of an atom. As a result the upper energy limit of X-ray quanta is identical with the primary electron energy. 

Inverse Photoemission Spectroscopy (IPES) and Bremsstrahlung Isochromat Spectroscopy (BIS)... 

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