High-Z materials

X-ray tubes are used in a broad variety of technical applications the classical application certainly is the radiographic inspection. For the penetration of high-Z materials, relatively high power is required. This lead to the development of X-ray tubes for laboratory and field use of voltages up to 450 kV and cp power up to 4,5 kW. Because of design, performance and reliability reasons, most of these maximum power stationary anode tubes are today made in metal-ceramic technology. 

Here Pyj is the structure factor for the (hkl) diffiaction peak and is related to the atomic arrangements in the material. Specifically, Fjjj is the Fourier transform of the positions of the atoms in one unit cell. Each atom is weighted by its form factor, which is equal to its atomic number Z for small 26, but which decreases as 2d increases. Thus, XRD is more sensitive to high-Z materials, and for low-Z materials, neutron or electron diffraction may be more suitable. The faaor e (called the Debye-Waller factor) accounts for the reduction in intensity due to the disorder in the crystal, and the diffracting volume V depends on p and on the film thickness. For epitaxial thin films and films with preferred orientations, the integrated intensity depends on the orientation of the specimen. 

Rutherford scattered electrons (Figure 9.12). The intensity of these electrons is proportional to Z2 (where Z is the atomic number of scattering atom) so that the experimental method is most suitable for high-Z materials distributed over low-Z supports. 

Bethe s formula requires that the velocity of the incident particle be much larger than that of the atomic electrons, a condition not easily fulfilled by the K-electrons except in the lightest elements. The required correction, called the shell correction, is denoted by subtracting a quantity C from the stopping number. In the penetration of high-Z material, even L-shell correction may be required. In that case, C denotes the sum total of all shell corrections. The subject of shell correction has been extensively treated by several authors, and various graphs and formulas are available for its evaluation (see, e.g., Bethe andAshkin, 1953). 

Tungsten as a high-Z material with low sputter yield and high melting temperature will be used for the baffle regions of the target and the surface of the divertor dome. 

For charged particles with energies considered here, the kinetic energy lost as bremsstrahlung might be important for electrons only. Even for electrons, it is important for high-Z materials like lead (Z = 82). For more detailed treatment of the emission of bremsstrahlung, the reader should consult the references listed at the end of the chapter. 

The second term in brackets indicates correction terms of the first order in Z. Figure 4.17 shows how the photoelectric coefficient changes as a function of E and Z. Fig. 4.17 and Eq. 4.46 show that the photoelectric effect is more important for high-Z material, i.e., more probable in lead (Z = 82) than in A1 (Z = 13). It is also more important for = 10 keV than = 500 keV (for the same material). Using Eq. 4.46, one can obtain an estimate of the photoelectric coefficient of one element in terms of that of another. If one takes the ratio of t for two elements, the result for photons of the same energy is... 

For thin detectors, or detectors made of high-Z material—e.g., CdTe or Hglj—some X-rays may escape, thus forming the so called escape peaks (see Sec. 12.8). 

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-... 

This shows that P impurity is a suitable co-dopant for LiBF4 Cu compound for enhancement of the TL response. The sensitivity of LiBF4 Cu,P is 3.3 times less than that of standard CaS04 Dy TLD phosphor which is high Z material (Z f= 15.3). 

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