ASNT

The choice of target material is determined by the need for a high-brightness source of photons in the energy range appropriate to the imaging task. Energetic electrons move from the surface of the target to rest through collisions with atomic electrons in the material. Bremsstrahlung radiation makes up the largest part of the X-ray spectrum and is favored by electron interaction with atoms of large Z , or atomic number. These interactions take place at different depth of penetration in the solid target; the radiation must emerge through the target material to be useful in illuminating the object under inspection. This produces a hardening of the spectrum compared to thin target spectra. Heat management is an important consideration for target material selection; refractory metals with a high atomic number are most often used—tungsten is the target for most radiographic inspection tubes in the range of 100 to hundreds keV energy.

ATOMIC STRUCTURE AFFECTING X-RAY SPECTRUM The feature of the atomic model that most affects the spectrum of the X-rays emerging from a typical X-ray source is the energy of the electrons in the atom’s ground state (Figure 1). The values of the energies of electrons in these states depend upon the charge of the nucleus of the atom and the main quantum numbers labeling the electron in its orbital: n (principal quantum number), l (orbital angular momentum), m (projection of the orbital angular momentum), and s (spin). Therefore, E = f (n, l, m, s). Features of the spectrum shape are due primarily to n, l, and the transition energy between the different energy levels in the target atom.

CHAPTER 2

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Part 1