ASNT

PART 2 Electromagnetic Radiation Characteristics

temperature of the solid; at 100 keV only a percent of the electron’s incident or initial energy appears as X-rays. Target design is largely responsible for engineering effective heat dissipation means through conduction and radiation since convection is not available due to the vacuum that must be maintained in the X-ray tube insert. Characteristic X-rays. An incident electron ejects a target electron from its state within the atom (Figure 2). A prompt cascade of target atom

DIFFERENT MECHANISMS FOR PRODUCING X-RAYS FROM ELECTRON BOMBARDMENT

An energetic electron of some 10s keV to 500 keV encounters a solid target and energy is lost to the solid through elastic and inelastic electron-electron collisions and electron-nucleus interactions. Many of these collisions result in loss of kinetic energy to heat the target. The energy lost raises the

Incident electron

M shell

L shell

Large impact parameter — low energy photon

Incident electron

K shell

Small impact parameter — higher energy photon

Complete conversion — highest energy photon

Ejected electron

Figure 2 Schematic of high-energy electron interacting with atom’s nucleus and ground-state electron configuration. Electron partially penetrates atom’s surrounding electrons and loses energy through bremsstrahlung electromagnetic radiation due to acceleration in the coulomb electric field generated by the nucleus’ charge. Longer-wavelength radiation indicates that a small fraction of the electron’s energy leaves as a lower photon energy. Impact parameter is smaller and the coulomb field interaction has greater effect, so radiation has a higher energy, shown by shorter wavelength. Initial kinetic energy of incident electron completely converted to photon, producing highest possible photon energy. Electron interacts with atomic electron in highest-energy K shell and ejects electron from orbital.

CHAPTER 2

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Part 2