ASNT

electrons acts to fill the vacancies thus created. An L-shell electron that loses energy to fill a K-shell vacancy does so by creating a photon having the energy difference E K - E L = E (K α 1 ) = 59.31 keV for tungsten. K α 2 refers to the transition from the next p-orbital of L-shell. Thus, E (K α 2 ) = 57.97 keV. K β 1 and K β 2 are 67.23 keV and 69.1 keV in tungsten (Behling 2016). X-ray photons thus created are called “characteristic” since a precise measurement of their energy would identify the target material. These photon emissions appear as spikes on top of the bremsstrahlung feature of the X-ray spectrum in Figure 3. Features of the spectrum as shown are typical. The blue line indicates a spectrum measured outside the X-ray tube with a low-energy resolution detector. The wedge-shaped portion falling to zero intensity at maximum photon energy is due to the bremsstrahlung process, for which many photon energies are possible depending upon the electron impact parameter and interaction with the nuclear coulomb field. The peaks K α and K β indicate the well-defined energy of the radiation that is characteristic of the target material. The shaded area of the spectrum is filtered. (Much higher intensity is expected of an in-vacuum vessel measurement of X-rays by an idealized X-ray detector capable of sensing very low-energy photons.) The term "filtered" applies to the photons that have been removed from the emerging

radiation by thicknesses of material representing the X-ray tube window, casing window, dielectric oil, air, and any other intervening material. Continuous X-rays. Bremsstrahlung energy loss by the electron when interacting with the target atom nuclear coulomb electric field is not discrete but continuous in the photon energy emitted. The greatest energy that can be lost is the total incident electron energy, so the end point of the bremsstrah- lung spectrum is a measure of the tube voltage—the voltage difference through which the electrons are accelerated onto the target. As the bremsstrahlung spectrum in Figure 3 shows, the bulk of the X-rays from an electronic source of X-rays is at an energy lower than the peak acceleration voltage of the source. The spectrum is often characterized by the kilovoltage peak (kVp), which is the maximum of the electron acceleration voltage in the source. There will actually be no X-rays at energies equal to the kilovoltage peak. Rather there are many X-rays over a spectrum of energies less than the kilovoltage peak . When a bremsstrahlung spectrum X-ray beam passes through matter, the overall intensity of the beam will be reduced with the lower energy X-rays attenuated more quickly than the higher energy X-rays. This causes the transmitted beam spectrum to be shifted to a higher energy distribution. For estimating the attenuation of a bremsstrahlung X-ray beam in

Filtered radiation

Characteristic radiation

K α

Bremsstrahlung radiation

K β

Number of X-ray

Maximum photon energy

photons (intensity)

0

50

100

150 X-ray energy (keV)

200

250

300

Figure 3 Schematic spectrum of X-ray emission from electron bombardment of a solid target shows the number of X-ray photons (intensity) versus energy of the photon.

CHAPTER 2

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Part 2