![]() ![]() ![]() For example, a M-shell electron of binding energy of 2.3 keV that fills the K shell vacancy produces a characteristic x-ray of 67.2 keV. Orbital electrons from outer shells (M, N, …) with lower binding energy will produce characteristic x-rays of higher energy. Electrons that transition from the L to an empty K-shell orbital produce discrete characteristic x-rays equal to the difference of 69.5 K-shell binding energy and the respective L-shell binding energies: 59.3, 58, and 57.4 keV. 6-4).įor tungsten, L-shell electrons have binding energies of 10.2, 11.5, and 12.1 keV. As discussed in Chapter 2, an outer shell electron with less binding energy immediately transitions to fill the vacancy, and a characteristic x-ray is emitted with an energy equal to the difference in the electron binding energies of the two shells ( Fig. When the kinetic energy of an incident electron exceeds the binding energy of an electron shell in a target atom, an interaction can eject an electron from its shell, creating a vacancy. Table 6-1 lists the common anode target materials and the binding energies of the K, L, and M electron shells that are “characteristic” of the element. The innermost shell is designated the K shell and has the highest electron binding energy, followed by the L, M, and N shells, with progressively less binding energy. Electrons in an atom are distributed in orbital “shells” and have specific electron binding energies to maintain equilibrium. In addition to the continuous bremsstrahlung x-ray spectrum, discrete x-ray energy peaks called “characteristic radiation” are present, with x-ray energies depending on the elemental composition of the anode and the applied x-ray tube voltage. Filtration in this context refers to the removal of x-rays by attenuation in materials that are inherent in the x-ray tube ( e.g., the glass or metal window of the tube insert), as well as by materials that are intentionally placed in the beam, such as thin aluminum and copper sheets to preferentially attenuate lower energy x-rays and to therefore tune the x-ray spectrum for low-dose imaging (see Beam Filtration in Section 6.5). 6-3b) has no x-rays present below about 10 keV the fluence increases to a maximum at about one third to one half the maximal x-ray energy and then drops off to zero at just beyond the peak x-ray energy. A typical filtered bremsstrahlung spectrum ( Fig. 6-3a) shows an inverse linear relationship between the fluence and energy of the x-rays produced, with the highest x-ray energy, x keV, determined by the peak voltage ( x kV) applied across the x-ray tube. The unfiltered bremsstrahlung spectrum ( Fig. A bremsstrahlung spectrum is the probability distribution of x-ray photon fluence produced as a function of energy, expressed in keV. For the closest electron-nucleus interactions ( i.e., r = 0), the highest x-ray energy is produced at extremely low probability and small number of x-rays. Increasing the radius in steps of d r from the nucleus defines an incrementally increasing annulus area thus electron-nucleus interactions generate incrementally larger numbers of x-rays at incrementally decreasing energies. The probability of electron interactions that produce x-rays of energy E depends on the radial interaction distance, r, from the nucleus, which defines an annulus of inner diameter 2 r, and outer diameter 2( r + d r), where d r is a small fixed radial increment. The exposure rate is the increment of exposure in a time interval (unit of C/kg/s). The term exposure is a dosimetric quantity describing the amount of charge in coulombs (C) released in a known mass of air (unit of C/kg) and is expressed in terms of the energy fluence and energy absorption integrated over energy. In terms of nomenclature used in this chapter, the fluence and energy fluence are scalar radiometric quantities. These components work in concert to produce a beam of x-ray photons with controlled fluence (number of incident photons per unit area), energy fluence (energy weighted number of photons per unit area), and a well-collimated trajectory. The x-ray generator supplies the tube potential to accelerate the electrons, a filament circuit to control tube current, and an exposure timer. The x-ray tube insert is mounted within a tube housing, which includes a metal enclosure protective radiation shielding x-ray beam filters for shaping the x-ray spectrum and collimators, which define the size and shape of the x-ray field. The x-ray tube insert contains an electron source, a vacuum environment, and a target electrode an external power source provides high voltage (potential difference) to accelerate the electrons. X-rays are produced when highly energetic electrons interact with matter, converting some or all of their kinetic energy into electromagnetic radiation. ![]()
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