Atomic Structure and Atomic Radiation - Atoms, Radiation, and-Radiation Protection - page 41

Fig. 2.8 Schematic diagram of modern X-ray tube with fixed target anode.

in the target, causing the ionization and excitation of atoms. In addition, they can
be sharply deflected in the vicinity of the atomic nuclei, thereby losing energy by
irradiating X-ray photons. Heavy nuclei are muchmore efficient than light nuclei in
producing the radiation because the deflections are stronger. A single electron can
emit an X-ray photon having any energy up to its own kinetic energy. As a result, a
monoenergetic beam of electrons produces a continuous spectrum of X rays with
photon energies up to the value of the beam energy. The continuous X rays are also
called bremsstrahlung, or “braking radiation.”
A schematic diagram, showing the basic elements of a modern X-ray tube, is
shown in Fig. 2.8. The tube has a cathode and anode sealed inside under high
vacuum. The cathode assembly consists of a heated tungsten filament contained
in a focusing cup. When the tube operates, the filament, heated white hot, “boils
off” electrons, which are accelerated toward the anode in a strong electric field
produced by a large potential difference (high voltage) between the cathode and
anode. The focusing cup concentrates the electrons onto a focal spot on the anode,
usually made of tungsten. There the electrons are abruptly brought to rest, emitting
continuous X rays in all directions. Typically, less than 1% of the electrons' energy
is converted into useful X rays that emerge through a window in the tube. The
other 99+ % of the energy, lost in electronic collisions, is converted into heat, which
must be removed from the anode. Anodes can be cooled by circulating oil or water.
Rotating anodes are also used in X-ray tubes to keep the temperature lower.
Figure 2.9 shows typical continuous X-ray spectra generated from a tube operated
at different voltages with the same current. The efficiency of bremsstrahlung pro-
duction increases rapidly when the electron energy is raised. Therefore, the X-ray
intensity increases considerably with tube voltage, even at constant current. The
wavelength of an X-ray photon with maximum energy can be computed from Eq.
(2.26). For the top curve in Fig. 2.9, we find λ min =

12400/50000 = 0.248 Å, where