Interaction of Photons with Matter - Atoms, Radiation, and-Radiation Protection - page 187

Fig. 8.9 Mass attenuation coefficients for various materials.
[Reprinted with permission from K. Z. Morgan and J. E. Turner,
eds., Principles of Radiation Protection, Wiley, New York (1967).
Copyright 1967 by John Wiley & Sons.]

cient to eject a photoelectron from the K shell of the atom. The curves for the other
elements show the same structure at lower energies. When the photon energy is
several hundred keV or greater, the binding of the atomic electrons becomes rela-
tively unimportant and the dominant interaction is Compton scattering. Since the
elements (except hydrogen) contain about the same number of electrons per unit
mass, there is not a large difference between the values of the mass attenuation
coefficients for the different materials. Compton scattering continues to be impor-
tant above the 1.022-MeV pair-production threshold until the latter process takes
over as the more probable. Attenuation by pair production is enhanced by a large
nuclear charge of the absorber.

Example
What thickness of concrete and of lead are needed to reduce the number of 500-
keV photons in a narrow beam to one-fourth the incident number? Compare the
thicknesses in cm and in g cm -2 . Repeat for 1.5-MeV photons.

Solution
We use Eq. (8.43) with N ( x )/ N 0 = ob-

tained from Figs. 8.8 and 8.9 are shown in Table 8.2. At 500 keV, the linear attenu-

0.25. The mass attenuation coefficients µ / ρ