Interaction of Photons with Matter - Atoms, Radiation, and-Radiation Protection - page 194

Fig. 8.13 Linear attenuation and energy-absorption coefficients
as functions of energy for photons in water.

It is instructive to see how the individual physical processes contribute to the in-
teraction coefficients as functions of the photon energy. Figure 8.13 for water shows
τ , σ s , σ tr ,and κ as well as the coefficients µ and µ en . Also shown for comparison
is the attenuation coefficient σ r for Raleigh scattering, which we have ignored. At
the lowest energies ( < 15 keV), the photoelectric effect accounts for virtually all of
the interaction. As the photon energy increases, τ drops rapidly and goes below σ s .
Between about 100 keV and 10 MeV, most of the attenuation in water is due to the
Compton effect. Above about 1.5 MeV, σ tr >

σ s . The Compton coefficients then fall
off with increasing energy, and pair production becomes the dominant process at
high energies.

8.9
Calculation of Energy Absorption and Energy Transfer

It remains to show how the coefficients µ tr and µ en are used in computations.
We consider again the idealized broad, parallel beam of monoenergetic photons in
Fig. 8.10 and ask how one can determine the rate at which energy is absorbed in
the slab, given the description of the incident photon field.
We begin by assuming that the slab is thin compared with the mean free paths
of the incident and secondary photons, so that (1) multiple scattering of photons in
the slab is negligible and (2) virtually all fluorescence and bremsstrahlung photons
escape from it. On the other hand, we assume that the secondary electrons pro-