Biomedical Engineering Reference
In-Depth Information
verse the slab without interacting will be detected. One can measure the relative
rate at which photons reach the detector as a function of the absorber thickness
and then use Eq. (8.43) to obtain the value of
µ
(Problem 33).
The linear attenuation coefficient for photons of a given energy in a given mater-
ial comprises the individual contributions from the various physical processes that
can remove photons from the narrow beam in Fig. 8.7. We write
(8.44)
µ
=
τ
+
σ
+
κ
,
where
τ
,
σ
,and
κ
denote, respectively, the linear attenuation coefficients for the
photoelectric effect, Compton effect [Eq. (8.40)], and pair production. The respec-
tive mass attenuation coefficients are
τ
ρ
for a material of density
ρ
.
We could also add the (usually) small contributions to the attenuation due to pho-
tonuclear reactions and the Raleigh scattering, but we are neglecting these.
Figures 8.8 and 8.9 give the mass attenuation coefficients for five chemical el-
ements and a number of materials for photons with energies from 0.010 MeV to
100 MeV. The structure of these curves reflects the physical processes we have been
discussing. At low photon energies the binding of the atomic electrons is important
and the photoelectric effect is the dominant interaction. High-Z materials provide
greater attenuation and absorption, which decrease rapidly with increasing photon
energy. The coefficients for Pb and U rise abruptly when the photon energy is suffi-
/
ρ
,
σ
/
ρ
,and
κ
/
Fig. 8.8
Mass attenuation coefficients for various elements.
[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.]
