Biomedical Engineering Reference
In-Depth Information
Fig. 8.11
Mass energy-absorption 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.]
and pair production) and indirectly (as Auger electrons). Except for the subsequent
bremsstrahlung that the electrons might emit, the energy absorbed in the imme-
diate vicinity of the interaction site would be the same as the energy transferred
there.
3)
Letting
g
represent the average fraction of the initial kinetic energy transferred
to electrons that is subsequently emitted as bremsstrahlung, one defines the mass
energy-absorption coefficient as
µ
en
ρ
=
µ
tr
ρ
(8.58)
(1 -
g
).
Generally, the factor
g
is largest for materials having high atomic number and for
photons of high energy. Figures 8.11 and 8.12 show the mass energy-absorption
coefficients for the elements and other materials we considered earlier.
Differences in the various coefficients are illustrated by the data shown for water
and lead in Table 8.3. It is seen that bremsstrahlung is relatively unimportant in
water for photon energies less than
∼
10 MeV (i.e.,
µ
tr
/
ρ
is not much larger than
µ
en
/
ρ
). In lead, on the other hand, bremsstrahlung accounts for a significant differ-
3 A positron that annihilates in flight will also
cause the absorbed energy to be less than the
energy transferred. We ignore this usually
small effect.
