Biomedical Engineering Reference
In-Depth Information
Fig. 8.10
Broad, uniform, parallel beam of monoenergetic
photons normally incident on an absorber of thicknesses
x
.
Incident energy fluence rate is
˙
0
, and transmitted energy
˙
fluence rate is
.
˙
flux density:
= d
0
/d
t
(=
ψ
0
)
. This quantity is also called the intensity. For the
special beam in Fig. 8.10,
0
˙
=
˙
and
(8.52)
0
=
0
h
ν
0
h
ν
.
0
Energy fluence rate (intensity) can be expressed in Jm
-2
s
-1
=
Wm
-2
.
To infer the rate of energy absorption in the slab from the uniform beam in
Fig. 8.10, one can compare the intensity
˙
of the radiation reaching a detector
placed right behind the slab to the incident intensity
˙
0
. Figures 8.7 and 8.10 to-
gether indicate that, under the broad-beam conditions, the detector receives uncol-
lided as well as scattered and other (e.g., bremsstrahlung and fluorescence) pho-
tons. Thus, not all of the energy of the incoming photons that interact in the slab is
necessarily absorbed there. The decrease in beam intensity with increasing
x
can
be expected to be
less
than that described by the linear attenuation coefficient, e
-
µx
.
We consider each of the principal energy-loss mechanisms in turn, discussing first
the energy-transfer coefficient and then the energy-absorption coefficient.
In the photoelectric effect, absorption of a photon of energy
h
ν
by an atom pro-
duces a secondary electron with initial kinetic energy
T
-
B
, where
B
is the
binding energy of the ejected electron. Following ejection of the photoelectron, the
inner-shell vacancy in the atom is immediately filled by an electron from an upper
=
h
ν
