Biomedical Engineering Reference
In-Depth Information
FIGURE 4.1: Different contributions to the total cross section
tot
(circles)
in carbon over photon energies ranging from 10 eV to 100 GeV. : absorption
edege,
COH
: coherent scattering,
INCOH
: Compton scattering,
n
: nuclear
field pair production,
e
: electron field pair production, and
PH:N:
: nuclear
photo absorption. (From [4].)
some of their energy. The differential cross section for an unbound electron
and a photon is given by the Klein{Nishina formula depending on the photon
angular distribution:
!
2
1 + cos
2
c
2
d
d
1
= Zr
e
E
m
e
c
2
(1 cos
c
)
1 +
!
2
(1 cos
c
)
2
(1 + cos
2
c
)(1 +
1 +
(4.2)
E
m
e
c
2
(1 cos
c
))
where
c
is the scatter angle, r
e
is the classical electron radius, the ne
structure constant, E
the photon energy, m
e
the electron mass, and c the
speed of light. Compton scattering per gram is nearly the same for differ-
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