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Photons deposit energy in the medium through a two step process:
1. energy is transferred to charged particles released in the medium
2. the released charged particles act as directly ionizing radiation
KERMA
:
K
inetic
E
nergy
R
eleased per unit
Ma
ss (J/kg = Gy)
Sum of initial kinetic energies transferred from
indirectly ionizing
radiation to
charged particles (electrons) in a sample of matter
Photons are indirectly ionizing
•
initial kinetic energies of e
-
1
and e
-
2
contribute
to the
KERMA
as both were generated in V, e
-
3
was generated outside V, so does not contribute
•
e
-
1
and e
-
3
contribute to the
Dose
in V, while e
-
2
deposits energy outside V, so does not
contribute
e
-
1
e
-
2
e
-
3
V