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Let us consider a volume element dV filled by a material of mass dm at a point in
photon radiation field. The quantity
kerma
refers to the
kinetic energy of charged
particles such as electrons and positrons that have been liberated by uncharged
particles
(photons). Kerma does not include the energy that has been expended
against the binding energies of these charged particles even if this is usually a
relatively small component. The name kerma for this quantity is derived from the
Kinetic Energy Released per unit Mass.
If dE
tr
is
the sum of the initial kinetic energies of all charged particles liberated by
uncharged particles
within a volume element dV of a material containing a mass
dm
=ρ.dVof that material, then the
kerma
K
(J.kg -1
) is given by
Kerma K
dV
dE
dm
dE
K
tr
tr
.
1
The name of the SI unit for kerma is gray (Gy): 1 Gy = 1
J.kg-1
. Considering dE
tr
it
has to be mentioned that this includes also the kinetic energy of Auger electrons.
dm
=ρ.dVair