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Let us consider a volume element dV filled by air of mass dm at a point in a
photon radiation field. If dQ is the absolute value of the total charge of the
ions of one sign produced in air when all the electrons and positrons
liberated or created by photons in the air filled volume element dV are
completed stopped in this volume
, then the
exposure
X
(
C.kg -1
) is defined
as
dV
dQ
dm
dQ
X
a
.
1
where ρ
α
is the density of air in volume dV. It has to be mentioned that
according to this definition ionisation produced by Auger electrons are
included in dQ where ionisation produced by radiative processes such as
bremsstrahlung and fluorescence photons is not included in dQ
.
The SI unit of Exposure is 1
C.kg-1
.
Exposure X
dm=ρ
α
.dV
air