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Let us consider a source of irradiation being a
point in free space (vacuum)
,
an
ideal point source
, consisting of a specific radionuclide and having a activity A.
Air Kerma-rate constant Γ
δ
If is the air kerma rate due to photons of energy greater than a certain cut-off
value
δ
at a distance r from that point source, then the
air kerma-rate constant
Γ
δ
of that radionuclide is defined as:
A
K r
δ
δ
&
⋅
=Γ
2
If the unit gray (Gy) for the kerma and becquerel (Bq) for the activity are used,
then the unit of the air kerma-rate constant is
Gy.s
-1
.Bq
-1
.m²
.