The AIR KERMA RATE in air ($K_{air}$) is given by the following relation:
(1)where A is the source activity
and d is the distance between the point of interest and the point source.
The REFERENCE AIR KERMA RATE is the recommended specification for brachytherapy sources:
(2)which is defined by the ICRU as the air kerma rate in air at a reference distance of 1 m, corrected for air attenuation and scattering.
The APPARENT ACTIVITY [ $A_{app}$ ] for a given brachytherapy source is defined as the activity of a hypothetical unfiltered point source of the same radionuclide that would give the same air kerma rate in air at a reference distance (typically 1 m) along the perpendicular bisector of the source.
(3)