12.3 - Measurement Of Sealed Sources

Specification of Sealed Source Strength

Measurement of sealed sources is a very complex topic.
The two most common methods of specifying source strength are:

  • Reference Air Kerma Rate (used outside of the USA), which measures the air kerma rate at a distance of 1 m from the source. The units are $\mu \text{Gy}/h$
  • Air Kerma Strength (used in the USA), which measures that air kerma rate at a distance of 1 m from the source, which is equal to the air kerma rate multiplied by the distance squared (hence, still 1). The units are $\mu \text{Gy}.m^2/h$

These two quantities have an equal number but different units. The sample measurement scenario is shown below:

The measurement gives the kerma (in Gy, J/kg) per hour at a distance of 1 m. This is the reference air kerma rate. To obtain the air kerma strength, the reference air kerma rate is multiplied by the square of the distance.

Dose Distribution


Anisotropy is the phenomen of having different measurements in different planes. A perfect sphere is isotrophic, as the radius from the centre to the edge is identical in all directions. An egg is anisotropic as it the radius is longer in one direction than another. Anisotropy of brachytherapy sources occurs due the cylindrical nature of most sources. This gives an increased dose rate lateral to the long axis of the cylinder, with less dose at each end. When determining the dose at a point near the sealed source, and anisotropy factor is used to adjust the dose based on the known distribution of the source.

Determining Dose at a Point

Once the reference air kerma rate is determined, it is important to calculated the dose rate at a more useful distance (usually a few centimetres).

I'm not sure how this is done at this moment in time but I'll keep working on it!