While air kerma is no longer of significant clinical interest at 60Co, the quantity is still relevant to some 192Ir dosimetry and is used for protection-level measurements, as well as remaining important for quality assurance and continuity purposes.
ARPANSA uses a graphite cavity chamber (Figure 1) to determine air kerma at 60Co. The chamber is described in an ARPANSA technical report (TR155). The air kerma is determined from a measurement of the ionisation in a known volume of air. Small corrections are provided for the graphite walls and electrode.
Due to some uncertainty around the volume of this chamber, a new chamber has been purchased, measured and assembled (Figure 2). The chamber was constructed by the Bureau International des Poids et Mesures (BIPM). The dimensions were measured in Australia before and after the final assembly at the BIPM.
Figure 1: The graphite cavity chamber – the primary standard for air kerma at 60Co – in front of the 60Co source.