Ionisation Chamber Measurements of I123 Using a Copper Filter

Summary

The low energy X-ray emissions of ¹²³I lead to large variations in activity measurements using an ionisation chamber. A copper filter 0.5mm thick removes these X-rays but only reduces the 159 keV emissions by 14%. However the ionisation chamber needs to be recalibrated for the copper filter attenuation. We outline a simple method using a measurement made on ⁹⁹Tc^m to determine the change in sensitivity of the ionisation chamber using the copper filter. With the copper filter, the variation of activity measurements when ¹²³I is in a syringe compared to those in a vial is reduced from 80% to 3%. The ratio of the manufacturers stated activity to measured activity was 102% and 111% for two manufacturers respectively.