Abstract
Neutron fields are always accompanied by photons. Because of the difference in relative biological effectiveness of these two radiation components, it is necessary to determine neutron and photon absorbed dose in the mixed field separately. The use of Mg/Ar or Al/Ar ionisation chambers for determination of the photon component of the total absorbed dose could provide specific advantages related to the energies of the photons in the mixed field. Therefore information is presented to enable the greatest accuracy to be obtained in the use of these chambers. The gas flow characteristics of an Mg/Ar ionisation chamber show a complex behaviour most likely due to the Jesse effect in Ar. The dependence of the reading on the wall thickness of the chamber might be influenced by the irradiation geometry. The presence in the beam of charged particles created in hydrogen-rich materials could be important, as well as the contribution of low-energy photons produced by charged particles in the target construction. In Ar, the phenomenon that only positive ions and electrons are produced by ionising particles determines the ion collection. A polarity effect is often observed. The relative neutron sensitivities of different chambers of the same design irradiated under similar conditions show considerable variations.
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