RESUMO
Extrapolation ionization chamber is normally used for absolute measurement of absorbed dose to tissue and in standardization of beta particle emitting sources in particular. However, in this technique, several correction factors are applied to the measured ionization current to evaluate the true value of the dose to the tissue from beta radiation. These correction factors are described in ISO 6980-2:2004. Out of these, ion recombination correction factor (ksat) and ambient air density correction factor (kad) are evaluated experimentally during measurement of ionization current. Other significant correction factors are backscatter correction factor (kba), correction factor for perturbation in electron spectrum due to ionization chamber side wall scatter (kpe), correction factor for difference in attenuation of beta spectrum due to change in ambience condition (kabs) and correction factor for radial non-uniformity of dose inside detector volume (kra). Experimental evaluation of correction factors kba, kpe and kabs are not easily achievable. In the present work four significant correction factors kba, kpe, kabs and kra are derived theoretically by Monte Carlo particle transport simulation. These correction factors are evaluated for beta emission spectrum of 85Kr, 90Sr-90Y and 106Ru-106Rh radioactive sources.
