A new approach to the determination of air kerma using primary-standard cavity ionization chambers

A consistent formalism is presented using Monte Carlo calculations to determine the reference air kerma from the measured energy deposition in a primary-standard cavity ionization chamber. A global approach avoiding the use of cavity ionization theory is discussed and its limitations shown in relation to the use of the recommended value for W. The role of charged-particle equilibrium is outlined and the consequent requirements placed on the calculations are detailed. Values for correction factors are presented for the BIPM air-kerma standard for Co-60, making use of the Monte Carlo code PENELOPE, a detailed geometrical model of the WPM Co-60 source and event-by-event electron transport. While the wall correction factor k(wall) = 1.0012(2) is somewhat lower than the existing value, the axial non-uniformity correction k(an) = 1.0027(3) is significantly higher. The use of a point source in the evaluation of k(an) is discussed. A comparison is made of the calculated dose ratio with the Bragg-Gray and Spencer-Attix stopping-power ratios, the results indicating a preference for the Bragg-Gray approach in this particular case. A change to the recommended value for W of up to 2 parts in 10(3) is discussed. The uncertainties arising from the geometrical models, the use of phase-space files, the radiation transport algorithms and the underlying radiation interaction coefficients are estimated.