Corrections for the recombination of ions and electrons before they are collected in the WAFAC were determined using multiple-voltage-extrapolation methods described in Lamperti et al.
Then the expected ionization current in the WAFAC can be evaluated approximately as
For example, for the effects of attenuation of the primary beam in the air path from the aperture plane through the WAFAC volume, the last terms in both [b.
The photon fluence was scored in regions within the WAFAC volume and converted to absorbed dose in the air through the use of photon mass energy-absorption coefficients.
Note that the effects of external scatter are much smaller than predicted by the usual build-up factors for air because of the partial shielding between the source and the WAFAC.