Figure 8.

The penumbra for each detector as measured during the scanning procedure, and calculated as the lateral distance between the 80% and 20% values (normalized to the central axis). The relatively large size of the detecting volume of the ionization chamber results in volume averaging, yielding larger penumbra values. As the field size increases, greater lateral scatter results in increasing penumbral widths. This increase is minimized at the initial depth of 1.4 cm, and increases with depth.