Experimental determination of wall correction factors. Part II: NACP and Markus plane-parallel ionization chambers

For pt.I see ibid., vol.35, p.835-46 (1990). The formalism to derive the absorbed dose to water from ionization chamber measurements in a phantom includes several wall correction factors, k_att, k_m and p_wall, that depend on the shape, size and composition of the ionization chamber. Wall correction factors have been measured for a number of NACP and PTW/Markus chambers. Significant deviations from calculated values occur due to uncertainties in the contribution to the total ionization from the different materials of these inhomogeneous ionization chambers. Experimental values of p_wall larger than one have been obtained for the NACP chamber indicating that the side and back wall material of this chamber reduce the ionization compared with the situation where all electrons originate from the water phantom.