Experimental determination of the overall perturbation factor for the NACP chamber in electron beams for dmax < d < or = d80.

In electron beam dosimetry with an ionization chamber, a factor that corrects for the cavity perturbation of the medium, Prepl, and one to account for the disturbance due to the chamber wall material differing from the medium, Pwall, are required. The overall perturbation correction factor, p(q) = PreplPwall, has been introduced because of the difficulty in separately measuring these two components. An advantage of parallel-plate ionization chambers is that p(q) has been shown to be close to unity at dmax. However, many dosimetry applications require knowledge of the overall perturbation factor at depths greater than dmax. We determined p(q) for the NACP chamber at depths beyond dmax by intercomparing percentage depth dose measurements made with it with those obtained with a PTW/diamond detector for which p(q) was taken as unity at all the measurement depths. Data were obtained at depths corresponding to approximately the 90 and 80 per cent of the dose maxima for 20, 16, 12 and 6 MeV incident electrons. The beam energy at depth, Ed, and the percentage depth-dose gradient varied from 1.4 to 14.3 MeV and 0 to 5.8% mm(-1) respectively. Our results show that within the estimated uncertainty of 1.3%, p(q),NACP is unity over the range of energies and dose gradients studied.