Experimental determination of the overall perturbation factor for theNACP chamber in electron beams for dmax < d ⩽ d80

Abstract

In electron beam dosimetry with an ionization chamber, a factor that corrects forthe cavity perturbation of the medium, Prepl, and one to account forthe disturbance due to the chamber wall material differing from the medium,Pwall, are required. The overall perturbation correction factor, pq = PreplPwall, has been introduced because of the difficulty inseparately measuring these two components. An advantage of parallel-plateionization chambers is that pq has been shown to be close to unity atdmax. However, many dosimetry applications require knowledge of theoverall perturbation factor at depths greater than dmax. Wedetermined pq for the NACP chamber at depths beyond dmax byintercomparing percentage depth dose measurements made with it with thoseobtained with a PTW/diamond detector for which pq was taken as unity at allthe measurement depths. Data were obtained at depths corresponding toapproximately the 90 and 80 per cent of the dose maxima for 20, 16, 12 and6 MeV incident electrons. The beam energy at depth, Ed, and the percentagedepth-dose gradient varied from 1.4 to 14.3 MeV and 0 to 5.8% mm-1respectively. Our results show that within the estimated uncertainty of 1.3%,pq,NACP is unity over the range of energies and dose gradientsstudied.