Dose distribution in electron-irradiated PMMA: effect of dose and geometry

Depth-dose distributions for an electron beam are generally determined with either a stack irradiation geometry or a wedge (generally with an angle of about 50 degrees-60 degrees) made from an electrically-conducting material (such as, aluminium) or a non-conducting material (such as, PMMA). If the non-conducting wedge is thicker than the electron range, the stored charge in the material could influence the measured depth-dose distribution. This effect was investigated for 7-MeV electrons for PMMA with the wedge angle v arying from 0 degrees to 60 degrees. The maximum-to-surface dose ratio was used as a characteristic parameter of the shape of the distribution. The de pth-dose distribution measured by a dosimetry film placed inside the wedge- pair was similar to the standard shape when the wedge angle was larger than about 55 degrees (dose ratio similar to 1.5). However, as the angle was de creased, this ratio sharply increased almost linearly up to about 15 degree s, and then leveled off at about 3. We also studied the effect of the surfa ce dose on this dose ratio for the wedge angle of 0 degrees, when we found that the ratio increased with the dose. Both of these effects can be explai ned by the presence of the electrostatic field around the dosimetry film du e to stored charge in the non-conducting PMMA. (C) 1999 Elsevier Science Lt d. All rights reserved.