EXTRAPOLATION CHAMBER MEASUREMENTS OF SR-90-90 BETA-PARTICLE OPHTHALMIC APPLICATOR DOSE-RATES(Y)

Aspects of extrapolation chamber dose-rate measurements of Sr-90 + Y-9 0 beta-particle ophthalmic applicators are examined in this report, in cluding the proper choice of collector electrode size, the gap width o ver which the measurement should be done, the effect of the entrance w indow materials, and the stopping-power ratio. Experiments, a simple a nalytic model for the effect of chamber geometry and nonzero gap width , and more detailed Monte Carlo simulations were used. The variation o f the planar flux density as a function of angle for a thick Sr-90 + Y -90 source was measured and used as input for the model. From Monte Ca rlo simulation, the dose rate for tissue irradiation falls off by 8% b etween the surface and a depth of 7 mg/cm(2) The derivative of chamber ionization as a function of gap width, needed for the dose-rate calib ration, increases rapidly as the gap;width decreases, typically by a f actor of about 2 between gap widths of 1.5 and 0.15 mm. About half of this change is due to ionizing electrons leaving the collection volume at the larger gap widths as shown by the analytic model; the rest of the change is due to ionizing electrons which backscatter from the col lector electrode and its backing as shown by Monte Carlo simulations. The backscattering effect increases the derived surface dose by a fact or of 1.46. A satisfactory dose-rate extrapolation is obtained from ga p widths of 0.1-0.25 mm, where the total ionization current is observe d to be nearly linear in gap width.