Extrapolation chamber measurements of 90Sr + 90Y beta-particle ophthalmic applicator dose rates.

Aspects of extrapolation chamber dose-rate measurements of 90Sr + 90Y beta-particle ophthalmic applicators are examined in this report, including the proper choice of collector electrode size, the gap width over which the measurement should be done, the effect of the entrance window materials, and the stopping-power ratio. Experiments, a simple analytic model for the effect of chamber geometry and nonzero gap width, and more detailed Monte Carlo simulations were used. The variation of the planar flux density as a function of angle for a thick 90Sr + 90Y source was measured and used as input for the model. From Monte Carlo simulation, the dose rate for tissue irradiation falls off by 8% between the surface and a depth of 7 mg/cm2. The derivative of chamber ionization as a function of gap width, needed for the dose-rate calibration, increases rapidly as the gap width decreases, typically by a factor 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 collector electrode and its backing as shown by Monte Carlo simulations. The backscattering effect increases the derived surface dose by a factor of 1.46. A satisfactory dose-rate extrapolation is obtained from gap widths of 0.1-0.25 mm, where the total ionization current is observed to be nearly linear in gap width.