Calibration of beta-particle-emitting ophthalmic applicators

This service provides calibrations for ^^Sr + beta particle ophthalmic applicators. The calibration determines the average surface absorbed-dose rate to water over the active area of the applicator. The technique used is to measure current per unit mass of air at the active surface with an extrapolation ionization chamber, and to convert this into absorbed-dose rate with Bragg-Gray cavity ionization theory. The extrapolation chamber measurements are made in three parts. In the first part, a 1 mm diameter probe electrode is used to map the relative dose rate across the source surface, and to determine the active area. In the second part, the probe electrode is replaced by a 30 mm diameter electrode, and the test applicator is replaced by an NBS-owned applicator for quality assurance measurements. In the third part, the 30 mm electrode is used with the test applicator to determine the extrapolation curve slope at the source surface. The ratio of this slope to the source area, divided by the air density, yields current per unit air mass. The transformation to absorbed-dose rate is described. The component uncertainties are listed and are shown to yield a total uncertainty of ± 15%, with the approximate significance of a 95% confidence limit. Data book measurements and a calibration report are given for one particular applicator.