Spherical Extrapolation Chamber

Design and construction of a spherical-extrapolation chamber for the calibration of concave-ophthalmic applicators
Graduate Investigator: Jon Hansen
Summary of Research:  Ophthalmic applicators are sealed beta-emitting sources used by ophthalmologists to treat surface lesions of the eye including those present within a few millimeters from the surface. The beta-emitting source is typically enclosed in either a plane-geometry or concave-geometry metal housing. The calibration of these applicators with the highest precision requires the use of special ionization chambers called extrapolation chambers. These instruments are simply ionization chambers with the added ability of having an adjustable air-gap spacing between its electrodes. This allows one to obtain the necessary conditions to properly measure the surface strength from these applicators. Primary calibration standards currently exist for planar applicators due to the relative ease in constructing an extrapolation chamber with flat electrodes. The National Institute of Standards and Technology, the national-standards laboratory in the United States, holds the primary-calibration standard for planar applicators with their custom-designed plane-extrapolation chamber. Concave applicators, however, have no calibration standard due to the difficulties in constructing the curved electrodes necessary to conform to the geometry of these sources. This project is focused on designing and building a spherical-extrapolation chamber which would allow the surface dose from concave applicators to be determined with relatively high precision.