Hot Particle Dosimetry Using Extrapolation Chambers and Radiochromic Foils

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Abstract
Results are presented of a comparison between extrapolation chambers and radiochromic foils for the dosimetry of small radioactive ('hot') particles. While it is possible to measure average dose over arbitrary areas using both methods, the radiochromic foil method has the advantage of displaying full two-dimensional dose distributions with a single measurement. With multiple layers of foils, three dismensional dose distributions are measurable. The foils used employ a new, relatively sensitive emulsion which is 6-8 µm thick and coated on a 0.1 mm thich polyester base. They require no processing; upon irradiation a very fine-grained, stable, blue image forms which is nearly linear over a absorbed dose range of 0.05 to 1.2 kGy. The readout is done with a scanning laser microdensitometer at a wavelength of 633 nm; a 100 µm diameter spot size is used which can be stepped in two dimensions in increments as small as 40 µm. The foils used for this study were calibrated using 90Sr/90Y beta particle spectra and were found to respond nearly identically to 60Co gamma rays. Results from such radiochromic foil measurements are compared with measurements with the same source using an extrapolation chamber. Pitfalls in the use of extrapolation chambers for these surface dose rate measurements are explored, including the effect of non-linear current relative to air gap functions on extrapolated slopes.

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