Measurements of Field Size and Angular Dependence of Absorbed Dose Rate in Beta Particle Reference Radiation Fields with TLDs and Radiochromic Film

C.G. Soares¹; F. Ingellis²; R. Ward²; and V. Pagonis² (¹National Institute of Standards and Technology, Gaithersburg, MD; ²McDaniel College, Westminster, Maryland)

The International Organization for Standardization (ISO) specifies a series of beta-particle reference radiation fields in ISO 6980, Parts 1-3. The field sizes are specified in these documents, as well as conversion factors for use at non-perpendicular angles of incidence. The primary standardization of the dosimetry of these beams is performed with extrapolation chambers, which require corrections for non-uniform radiation fields. These corrections have up until now only been specified at normal incidence. Traditional methods for determining field sizes and conversion factors involve the use of extrapolation chambers, thermoluminescence dosimeters (TLDs), and silver halide photographic film. In this work, a novel, high-sensitivity radiochromic film is used to measure field sizes at a number of different angles for the ⁹⁰Sr/Y and ⁸⁵Kr reference radiation fields. These measurements are supplemented by TLD measurements using 2 mm diameter by 0.5 mm thick LiF:Mg,Ti chips placed at 5 mm intervals on the x- and y-axes on the surface of a slab phantom. The measurements indicate that the unfiltered ⁹⁰Sr/Y fields at 11 cm, 20 cm and 30 cm are asymmetric, even at normal incidence, possibly due to the geometry of the collimator system or the source itself, and thus the position of the distance and centering device of the secondary standard irradiator used does not predict the resulting radiation field center, being off center by about 1 cm along the horizontal axis. The film and TLD measurements also indicate that significant field size corrections for the primary standard extrapolation chamber are necessary for off normal incidence calibration measurements. Field size corrections for use with the standard extrapolation chamber obtained from these film and TLD measurements are given, and comparison of the measurement results with the predictions of simple inverse square dependence is also presented.

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