Experimental determination of 32
P dose backscatter factors at and near soft-tissue boundaries
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Beta-ray dose backscatter factors with respect to soft tissue were measured using an extrapolation chamber. The beta-ray dose backscatter factor is a measure of the change effected in absorbed dose to a soft-tissue medium when part of the medium is replaced by a material other than soft tissue (i.e., a scatterer); the source is located at the boundary between the two media. The dependencies of backscatter factor on scatterer atomic number and on source geometry are investigated, and the variation of backscatter factor with distance from the boundary is determined. For a 32P point source, backscatter factors with respect to Mylar, a soft-tissue substitute, at 0.55 mg/cm2 from the boundary, are, 29.65[0.12]%, 31.07[0.24]%, 19.30[0.48]%, 16.27[0.35]%, 5.46[0.11]% and -26.44[0.02]% for bismuth, tungsten, cadmium, copper, aluminum, and air scatterers, respectively. Backscatter factors measured for a 32P planar source are generally smaller than those for a point source. The variation of backscatter factor with distance from the boundary is well represented analytically by sums of exponentials. Therefore, the rate of decrease of backscatter factor with distance can be specified by a relaxation length, defined as the depth through which the backscatter factor is reduced by 1/e, where e is the base of the natural logarithm. For example, with a 32P planar source, relaxation lengths in Mylar are 588 mg/cm2 and 238mg/cm2 for bismuth and aluminum scatterers, respectively. Qualitative interpretation of backscatter factor depth profiles is presented. In addition, the variations of backscatter factor with scatterer atomic number and with source geometry are discussed with reference to existing experimental findings on beta particle reflection.
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