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Experimental determination of 32 P dose backscatter factors at and near soft‐tissue boundaries
Author(s) -
Nunes Josane,
Prestwich William V.,
Kwok Cheuk S.
Publication year - 1993
Publication title -
medical physics
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.473
H-Index - 180
eISSN - 2473-4209
pISSN - 0094-2405
DOI - 10.1118/1.597089
Subject(s) - backscatter (email) , dosimetry , medical imaging , nuclear medicine , physics , optics , materials science , medicine , radiology , computer science , telecommunications , wireless
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 32 P point source, backscatter factors with respect to Mylar, a soft‐tissue substitute, at 0.55 mg/cm 2 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 32 P 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 32 P planar source, relaxation lengths in Mylar are 588[7] mg/cm 2 and 238[2]mg/cm 2 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.