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Monte Carlo modeling of the response of NRC's Sr 90 ∕ Y 90 primary beta standard
Author(s) -
Selvam T. Palani,
Saull P. R. B.,
Rogers D. W. O.
Publication year - 2005
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 - 10.1118/1.1997347
Subject(s) - extrapolation , ionization chamber , monte carlo method , dosimetry , physics , beta (programming language) , materials science , absorbed dose , radiation , computational physics , nuclear medicine , optics , nuclear physics , ionization , mathematics , statistics , ion , computer science , medicine , quantum mechanics , programming language
The BEAMnrc/EGSnrc Monte Carlo code system is employed to develop a model of the National Research Council of Canada primary standard of absorbed dose to tissue in a beta radiation field, comprising an extrapolation chamber andSr90 ∕ Y90beta source. We benchmark the model against the measured response of the chamber in terms of absorbed dose to air, for three different experimental setups when irradiated by theSr90 ∕ Y90source. For the first setup, the chamber cavity depth is fixed at 0.2 cm and the source‐to‐chamber distance varied between 11 and 60 cm . In the other two cases, the source‐to‐chamber distance is fixed at 30 cm . In one case the response for different chamber depths is studied, while in the other case the chamber depth is fixed at 0.2 cm as different thicknesses of Mylar™ are added to the front surface of the extrapolation chamber. The agreement as a function of distance between the calculated and measured responses is within 0.37% for a variation in response of a factor of 29. In the case of dose versus chamber depth, the agreement is within 0.4% for the ISO‐recommended nominal depths of 0.025 – 0.25 cm . Agreement between calculated and measured responses is very good (between 0.02% and 0.2%) for added Mylar foils of thicknesses up to 10.8 mgcm − 2. For larger Mylar thicknesses, deviations of 0.6%–1.2% are observed, which are possibly due to the systematic uncertainties associated with the restricted collisional stopping powers of air or Mylar used in the calculations. We conclude that our simulation model represents the extrapolation chamber andSr90 ∕ Y90source with adequate accuracy to calculate correction factors for accurate realization of dose rate to tissue at a depth of 7 mgcm − 2in an ICRU tissue phantom, despite the fact that the uncertainties in the physical characteristics of the source leave some uncertainty in certain calculated quantities.