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Title: Sci—Fri AM: Mountain — 03: Current status of the NRC primary standard for {sup 192}Ir HDR brachytherapy sources

The Canadian primary standard for {sup 192}Ir HDR brachytherapy sources has been recently revised in a more accurate manner allowing for more realistic uncertainty estimation. Air-kerma strength S{sub k} is derived from measurements of the source's output using a graphite-walled spherical ionization chamber (2S) at several distances. Traceability to NRC primary standards for the {sup 192}Ir calibration coefficient N{sub k} is insured by estimating it as the inverse arithmetic mean of the inverse of the calibration coefficients for a {sup 137}Cs beam and the medium energy x-ray beam quality N250, both of which are traceable to NRC primary standards. The multiple-distance method is combined with a non-linear least squares fit to determine St, while at the same time removing the effects of room scatter and position offset. The previously used shadow-cone method for directly measuring the room scatter is found to be inadequate due to the increased scatter contribution from the lead cone itself, especially at short source-detector distances. Rather than including the reported 1% difference in source strength for {sup 192}Ir HDR sources of different construction into the total uncertainty, users are cautioned that the calibration coefficient provided by NRC is only valid for a microSelectron V2 model. Amore » comprehensive uncertainty budged shows that the total one sigma uncertainty of the standard is actually 0.6% rather than the previously assumed 1.2%. NRC measured S{sub k} agrees within 0.03% of the manufacturer's value.« less
Authors:
;  [1]
  1. National Research Council of Canada (Canada)
Publication Date:
OSTI Identifier:
22407700
Resource Type:
Journal Article
Resource Relation:
Journal Name: Medical Physics; Journal Volume: 41; Journal Issue: 8; Other Information: (c) 2014 American Association of Physicists in Medicine; Country of input: International Atomic Energy Agency (IAEA)
Country of Publication:
United States
Language:
English
Subject:
07 ISOTOPES AND RADIATION SOURCES; BRACHYTHERAPY; CALIBRATION STANDARDS; CANADA; CESIUM 137; DOSIMETRY; IONIZATION CHAMBERS; IRIDIUM 192; KERMA; SPHERICAL CONFIGURATION