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SU‐F‐T‐64: An Alternative Approach to Determining the Reference Air‐Kerma Rate from Extrapolation Chamber Measurements
Author(s) -
Schneider T
Publication year - 2016
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 - 10.1118/1.4956199
Subject(s) - kerma , extrapolation , ionization chamber , calibration , ionization , physics , dosimetry , computational physics , percentage depth dose curve , range (aeronautics) , primary standard , atomic physics , nuclear physics , optics , materials science , nuclear medicine , mathematics , ion , statistics , medicine , composite material , quantum mechanics
Purpose: Since 2008 the Physikalisch‐Technische Bundesanstalt (PTB) has been offering the calibration of 125 I‐brachytherapy sources in terms of the reference air‐kerma rate (RAKR). The primary standard is a large air‐filled parallel‐plate extrapolation chamber. The measurement principle is based on the fact that the air‐kerma rate is proportional to the increment of ionization per increment of chamber volume at chamber depths greater than the range of secondary electrons originating from the electrode x 0 . Methods: Two methods for deriving the RAKR from the measured ionization charges are: (1) to determine the RAKR from the slope of the linear fit to the so‐called 'extrapolation curve’, the measured ionization charges Q vs. plate separations x or (2) to differentiate Q(x) and to derive the RAKR by a linear extrapolation towards zero plate separation. For both methods, correcting the measured data for all known influencing effects before the evaluation method is applied is a precondition. However, the discrepancy of their results is larger than the uncertainty given for the determination of the RAKR with both methods. Results: A new approach to derive the RAKR from the measurements is investigated as an alternative. The method was developed from the ground up, based on radiation transport theory. A conversion factor C(x 1 , x 2 ) is applied to the difference of charges measured at the two plate separations x 1 and x 2 . This factor is composed of quotients of three air‐kerma values calculated for different plate separations in the chamber: the air kerma Ka(0) for plate separation zero, and the mean air kermas at the plate separations x 1 and x 2 , respectively. The RAKR determined with method (1) yields 4.877 µGy/h, and with method (2) 4.596 µGy/h. The application of the alternative approach results in 4.810 µGy/h. Conclusion: The alternative method shall be established in the future.