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An Alternative Approach to Determining the Reference Air-Kerma Rate From Extrapolation Chamber Measurements

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T Schneider

T Schneider*, Physikalisch-Technische Bundesanstalt, Braunschweig, Niedersachsen


SU-F-T-64 (Sunday, July 31, 2016) 3:00 PM - 6:00 PM Room: Exhibit Hall

Since 2008 the Physikalisch-Technische Bundesanstalt (PTB) has been offering the calibration of ¹²⁵I-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 x₀.

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.

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(x₁, x₂) is applied to the difference of charges measured at the two plate separations x₁ and x₂. 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 x₁ and x₂, 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.

The alternative method shall be established in the future.

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