Monte Carlo calculation of Spencer-Attix and Bragg-Gray stopping-power ratios of tissue-to-air for ISO reference beta sources — an EGSnrc study

2020

Several correction factors for two new reference beta-particle radiation fields complementing ISO 6980 have been measured and calculated for both primary beta dosimetry and the operational quantities in radiation protection. The following correction factors for primary beta dosimetry for the determination of the absorbed dose to tissue, D t, have been determined by calculations: k ba for backscatter from the collecting electrode, k pe for perturbation by the chamber’s side walls, k ih for inhomogeneity inside the collecting volume, k Sta for the stopping power ratio at different phantom depths, and k SA for the use of the Spencer-Attix theory, while the following have been measured: k br for the effect of bremsstrahlung and k abs for variations in the attenuation and scattering of beta particles between the source and the collecting volume due to variations from reference conditions and for differences of the entrance window to a tissue-equivalent thickness of 0.07 mm. Furthermore, calculations were undertaken to determine the following correction factors to assess the operational quantities H p(0.07), H p(3), H′(0.07), and H′(3): k rod(0.07) for the rod instead of the slab phantom, k cyl(3) for the cylinder instead of the slab phantom, as well as k′(0.07) and k′(3) for the ICRU sphere instead of the slab phantom, while the correction factor for oblique radiation incidence has been measured. The newly determined correction factors were determined in the same way as those for several well-established beta-particle radiation fields described in two earlier publications by the author. Furthermore, they are ready to be implemented in an updated version of the ISO 6980 series and in the software of the Beta Secondary Standard BSS 2.

Section: Use Of the Spencer-attix Theory Via Correction Factor K Samentioning

confidence: 99%

Correction factors for two new reference beta radiation fields

2020

“…The parameters c 0 , c 1 and c 2 obtained from the regression are listed for several sources in table C5 in appendix C for BPS1. Figure 8 shows the data from the literature [23,24] and the corresponding polynomial regressions according equation (15). For a chamber depth of 1000 µm this results in values for k SA of 1.0036, 1.0045, 1.0057 and 1.0074 for 147 Pm at a distance of 20 cm, 85 Kr at a distance of 30 cm, 90 Sr/ 90 Y at a distance of 30 cm and 106 Ru/ 106 Rh also at a distance of 30 cm, all with a beam flattening filter, respectively.…”

Section: Use Of the Spencer-attix Theory Via Correction Factor K Samentioning

confidence: 99%

Correction factors for primary beta dosimetry

2020

Several correction factors for primary beta dosimetry in radiation protection have been determined by means of measurements and simulations for both the well-known Böhm primary standard extrapolation chamber developed at PTB (which has been commercially available for a long time) and a new primary standard extrapolation chamber also developed at PTB. The correction factors k el for electrostatic attraction of the entrance window due to the collecting voltage and k sat for ionisation collection losses due to ionic recombination have been measured. The following correction factors have been determined by simulations for normal (0°) as well as oblique (15°, 30°, 45° and 60°) radiation incidence: k ba for backscatter from the collecting electrode, k pe for perturbation by the chamber’s side walls, and k ra, k ac and k di for inhomogeneity inside the collecting volume. The latter three have been combined to k ih. Six more correction factors adopted from the literature are described to complete the necessary corrections. A comparison with values given in ISO 6980-2:2004 (for 0°), all determined at that time by means of measurements, is shown as well as the application of the new factors to measured data. The newly determined correction factors are ready to be implemented in an updated version of ISO 6980-2.

“…The Spencer-Attix cavity theory is considered to be more accurate than the Bragg-Gray one as it accounts for the variation in the response measured as a function of cavity dimension whereas the Bragg-Gray theory does not. Therefore, the Spencer-Attix stopping power ratios depending on the chamber depth have been calculated [43,44]. Corresponding correction factors were determined [33,34] and added in ISO 6980-2.…”

Section: Inclusion Of a Correction Factor For Primary Dosimetry To Us...mentioning

confidence: 99%

New beta primary standard (BPS2) and revised ISO 6980 at PTB

2023

Firstly, in this work, the main characteristics of PTB’s new beta primary standard (BPS2) are presented in comparison to its previous one (BPS1), i.e., to the Böhm chamber. Secondly, the main changes of the revised ISO standard series on beta reference radiation fields, ISO 6980, are described. Finally, the application of the revised ISO 6980 to both PTB’s new and old beta primary standard is implemented. It turned out that both devices, BPS2 and BPS1, agree with each other within theier one sigma statistical uncertainties, i.e., approximately 3 % for 147Pm and 1.5 % to 1.8 % for 85Kr, 90Sr/90Y and 106Ru/106Rh radiation qualities. Therefore, as of approximately mid 2023, PTB will change from the 2004 to 2006 version of ISO 6980 to the revised version from 2022 and from the old beta primary standard, BPS1 (i.e., the Böhm chamber) to the new one, BPS2.