Improved new beta primary standard (BPS2) and application of updated ISO 6980 in beta dosimetry

Beta radiation is used both in medicine (diagnosis and therapy) and in industry (for example in the control of material layer thicknesses and filling levels in manufacturing and in the dismantling of nuclear power plants). Compliance with dose limits is essential for the staff concerned. Therefore, PTB has been operating a primary standard measuring device for high‑precision measurements of the dose rate from beta radiation sources (beta primary standard or standard, BPS1) for more than 30 years.

In the past years, PTB has developed a new beta primary standard (BPS2), which is similar to the previous one (BPS1), the Böhm chamber, but is improved in some important respects. The new BPS2 has an absorber changer which allows an automatic measurement sequence of the entire calibration of a beta radiation source. Furthermore, the exact geometry of the measurement chamber can be controlled because the plane‑parallelism of its spatial dimensions can be now measured and adjusted. In addition to a gain in efficiency of operation, more detailed investigations of the applicability of correction factors can be carried out.

All correction factors required for such measurements are included in the basic ISO 6980 series of standards on beta reference radiation fields. This series of standards has been updated in recent years. Improved methods for determining correction factors have been introduced into the standard. The values resulting from this update were applied to BPS2 and BPS1. It was found that the results of both primary standards agreed on average to about ± 1 %. This deviation is within the statistical uncertainty (k = 1) of the measurements of about ± 3 % for ¹⁴⁷Pm and nearly ± 2 % for ⁸⁵Kr, ⁹⁰Sr/⁹⁰Y and ¹⁰⁶Ru/¹⁰⁶Rh radiation qualities, see box within Figure 1. This is a significant improvement compared to the application of the previous edition of ISO 6980: when using this edition, the deviations are also only up to 1.7 % on average, but individual extreme values occur up to almost 5 %, compared to a maximum 2.5 % when using the updated series of standards (see [1] for more details).

Since mid‑2023 PTB applies the updated ISO 6980 of 2022 and uses the new beta primary standard BPS2.

Figure 1: Overview of the ratio of the dose rate measured with BPS2 and BPS1 depending on the beta mean energy. To both BPS2, the correction factors from the revised ISO 6980 were applied. The uncertainty bars (k = 1) contain the uncorrelated contributions only.

Literature

[1]   R. Behrens: New Beta Primary Standard (BPS2) and revised ISO 6980 at PTB.
Metrologia 60 (2023) 045005, https://doi.org/10.1088/1681-7575/acd6fc

[2]   ISO 2023 Reference beta‑particle radiation – Part 1: Methods of production ISO 6980‑1: https://www.iso.org/standard/87213.html

[3]   ISO 2023 Reference beta‑particle radiation – Part 2: Calibration fundamentals related to basic quantities characterizing the radiation field ISO 6980‑2: https://www.iso.org/standard/87214.html

[4]   ISO 2023 Reference beta‑particle radiation – Part 3: Calibration of area and personal dosemeters and the determination of their response as a function of beta radiation energy and angle of incidence ISO 6980‑3: https://www.iso.org/standard/87215.html

Further information on the beta primary standards BPS1 and BPS2

Contact

Rolf Behrens, Department 6.3, Working Group 6.34