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Dosimetric quantities for beta radiation are the personal dose equivalent at a depth of 0.07 mm Hp(0.07) and the personal dose equivalent in the eye lens at a depth of 3 mm Hp(3). To ensure their metrological traceability in the Slovak Republic, it is necessary to build a new national standard for beta radiation dosimetric quantities. In March 2023, the Slovak Institute of Metrology began its construction by providing a measuring bench containing three beta sources 147Pm, 85Kr and a 90Sr/90Y covering beta radiation mean energies from about 0.07 MeV to 0.8 MeV, these sources are traceable to the German National Metrology Institute, Physikalisch-Technische Bundesanstalt (PTB). For extension of the national standard by a primary method, a Böhm extrapolation chamber was used. By moving the electrode behind the entrance window and placing polyethylene terephthalat (PET) and polymethyl methacrylate (PMMA) absorbers of various thicknesses in front of the extrapolation chamber, the relationship between the measured ionization current and the distance between the entrance window and the electrode was analyzed. A linear extrapolation of this relationship to zero allows to determine the reference value of Hp(0.07).
For the correct determination of the reference value, it is necessary to know several correction factors of the extrapolation chamber. These are mainly corrections for backscatter, perturbation by the chamber wall, bremsstrahlung, air density, ionization losses due to recombination, and others (see ISO 6980 2:2023). For beta sources located inside the measuring bench at the reference distances according to ISO 6980 1:2023, these corrections were calculated and used in the calculation of the reference value of Hp(0:07). Measurements show good agreement with PTB reference values, except for 147Pm which will undergo further measurements using the SMU extrapolation chamber directly at PTB.
| Preferovaná sekcia | Metrológia, meranie a prístrojová technika |
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