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Simulation of the response of an ionization chamber to 214Bi emission. Application to the measurement of 222Rn

Sylvie Pierre 1 Cheick Thiam 1 Philippe Cassette 1 Xavier Mougeot 1 Abhilasha Singh 1
1 LNHB - Laboratoire National Henri Becquerel
DM2I - Département Métrologie Instrumentation & Information : DRT/LIST/DM2I
Abstract : Well-type re-entrant ionization chambers (IC) are used in radionuclide metrology for secondary measurements of radioactive sources. Generally this technique is used for solutions of gamma-emitting radionuclides, but was also extended to high-energy beta-emitting radionuclides like 90Y. In this case, the energy transfer to the gas is made through Bremsstrahlung photons. The measurement of gaseous 222Rn sources with an IC was also undertaken in the framework of the Système International de Référence (SIR) of the Bureau International des Poids et Mesures (BIPM). In this case, we observed an important influence of the thickness of the walls of the glass container of the source. We analysed this effect as the consequence of the direct interaction of high-energy electrons from 214Bi (maximum beta energy of 3.27 MeV) with the counting gas. As the efficiency of the direct ionization of a gas by electrons is several orders of magnitude more important than ionization by Bremsstrahlung, and as energy absorption in the walls of the ampoule is much more sensitive to the thickness of the walls than absorption of the Bremsstrahlung, this could explain the observed phenomenon. It is worth noticing that direct interaction of electrons with the IC gas is never expected when measuring solutions, and this phenomenon was to our knowledge, never addressed in the past. In order to analytically study this phenomenon, we undertook a Monte Carlo simulation of a Vinten 671 IC, with a radon source in a glass vial, where the solid daughters in secular equilibrium with the radon are adsorbed on the internal walls of the gas ampoule. The influence of the glass wall thickness on the response of the IC was studied vs. the wall thickness using the PENELOPE 2016 package and allowed us to determine the acceptable variability of the thickness of the ampoules. Similar simulations were also undertaken with metal gas containers, showing that they could solve measurement problems of 222Rn in the SIR system.
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Submitted on : Thursday, February 13, 2020 - 9:56:38 AM
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Sylvie Pierre, Cheick Thiam, Philippe Cassette, Xavier Mougeot, Abhilasha Singh. Simulation of the response of an ionization chamber to 214Bi emission. Application to the measurement of 222Rn. 22nd International Conference on Radionuclide Metrology ICRM'19, May 2019, Salamanca, Spain. pp.27 - 31, 2019. ⟨cea-02476974⟩

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