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109Cd is an electron-capture radionuclide used to calibrate gamma spectrometer in the low energy domain thanks to its 88 keV gamma-rays. Therefore, it is critical to produce standard solutions of 109Cd with a high accuracy. However, the specific decay scheme of 109Cd is a challenge for metrology. It indeed disintegrates by electron capture into 109mAg whose metastable state makes it impossible to implement 4πβ-γ coincidence or anti-coincidence techniques. One possibility is to count internal conversion electrons (CE) from the isomeric transition of 109mAg. Competing with the 88 keV gamma rays transition, these electrons are generated with an emission probability equal to 0.9634(5) [1]. The energies of the conversion electrons are of 62.5 keV for the K shell, and between 84.2 keV and 88 keV for the L, M, N shells, while Auger electrons and x-rays are all with energies below 25.5 keV. Moreover, the 88 keV gamma rays interact mainly by Compton scattering in the scintillating liquid , depositing a maximum energy of 22.5 keV. Assuming a 100% detection efficiency of these conversion electrons in a liquid scintillation (LS) counter, it is possible to separate conversion electrons (> 62.5 keV) from the other components (<25.5 keV) by analysing the energy distribution. This approach has originally been developed by the PTB in 2006 [2].
In the framework of the CCRI(II)-K2.Cd-109.2021 international key comparison , the BIPM has implemented this approach using the 3-photomultiplier tubes system of the ESIR, an international comparator of primary standards under development [3]. Much of the uncertainty comes from the overlap between the peak of the conversion electrons peak and lower energy peak due to the other decay products. The energy resolution of the LS system is therefore the most critical parameter to get an accurate measurement. The present study demonstrates the advantage of summing the 3 PMTs signal to enhance the energy resolution and then reduce the peak overlap. In addition, the energy distribution has been processed by a specific unfolding method to properly separate the spectral components. Thanks to the optimization introduced in this study, an activity estimation has been performed with a relative standard uncertainty of 0.5 %.
[1] Bé M-M et al. 2016 Table of Radionuclides. vol. 8
[2] Kossert K et al. 2006 Applied Radiation and Isotopes 64 1031–5
[3] Coulon R et al. 2022 Journal of Radioanalytical and Nuclear Chemistry
