Standardization of 109Cd Using CIEMAT/NIST Method and Internal Conversion Electron Counting

Cd-109 is a very important radionuclide and frequently used for calibration of gamma ray spectrometers in low energy region due to its emission of 88 keV photons and long half-life of 461.9 +/- 0.4 days. Due to complex decay scheme standardization of Cd-109 is very challenging and was selected for international intercomparison by Bureau International des Poids et Mesures (BIPM) in 2021.Cd-109 decays via electron capture to the excited state of Ag-109m which decays mainly with emission of conversion electrons with half-life of 39.6 s. Due to this standardization of Cd-109 using conventional 4 pi beta-gamma coincidence method is extremely difficult. Some of the methods for standardization of Cd-109 were measurement of conversion electron emission rate using liquid scintillation spectrometer and CIEMAT/NIST method. BARC is the designated institute (DI) for ionizing radiation of India. In BARC, RSS, RSSD has been entrusted with the responsibilities related to activities of DI for ionizing radiation and hence develops, establishes, maintains and upgrades the primary and secondary standards for ionizing radiation quantities to provide traceability to the ionizing radiation measurements in the country. To establish the degrees of equivalence with the NMIs/DIs of other countries, BARC participated in international intercomparison of activity measurements of Cd-109 (CCRI(II)-K2.Cd-109 key comparison) piloted by BIPM with support of LNE-LNHB, France. The standardization was carried out by CIEMAT/NIST efficiency tracing using H-3 as a tracer and 4 pi liquid scintillation conversion electron measurements. The activity concentration obtained by CIEMAT/ NIST efficiency tracing has the deviation of 0.59% with respect to the value obtained using 4 pi liquid scintillation conversion electron measurements which is within uncertainty limits.