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This paper presents the results obtained with two primary techniques (4pibeta-gamma and TDCR methods) in the case of the activity standardization of two ߯ emitters (60Co and 106Ru/106Rh). In particular, the impact of the calculated ß-spectra implemented for the activity calculation with the TDCR statistical model is addressed.
Cobalt-60 (T½ ~ 5.2711 (8) a) disintegrates by ߯ emission to excited levels of 60Ni followed by two coincident ß-transitions (1173 keV and 1332 keV). Ruthenium-106 disintegrates through ߯ emission with max. beta energies of 39.4 keV (T½(106Ru) ~ 371.5 (21) d). The daughter radionuclide 106Rh also disintegrates by ߯ emission with maximum ß-energies ranging from 144 keV to 3.5 MeV (T½(106Rh) ~ 30.1 (3) s) to the ground state (~78.8%) and to 36 excited levels of 106Pd. The main gamma-photon emissions are: ~ 512 keV (~ 20.5%), ~ 622 keV (~ 9.9%) and ~ 1050 keV (~ 1.5%).
For both radionuclides, the TDCR method was applied using a stochastic approach based on the Geant4 simulation code for the calculation of energy deposition in the liquid scintillation vial. As already investigated in [1], the assessment of activity is sensitive to the calculated ß-spectra introduced in the statistical modelling and more accurate results are obtained when atomic screening and exchange effects [2] are considered in the spectrum computation.
The TDCR results were compared to those obtained with the 4pibeta-gamma coincidence method. The measurements of 60Co were carried out by means of a 4pibeta(PC)-gamma coincidence system using a proportional counter (CH4 at atmospheric pressure) in the beta-channel. In the case of 106Ru/106Rh, the measurements were performed using a 4pibeta(LS)gamma coincidence detection system equipped with a 3-PMTs TDCR set-up in the beta-channel and by setting a detection threshold to avoid counting from 106Ru decay emission. The TDCR method was carried out by measuring both 106Ru and 106Rh decay emission using the same 3-PMTs apparatus applied in the LS beta-channel.
For both radionuclides, a better agreement is obtained when atomic screening and exchange effects are considered in the computation of ß-spectra used in the TDCR modelling. Without those corrections, the deviation between both primary methods is about 0.2% for 60Co and about 0.4% for 106Ru/106Rh. These results confirm those describes in [1] in the case of 60Co.
1. Kossert K. et al., Activity determination of 60Co and the importance of its beta spectrum, ARI 134, 212 (2018).
2. Mougeot X., Bisch C., Consistent calculation of the screening and exchange effects in allowed ß- transitions. Physical Review A 90, 012501 (2014).
