Speaker
Description
Technetium-99m, the short-lived metastable nuclear isomer of Tc-99 is one of the most relevant medical radionuclides world-wide, due to its extensive use in medical imaging and functional studies of various organs. Nevertheless, from a metrological point of view, the absolute standardization of Tc-99m is far from straightforward due to its short half-life and its low energy conversion electron spectrum. Recently, our working group at PTB successfully realized an absolute activity standardization of a Tc-99m solution by means of digital coincidence counting in the frame of a key comparison involving the International Reference System’s Transfer Instrument (SIR-TI) organized by BIPM. The solution was obtained from a commercial Mo-99/Tc-99m generator and standardized by detecting the 140.5 keV gamma radiation from the decay of Tc-99m in coincidence with the preceding emission of conversion electrons with energies around 2 keV. In order to maximize the detection efficiency for these low energy electrons, the measurements were performed using a custom-built liquid scintillation counter equipped with a CeBr3 inorganic scintillator as dedicated gamma detector. Furthermore, the light yield of the scintillation cocktail was optimized by using a 13:2 mixture of Ultima Gold F und Ultima Gold LLT scintillators. By simply changing the ratio of the two scintillators, or by addition of small amounts of a chemical quenching agent, detection efficiency for conversion electrons could be varied. The activity of the solution was obtained by the efficiency extrapolation technique. The overall uncertainty was less than 0.8% (k = 1). To validate the results of this novel approach, an independent standardization of the same Tc-99m solution was also carried out using a proportional counter based conventional coincidence setup. The samples were prepared by drop deposition on thin polyvinyl chloroacetate (VYNS) films. In this case, the efficiency variation was achieved by adding droplets of a colloidal silica solution (LUDOX) on the top of the samples. The obtained activity was consistent with the one derived by the liquid scintillation-based approach, although, its uncertainty was higher due to the lower overall detection efficiency for conversion electrons.
