Accurate activity measurement of Lu-177 by the liquid scintillation 4πβ–γ coincidence counting technique

This paper reports on absolute measurements made at the NMISA by the 4π[LS]β–γ coincidence extrapolation technique, which formed part of a key comparison of activity measurements of the radionuclide lutetium-177, a beta–gamma emitter. A detection efficiency analysis based on the decay scheme indicated that the coincidence extrapolation method is feasible for extracting the source disintegration rate of 177Lu. A simulation was undertaken utilizing the efficiency equations to give an indication of the expected variation of the source count rate with efficiency for different gamma-ray windows. Both the simulation and experiment indicated that the window giving the highest counting efficiency was centered about the 208.4 keV photopeak, with the data showing linear variation in the upper efficiency range. The extracted activity concentration of the 177Lu solution proved to be highly accurate, being higher than the proposed comparison reference value by 0.15%, well within one standard uncertainty (σ=0.26%) as specified by the NMISA. - Highlights: ► Activity of lutetium-177 was determined by liquid scintillation coincidence counting. ► The measurements formed part of a key comparison of absolute activity measurements. ► A source rate simulation utilizing detection efficiency equations was undertaken. ► The simulation indicated the expected variation of rate with counting efficiency. ► The extracted activity concentration of the solution proved to be highly accurate.