Concurrent spectrometry of annihilation radiation and characteristic gamma-rays for activity assessment of selected positron emitters

Abstract A method is described to determine the activity of non-pure positron emitters in a radionuclide production environment by assessing the 511 keV annihilation radiation concurrently with selected γ -lines, using a single High-Purity Germanium (HPGe) detector. Liquid sources of 22 Na, 52 Fe, 52 m Mn, 61 Cu, 64 Cu, 65 Zn, 66 Ga, 68 Ga, 82 Rb, 88 Y, 89 Zr and 132 Cs were prepared specifically for this study. Acrylic absorbers surrounding the sources ensured that the emitted β + -particles could not escape and annihilate away from the source region. The absorber thickness was matched to the maximum β + energy for each radionuclide. The effect on the 511 keV detection efficiency by the non-homogeneous distribution of annihilation sites inside the source and absorber materials was investigated by means of Monte Carlo simulations. It was found that no self-absorption corrections other than those implicit to the detector calibration procedure needed to be applied. The medically important radionuclide, 64 Cu, is of particular interest as its strongest characteristic γ -ray has an intensity of less than 0.5%. In spite of the weakness of its emission intensity, the 1346 keV γ -line is shown to be suitable for quantifying the 64 Cu production yield after chemical separation from the target matrix has been performed.