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

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 22Na, 52Fe, 52mMn, 61Cu, 64Cu, 65Zn, 66Ga, 68Ga, 82Rb, 88Y, 89Zr and 132Cs 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, 64Cu, 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 64Cu production yield after chemical separation from the target matrix has been performed.