The potential radio-immunotherapeutic α-emitter 227Th – part II: Absolute γ-ray emission intensities from the excited levels of 223Ra

Abstract At the time of publication, radiopharmaceuticals labelled with thorium-227 are in clinical trials in Europe for the treatment of various types of cancer. In part I of this two-part series the primary standardisation of an aqueous solution of 227 Th was reported. In part II, the activity derived from the recommended absolute γ-ray emission intensities have been compared to that from the primary standardisation techniques. This comparison showed a negative bias of 4% in the determined activity per unit mass with an 11% spread in the activities determined for the eight most intense γ-ray emissions ( I γ > 1%) from the 227 Th α decay. Using the standardised 227 Th, measurements of the characteristic γ-ray emissions from the 223 Ra excited states were made using a calibrated HPGe γ-ray spectrometer. This has enabled the absolute intensities of 70 γ ray emissions from the 227 Th α-decay to be experimentally determined. A significant improvement over the precision of the recommended normalisation scaling factor has been made, with a value of 12.470 (35) % determined. Typically, the precision of the intensities has been improved by an order of magnitude or greater than current recommended values. The correlation matrices for pairs of the most intense γ-ray emission intensities are presented.