Evaluating the intensity of the 'prompt' 140.5 keV γ-ray of ⁹⁹Mo using a 4παβ(LS)-γ(HPGe) measurement system

The absolute intensity for the 'prompt' 140.5 keV gamma-ray of ⁹⁹Mo was evaluated using the β-γ coincidence technique. A liquid sample of ⁹⁹Mo was prepared from a⁹⁹Mo/^99mTc generator and measured in a 4παβ(LS)-γ(HPGe) system that comprises a Liquid Scintillator (LS) detector and a High-Purity Germanium (HPGe) detector. The sample was introduced into scintillation fluid embedded in a photo-reflector assembly that provides almost 100% efficiency for detecting β particles (in the energy range of intreset). The combination of the HPGe and the LS detectors provided a highly effective rejection mechanism for non-coincident events. Thereby, the distinction between the detected 140.5 keV events originating from decays of ^99mTc (IT) and those from transitions bypassing the metastable state could be obtained and the 'prompt' intensity was evaluated directly. The system was calibrated for detecting β particles and γ-rays using radioactive sources of known activities and having identical geometry as the sample containing ⁹⁹Mo. The absolute intensity of the 'prompt' 140.5 keV was found to be (5.21 ± 0.02_stat±0.16_sys)%, in good agreement with results from more recently reported works.