Absolute intensity of the 140.5 keV gamma-ray of 99Mo

The induced activity of the 99Mo isotope is mostly determined via the 140.5 keV γ-line, which is the strongest gamma-ray of its daughter sup(99m)Tc. Some recent literature, however, indicates a direct feeding of this energy level from the mother isotope as well. Considering the importance of this line in practice as well as the large controversy and scattering in relevant nuclear data available at present, a combined effort was made to remeasure this questionable absolute intensity. A relative method of irradiating a Mo-target with reactor neutrons and repeatedly measuring its (n,γ) induced activity relative to the 181.1 keV and 739.5 keV gamma lines of 99Mo as internal references was used. The weighted average of different runs yielded γ(99Mo, 140.5 keV)=(5.07+-0.37)%. As a consequence, when the 140.5 keV gamma line is used, the contribution from the 99Mo mother isotope should always be taken into account, e.g. in neutron cross-section measurements and neutron activation analysis. (author)