Determination of the absolute intensity of the 1205 keV γ-ray emission from ⁹¹Y

A radiochemically pure solution of ⁹¹Y was produced by the thermal neutron fission of ²³⁵U followed by successive chemical separations to remove fission product impurities. The gamma emission rate of the ⁹¹Y 1205 keV gamma was measured using multiple high purity germanium gamma spectrometers previously calibrated for counting efficiency using a certificated mixed nuclide gamma standard. The activity concentration of the ⁹¹Y was subsequently standardised by liquid scintillation counting. From the combination the activity concentration and gamma emission intensity, the absolute intensity of the 1205 keV gamma emission was derived as 0.2297(39)%. This data agrees within the quoted uncertainties with the absolute intensity of 0.26(4)% published in nuclear data sheets A=91 (Baglin, 2013), but reduces the uncertainty by an order of magnitude.