| Summary: | The β<sup>+</sup>-emitting radionuclide <sup>86g</sup>Y (t<sub>1/2</sub> = 14.7 h) forms a matched-pair with the β<sup>−</sup>-emitting therapeutic radionuclide <sup>90</sup>Y (t<sub>1/2</sub> = 2.7 d) for theranostic application in medicine. This approach demands a precise knowledge of the positron emission probability of the PET nuclide which was until recently rather uncertain for <sup>86g</sup>Y. In this work, an <sup>86g</sup>Y source of high radionuclidic purity was prepared and a direct measurement of the positron emission intensity per 100 decay of the parent (hereafter “positron emission intensity”) was performed using high-resolution HPGe detector γ-ray spectroscopy. The electron capture intensity was also determined as an additional check by measuring the K<sub>α</sub> and K<sub>β</sub> X-rays of energies 14.1 and 15.8 keV, respectively, using a low energy HPGe detector. From those measurements, normalized values of 27.2 ± 2.0% for β<sup>+</sup>-emission and 72.8 ± 2.0% for <i>EC</i> were obtained. These results are in excellent agreement with values recently reported in the literature based on a detailed decay scheme study.
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