166mHo: a multi-γ standard for the calibration of Ge spectrometers

Efficiency calibration curves for germanium detectors are usually established by using a large number of standard γ-ray sources of specific radionuclides, that decay with few γ-rays, or radionuclides with complex decay schemes, such as 133Ba or 152Eu. But these multi-γ radionuclides cannot be used alone, because the γ-rays of high intensity are irregularly distributed across the energy spectrum. 166mHo is a more suitable single source for such calibrations, because this radionuclide decays by β--emission to the excited states of 166Er with the subsequent emission of about 20 strong and well distributed γ-rays ever the energy range 50-1000 keV. Moreover, with a relatively long half-life (1200 yr) and characteristic X-rays between 40 and 50 keV, this nuclide is a good standard for the calibration of germanium detectors. The X- and γ-ray emission probabilities of their main lines need to be known with good accuracy in order to resolve the discrepancies found in the literature. A combination of 4πβ-γ coincidence and X- and γ-ray spectrometric techniques have been used to determine 50 photon emission probabilities