Primary standardization and γ-ray emission intensity determination ^166mHo at NRC

Holmium-166m (^166mHo), with its long half-life, is valued in radionuclide metrology for its numerous well-distributed γ-ray emissions, and is suited for detector calibration and ionization chamber monitoring. The nuclear data recommended by the Decay Data Evaluation Project (DDEP) date from 1999, and growing evidence from recent absolute measurements point to inconsistencies in the emission intensity of the 184.4 keV normalization line, which underpins the entire decay scheme normalization. To address this, the ^166mHo activity concentration was established by primary liquid scintillation counting using the triple-to-double coincidence ratio method, achieving a relative standard uncertainty of 0.38%. Measurements of γ-ray emission intensities were derived for 45 transitions, corresponding to 48 transitions and including three unresolved doublets, spanning 80-1430 keV through absolute and relative determinations from the same spectral data. The relative determination used four reference lines distributed across the decay scheme, avoiding sole reliance on the 184.4 keV transition. Both determination methods are in excellent mutual agreement. Significant discrepancies with the DDEP are observed for the 119.0 keV and 121.2 keV transitions, located in a region where a structured Compton continuum complicates local background estimation. For the 184.4 keV line, consolidating all available absolute measurements through the power-moderated mean method yields a suggested emission intensity of 71.8(3)%, 0.7% below the DDEP value of 72.5(3)%. The impact on secondary standardization was assessed through Monte Carlo simulations of the Vinten 671 ionization chamber calibration coefficient, showing non-significant effects on activity determinations.