Primary standardization, half-life determination, and γ-ray emission probability of ²³³Pa at NRC

Protactinium-233 (²³³Pa) is an isotope of interest in nuclear forensics as a radiotracer for dating the ²³⁵U/²³¹Pa radiochronometric pair. While its half-life is known within 0.08%, γ-ray emission probabilities have high relative uncertainties, on average greater than 1 %, and in particular the 300.129 keV emission has a relative uncertainty of 3.2% (k=1). This study reevaluated emission probabilities for energies above 100 keV and reassessed the half-life of ²³³Pa. A sample of reactor-produced protactinium chloride (Naperstkow, 2018. doi.org/10.1007/s10967-018-6068-x) was standardized using the TDCR primary standard method, with efficiencies calculated via the MICELLE2 code (Kossert, 2010. doi.org/10.1016/j.apradiso.2009.11.048). The half-life of ²³³Pa was determined to be 26.95(3) days, within the range of the accepted value of 26.98(2) days. Gamma-ray emission probabilities were evaluated using a calibrated high-purity germanium detector, resulting in an average relative uncertainty of 1.3%. A secondary analysis method based on efficiency ratios reduced the uncertainty of the 300.129 keV emission probability. This approach is independent of source activities, allowing determination of the emission probability at the sub-percent level. The combined methods determined the 300.129(5) keV γ-ray emission probability of ²³³Pa to be 6.510(12) %, consistent with accepted values.