Experimental determination of the²³³U half-life

Abstract

The half-life of 233U was determined by a specific activity method, in the frame of an international collaboration project of four metrology institutes. A purified, highly enriched 233U solution was prepared with certified concentration of uranium isotopes. The isotopic composition of the uranium material was determined by thermal ionization mass spectrometry. The ingrowth of daughter nuclides was calculated and confirmed by α-particle and γ-ray spectrometry measurements. Quantitative sources were prepared from aliquots of the 233U solution and their disintegration rate was determined by means of α-particle counting at a defined small solid angle, liquid-scintillation counting and 4π counting in a pressurized proportional counter and in a CsI(Tl) sandwich spectrometer. A half-life value of 1.5867 (14) × 105 a was obtained, in which the relative standard uncertainty amounts to 0.09%. The result is lower than the currently recommended value of 1.592 (2) × 105 a and a new best estimate of T1/2(233U) = 1.5903 (13) × 105 a was calculated from a partially weighted mean of a set of published data.

Methods for primary standardization of activity

“…Alpha-particle counting at a defined low solid angle is amongst the most accurate primary standardization methods for alpha-emitting radionuclides, with attainable accuracies around 0.1% [88][89][90][91]. It has been applied in various standardization exercises [17,92,93], but also in the frame of half-life determinations of long-lived radionuclides [94][95][96][97][98][99][100]. A special application is the standardization of radon gas that is condensed on a cold circular disc [56,101,102].…”