As background significantly affects measurement accuracy and a detection limit in determination of the trace amountsof uranium, it is necessary to minimize the impurities in the filaments used for thermal ionization mass spectrometry (TIMS). Wehave varied the degassing condition such as the heating currents and duration times to reduce the backgrounds from the filamentsprepared with zone-refined rhenium tape. The most efficient degassing condition of the heating current and the duration time wasdetermined as 3.5 A and 60 min, respectively. The TIMS measurement combined with the isotope dilution mass spectrometry(IDMS) technique showed that the uranium backgrounds were determined to be in a few fg level from blank rhenium filaments. The background minimized filaments were utilized to measure the uranium isotope ratios of a U030 (NIST) standard sample. The excellent agreement of the measurement with the certified isotope ratios showed that the degassing procedure optimized inthis study efficiently reduced the impurity signals of uranium from blank rhenium filaments to a negligible level.
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