Application of a continuous heating method using thermal ionization mass spectrometry to measure isotope ratios of plutonium and uranium in trace amounts of uranium–plutonium mixture sample

Abstract

A method for measuring isotope ratios of plutonium and uranium in plutonium or uranium–plutonium (Pu/U) mixture samples was developed by using a continuous heating method of thermal ionization mass spectrometry, which was previously shown to be effective in measuring isotope ratios of trace uranium. In this method, temperature of an evaporation filament is increased sequentially. This leads to measure the ratios of Pu and U at different temperatures in which each element is evaporated efficiently. Under optimal measurement conditions, the intensity of 239Pu and the ratio of 240Pu/ 239Pu were measured down to sample amounts of 0.1 fg and 0.3 fg, respectively. In Pu/U mixture samples with pg-level masses and 0.01–10 Pu/U ratios, isotope ratios of 240Pu/ 239Pu and 235U/ 238U were measured simultaneously without chemical separation of samples. The relative standard deviation of the isotope ratios of 240Pu/ 239Pu and 235U/ 238U in the sample containing each 1 pg of plutonium and uranium were better than 2% and 4%, respectively. Moreover, 238Pu/ 239Pu ratios were obtained by using a correction in which the 238U intensity was estimated from the 235U intensity and the 235U/ 238U ratio. The results in this study suggest that the continuous heating method of thermal ionization mass spectrometry will be effective tool as a method for measuring isotope ratios in the Pu/U mixture samples.