Zhipu Xuebao (Jan 2023)
Modified Total Evaporation Method for Highly Precise Measurement of Uranium Isotopes Ratio
Abstract
Thermal ionization mass spectrometry (TIMS) is a well-established instrumental technique for accurately analyzing isotopes of elements with low first ionization potential. However, the measurement accuracy is affected due to the existence of the mass fractionation effect. Generally, applying the external standard method or total evaporation method is the most common way to correct the mass fractionation effect. Total evaporation (TE） technology is the most conventional method for uranium major isotope ratio measurement (235U/238U) using TIMS. However, due to the strong peak tailing of the major isotopes and other factors, the measurement precision and accuracy of minor isotope ratios (234U/238U and 236U/238U) are relatively low. In this study, the modified total evaporation (MTE) method for highly precise measurement of uranium isotopes ratios by TIMS was developed based on the research of dynamic heating procedures, dynamic and static receiving procedures, peak tailing correction, efficiency calibration of collectors and mass discrimination calibration. The accuracy and reliability of the MTE method were verified with different reference materials. The experiment results showed that the yield of uranium oxide should be monitored and the related parameters should be precisely controlled to reduce the yield of uranium oxidation during measurement. This action could guarantee the effectiveness of the MTE measurement and improve the uranium samples utilization. The initial strength of 187Re+ played an important role in the ionization efficiency of uranium and the yield of uranium oxide. The optimal signal of 187Re+ should be set in the range of 0.17-0.22 V. When uranium samples with 2% 235U isotopic abundance were measured by the MTE method using TIMS, the measurement precisions of 235U/238U, 234U/238U and 236U/238U were 0.024%, 0.06%, and 0.19%, and the bias between the measured value and reference value were 0.025%, 0.19%, and 0.38%, respectively. Compared to the TE method, the MTE method showed the same precision for the 235U/238U ratio measurement. As for the MTE measurement of 234U/238U and 236U/238U ratios, the major uranium isotopes (235U and 238U) were used as an internal standard to overcome the mass fractionation effect. When the tailing effects were eliminated, the measurement precisions of 234U/238U and 236U/238U ratios were improved by 5 to 10 times. As for sample amount, the TE method generally requires the uranium amount of less than 1 μg, while 5 μg is required for the MTE method.
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