Measurement of Trace Uranium Isotope Ratio Using Vacuum Carburized Rhenium Filament

Low ionization efficiency is one of the main constraints for accurate measurement of trace uranium isotope ratio by thermal ionization mass spectrometry (TIMS). Vacuum carburized rhenium filament is made under low pressure organic vapor and high temperature conditions. This filament can enhance the ionization efficiency of uranium so as to improve the ability of measuring trace uranium. There are three main factors affecting vacuum carburization: carburizing temperature, carburizing duration and vapor pressure. A simple carburization device was prepared, which was reformed on the basis of degassing device. The carburizing temperature and carburizing pressure could be controled by controlling the heating current of rhenium filament. This device was used to make carburized rhenium filaments under different conditions. The ionization efficiency and isotope ratio of trace uranium samples were measured by total evaporation method. The effects of carburizing conditions on the ionization efficiency and mass discrimination coefficient were compared. The results showed that the optimum carburizing conditions were as follows: benzene vapor pressure was 0.1 Pa, carburizing current was 2.5 A, and carburizing duration was 1 h. With carburized filament, the ionization efficiency of 10 pg uranium samples could reach 0.25%, which was about 5 times higher than that of double-filament method. For 10 pg samples, the external accuracy of ²³⁵U/²³⁸U was less than 1%, and that of ²³⁴U/²³⁸U and ²³⁶U/²³⁸U was less than 5%. The carburized filament could also measure 1-2 pg uranium samples. For 1 pg CRM020A and 2 pg IRMM184 samples, the external accuracy of ²³⁵U/²³⁸U measurement was less than 1%, and that of ²³⁴U/²³⁸U was less than 5%. In summary, carburized rhenium filament performed well in the measurement of trace uranium. It is worthwhile to make further study of its application in the measurement of environmental samples, uranium particles, and so on.