真空渗碳铼带用于痕量铀同位素比的测量

Measurement of Trace Uranium Isotope Ratio Using Vacuum Carburized Rhenium Filament

离子化效率低是制约热电离质谱(TIMS)准确测量痕量铀同位素比的主要因素之一。对铼带进行渗碳处理能够增强铀的离子化效率,从而提高TIMS测量痕量铀的能力。本研究采用简易的渗碳装置,在不同条件下对铼带进行渗碳处理。采用全蒸发法测量痕量铀样品的离子化效率及同位素比,对比不同渗碳条件对离子化效率及质量歧视系数的影响。较优的渗碳条件为:苯蒸气压0.1Pa,渗碳电流2.5A,渗碳时间1h。采用这种铼带对10pg铀样品的离子化效率可达0.25%,约是双带法的5倍。对于10pg样品,235U/238U测量的外精度在1%以内,234U/238U、236U/238U测量的外精度小于5%。渗碳铼带还能够实现对1~2pg铀样品的测量,对1pg的CRM020A、2pg的IRMM184样品,235U/238U测量的外精度小于1%,234U/238U测量的外精度小于5%。

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 235 U/238 U was less than 1%,and that of 234 U/238 U and 236 U/238 U 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 235 U/238 U measurement was less than 1%,and that of 234 U/238 U 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.