MC-ICP-MS测量铀中低丰度铀同位素比值

Determination of Minor Isotope Ratios in Uranium by Multiple Collector Inductively Coupled Plasma Mass Spectrometry

高精密度地测量铀材料中的铀同位素组成信息,特别是低丰度铀同位素234U和236U,是核取证研究的重要内容。本研究采用多接收电感耦合等离子体质谱(MC-ICP-MS)测量铀同位素比值,将两种黄饼样品的酸消解液制备成238U浓度约21000ng/g和180ng/g的待测样品,采用外标标准化法和标准样品交叉法校正质量分馏效应,MC-ICP-MS对238U浓度约180ng/g的样品中235U/238U测量的相对实验标准偏差可低于0.014%。为降低超档离子流信号对低丰度铀同位素分析的影响,建立了法拉第杯接地方法,使轰击到法拉第杯上的238U^+产生的电流在到达前置放大器之前被引入大地,该方法对238U浓度约21000ng/g的样品中234U/235U测量的相对实验标准偏差可低于0.020%,对浓缩铀GBW04234和GBW04238中236U/235U测量的相对实验标准偏差小于0.11%,测量结果与参考值在不确定度范围内一致。该方法的精密度较高、结果准确,可识别铀同位素组成存在一定差异的核材料,为核取证和核保障监督提供技术支持。

The isotopic fingerprints of uranium are of particular interest for nuclear forensics and nuclear safeguards.Especially the need for lower detection uncertainties of minor isotopes,including 234U and 236U,is considered to be of major importance for establishing relations between materials.This work described the utilization of multiple collector inductively coupled plasma mass spectrometry(MC-ICP-MS)for the determination of uranium isotope ratios of uranium materials.The EC nuclear reference material 199(EC-NRM199)were diluted to about 40.0 ng/g by 2%HNO3(V/V).The digested liquid samples of two kinds of yellow cakes were diluted to about 21000 ng/g and 180 ng/g,respectively.Mass fractionation was corrected by external standardization and standard sample bracketing(SSB)method.The analytical results of the 180 ng/g uranium sample solution showed that the precision for the major isotope ratio 235U/238U could be smaller than 0.014%.A faraday-ground-connect method was developed to measure the minor isotope ratios,such as 234U/235U,233U/235U and 233U/234U by faraday cups.The electric current produced by 238U^+ in the faraday cup was conducted to the ground potential before arriving the preamplifier.The relative uncertainty for 234U/235U could be smaller than 0.020% for the 21000 ng/g uranium sample solution under magnet-scan mode.Variation of the measured isotope signatures would help in identification and source attribution of different uranium materials.An alternative method for analyzing 236U/235U was to set the magnet value of the center faraday detector named Ax at mass 234.Through the method,the relative uncertainty of 236U/235U for both GBW04234 and GBW04238 was less than 0.11%,and the results agreed with the certificated values within uncertainty range.The results demonstrated that this technique is a precise and accurate method for the determination of uranium isotopic fingerprints in nuclear forensics and nuclear safeguards.Furthermore,the faraday-ground-connect method shows great potential in measuring minor isotope ratios of other elements for nuclear and geological applications.