Development of an analysis method of minor uranium isotope ratio measurements using electron multipliers in Thermal Ionization Mass Spectrometry

Abstract

A simple analytical procedure was developed to measure with high accuracy the isotope ratio of minor isotope of natural uranium present in small quantities using a thermal ionization mass spectrometer (TIMS). The reduction of quantities used for analysis and the measurement of non-abundant isotopes are of prime interest in the nuclear industry. Indeed it is necessary to reduce the analyst received dose and the effluent released, as well as realizing measurement at trace level. The new generation of TIMS is equipped with a multicollection system of electron multipliers: discrete dynode electron multiplier (SEM) and continuous dynode electron multiplier (MIC), that improve the sensitivity compared to faraday cups. The procedure developed was verified using Certified Reference Material IRMM 052. Results were evaluated relying on NF T 90-210 norm regarding method validation. First, the isotope ratio 234U/238U was examined by total evaporation using the SEM and MIC to measure 234U and the faraday cup to measure 238U. In a second approach, the isotope ratio 235U/238U was studied by total evaporation using the SEM to measure 235U and the faraday cup to measure 238U. The classical method with peak-jumping SEM measurement was also used. Total evaporation method employing only the faraday cup was used to confront the results obtained. The analyzable quantity was reduced from 250ng to 50ng for the 235U/238U isotope ratio and from 1270ng to 50ng for the 234U/238U isotope ratio with acceptable uncertainties thanks to the use of electron multipliers. For all experiments were the accuracy was achieved, the calculated uncertainties were below to 0.28% for the 235U/238U isotope ratio and 5% for the 234U/238U isotope ratio.