Application of Total Evaporation Technique Using Thermal Ionization Mass Spectrometry in Measurement of Plutonium Isotopes#br#

When measuring the isotope ratio of plutonium by thermal ionization mass spectrometry with conventional method, the plutonium isotope standard material is usually needed to calibrate the bias caused by the isotope fractionation which is the main factor that influences the accuracy of results. Recent years, the total evaporation technique has become a good choice because of its inherent characteristics that can obtain results with no significant bias without calibrate by external standard. In this work, the main factors were investigated that influence the total evaporation measurement of thermal ionization mass spectrometry, including the amplifier current gain calibration, sample amount, and the isobaric interference ²³⁸U and ²⁴¹Am. ²⁴¹Am was separated from the aged plutonium by UTEVA resin, it was shown that isotopic analysis of plutonium should be completed within a reasonable time period after separation from Americium to minimize interference of ²⁴¹Am in-growth from decaying ²⁴¹Pu, and the interval limiting between sample purification and isotopic analysis was about 20 days to make sure the accuracy and precision, there would be about 2% bias after 30 days of separation. The sample lost between integration due to the necessary to adjust the filament current and the residual sample after measuring was eliminated as less than 0.3%. Measured with total evaporation technique utilizing Faraday cups only, IRMM-086 with different sample loadings were studied respectively, the results showed that sample loading between 50 and 100 ng were likely enough for good accuracy and precision, but there were also slightly biases in the ²³⁸Pu/²³⁹Pu and ²⁴²Pu/²³⁹Pu due to the low signal of ²³⁸Pu and ²⁴²Pu influenced by the noise of Faraday cups. The results of IRMM-086 using total evaporation and conventional techniques were compared, and showed that the total evaporation technique could obtain better accuracies than the conventional method while sample loading for total evaporation and conventional were 50 ng and 200 ng respectively. There was no significant bias between measured and certified value when isotope ratio was more than 10-4. For IRMM-086, the relative standard deviations of ²⁴⁰Pu/²³⁹Pu, ²⁴¹Pu/²³⁹Pu, ²⁴²Pu/²³⁹Pu measured by total evaporation were 0.006 8%, 0.50% and 0.83%, respectively. Giving high precision and accuracy results without calibration, the total evaporation technique will be more widely used when we have a lower noise amplifier for Faraday collectors. In the field of spent fuel reprocessing analysis, plutonium isotope composition varies greatly due to different burn-up. Total evaporation measurement technology has a very good application prospect in the field of plutonium isotope composition measurement.