Detection and Correction of Uranium Hydride Mass Interference in Low-Level Materials
This overview discusses the revised MTE-TIMS method aimed at improving the accurate analysis of uranium safeguards samples, which plays a crucial role in verifying state declarations, assessing the quality of declared materials, and detecting potential undeclared nuclear material or activities. The methodology involves thermal ionization mass spectrometry (TIMS) and modified total evaporation (MTE) techniques, developed for routine safeguards measurements at the International Atomic Energy Agency (IAEA). The approach ensures precise measurements of isotopic ratios to evaluate the completeness and consistency of state declarations, incorporating features to enhance accuracy in contemporary sample analysis.
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Revised MTE-TIMS Method for the Detection and Correction of Uranium Hydride Mass Interference in Low-Level U-236 HEU Materials Andi Mundl-Petermeier, Andreas Koepf, Sergei Boulyga Office of Safeguards Analytical Services (SGAS), Department of Safeguards, International Atomic Energy Agency INMM | July 22nd2024
Thermal ionization mass spectrometry (TIMS) at IAEA Thermo-Scientific Triton (T2), Triton-Plus (T3 and T4) U and Pu isotopic compositions and elemental concentrations by IDMS Total Evaporation (TE) for Material Balance Evaluation (MBE) Modified Total Evaporation (MTE) for Material Characterization ~6000 7500 filaments measured per year
Material Characterization Safeguards goals: Support verification of State declarations Assess quality of declared material (data compared to industrial standards) Monitor process design (data compared to declarations, expected characteristics and earlier sampling results) Assess accidental gains of newly declared material (data compared to reference datasets) Look for indications of undeclared nuclear material or activities Address potential substitution of declared material by assessing its origin (data compared to reference datasets) Detect undeclared production of nuclear material at declared locations (data compared to earlier sampling results)
Thermal ionization mass spectrometry (TIMS) material characterization Modified total evaporation (MTE) Initially developed by Stephan Richter (NBL, now JRC Geel) with optimization for routine safeguards measurements at IAEA Precise and accurate measurements of n(U-234)/n(U-238) and n(U-236)/n(U-238) ratios in uranium safeguards samples to assess the completeness and consistency of State declarations Uses a combination of the secondary electron multiplier (SEM) and multi-collector Faraday detectors Corrects for peak tailing and background contributions for every sample measurement As the range of isotopic compositions in inspection samples has been extending, the IAEA recognizes a need for revision of their MTE-TIMS procedure to improve the accuracy of contemporary sample analysis
Thermal ionization mass spectrometry (TIMS) material characterization Analytical challenges in HEU (U-235 >20 atom%) materials: Generally high n(U-234)/n(U-238) ratios ranging from 10-3 to 10-1 In-run determination of SEM yield by measuring n(U-234)/n(U-235) not possible! Large variations in n(U-236)/n(U-238) ratios from <10-8 to 10-2 require different/new/improved analytical methods Low 236U signals (< 10 mV) analyzed with secondary electron multiplier (SEM) Corrections of interferences from major isotope signals
Modified total evaporation measurements Analytical challenges in HEU interference corrections: Mass signal interference from: Peak tailing effects from main U peak U-235 on U-234 and U-236 signal [V] distance from major isotope peak center [amu]
Modified total evaporation measurements Analytical challenges in HEU interference corrections: Mass signal interference from: Peak tailing effects from main U peak U-235 on U-234 and U-236 signal [V] signal [mV] distance from major isotope peak center [amu] distance from major isotope peak center [amu]
Modified total evaporation peak tail correction (Faraday detectors) 35000 Tail from main U-235 peak on n(U-234)/n(U-238) between 2x10-6 and 6x10-6 30 V 30000 m/z 234 25000 20000 signal [cps] cps 15000 20 V 10000 10 V 5000 0 -1.6 -1.4 -1.2 -1 -0.8 -0.6 distance from major isotope peak center in amu distance from major isotope peak center [amu]
Modified total evaporation peak tail correction (Faraday detectors) Tail from main U-235 peak on n(U-234)/n(U-238) between 2x10-6 and 6x10-6 Measured U-234 signal = actual U- 234 signal + signal from U-235 tail signal [cps] distance from major isotope peak center [amu]
Modified total evaporation peak tail correction (Faraday detectors) Tail from main U-235 peak on n(U-234)/n(U-238) between 2x10-6 and 6x10-6 Measured U-234 signal = actual U- 234 signal + signal from U-235 tail signal [cps] Logarithmic tail correction by measuring peak sides (+0.35 amu and -0.35 amu) and calculating tail contribution distance from major isotope peak center [amu]
Modified total evaporation peak tail correction (SEM with RPQ) 500 Tail from main U-235 peak on n(U-236)/n(U-238) between 1.2x10-9 and 3x10-9 with RPQ (energy filter) 450 30 V 400 350 signal [cps] 300 20 V 250 cps 200 150 10 V 100 50 0 0.4 0.6 0.8 1.0 1.2 1.4 distance from major isotope peak center in amu distance from major isotope peak center [amu]
Modified total evaporation peak tail correction (SEM with RPQ) 500 Tail from main U-235 peak on n(U-236)/n(U-238) between 1.2x10-9 and 3x10-9 with RPQ (energy filter) Measured U-236 signal = actual U-236 signal + signal from U-235 tail Small tail when using RPQ - correction by measuring peak sides (+0.35 amu and -0.35 amu) 450 30 V 400 U-236 total measured signal 350 signal [cps] 300 250 cps 200 actual U-236 signal 150 100 signal from U-235 tail 50 0 0.4 0.6 0.8 1.0 1.2 1.4 distance from major isotope peak center in amu distance from major isotope peak center [amu]
Modified total evaporation measurements Analytical challenges in HEU interference corrections: Mass signal interference from: Peak tailing effects from main U peak U-235 on U-234 and U-236 Uranium hydride (U1H) from U-235 on U-236 signal
Modified total evaporation mass interference corrections 400 400 Peak tailing effects and 235U1H interference on U-236: 350 350 300 300 signal [cps] signal [cps] Tail correction by measuring peak sides (+0.35 amu and -0.35 amu) Hydride correction by measuring signal on m/z 239 and convert to m/z 236 signal 250 250 cps cps m/z 236 m/z 236 200 200 150 150 100 100 235U1H 235U1H n(235U1H)/n(238U) up to 10-7! 50 50 235U tail 235U tail 0 0 0.4 0.4 0.5 0.5 0.6 distance from main mass in amu distance from main mass [amu] distance from main mass in amu distance from main mass [amu] 0.6 0.7 0.7 0.8 0.8 0.9 0.9 1.0 1.0 1.1 1.1 1.2 1.2 1.3 1.3
Modified total evaporation mass interference corrections 400 400 Peak tailing effects and 235U1H interference on U-236: 350 350 300 300 signal [cps] signal [cps] Tail correction by measuring peak sides (+0.35 amu and -0.35 amu) Hydride correction by measuring signal on m/z 239 and convert to m/z 236 signal 250 250 cps cps m/z 236 m/z 236 200 200 150 150 100 100 235U1H 235U1H n(235U1H)/n(238U) up to 10-7! 50 50 235U tail 235U tail actual actual U-236 signal could be higher than actual U-236 signal! U-236 signal 0 0 depends on sample matrix, lab humidity, instrument conditions 0.4 0.4 0.5 0.5 0.6 distance from main mass in amu distance from main mass [amu] distance from main mass in amu distance from main mass [amu] 0.6 0.7 0.7 0.8 0.8 0.9 0.9 1.0 1.0 1.1 1.1 1.2 1.2 1.3 1.3
Modified total evaporation hydride formation in TIMS instruments 239.05/U-238 (238U1H plus U-238 tail!) correlates with source vacuum at higher P Different instruments can produce different hydride rates at different times HV source pressure [bar]
Modified total evaporation hydride formation in TIMS instruments 239.05/U-238 (238U1H plus U-238 tail!) correlates with source vacuum at higher P Different instruments can produce different hydride rates at different times HV source pressure [bar]
Modified total evaporation hydride formation in TIMS instruments 239.05/U-238 (238U1H plus U-238 tail!) correlates with source vacuum at higher P Different instruments can produce different hydride rates at different times Strategies to lower hydride rates: HV source pressure [bar] Source housing bake-out (0.5 - 3 hrs.)
Modified total evaporation hydride formation in TIMS instruments 239.05/U-238 (238U1H plus U-238 tail!) correlates with source vacuum at higher P Different instruments can produce different hydride rates at different times Strategies to lower hydride rates: HV source pressure [bar] Source housing bake-out (0.5 - 3 hrs.)
Modified total evaporation Effect of hydride ions on n(U-236)/n(U-238) in HEU Can be significant for materials with n(U-236)/n(U-238) < 3x10-7 Sample 1 Sample 2 Sample 3
Modified total evaporation Method verification New working standards are needed for quality control. Such standards can be produced by blending the existing certified reference materials with the following characterization of the obtained isotopic composition by qualified NWAL members. available QC samples IRMM-3020 CRM U200 CRM U500 IRMM 3050 IRMM 3075 IRMM 3090 CRM U930 CRM U930D n(U-236)/n(U-238) 9.8x10-6 2.7x10-3 1.5x10-3 4.0x10-5 1.1x10-4 2.6x10-4 3.8x10-2 1.4x10-5 HEU QC samples are needed with n(U-234)/n(U-238) ratios of ~10-2 and n(U-236)/n(U-238) ratios of 10-8 to 10-7
Modified total evaporation Method verification Downblends of CRM U930D with CRM 112-A to more closely match samples U-234 and U-236 compositions n(U-236)/n(U-238) n(U-236)/n(U-238) average 235U1H/238U = 5.6E-08 average 235U1H/238U = 2.4E-08
Modified total evaporation Method verification Verification of hydride correction long-term reproducibility RSD: 1.57 % vs. 0.57 %
Conclusions: New sample types bring new analytical challenges HEU materials with low-level U-236 require development and implementation of new analytical methods/data corrections Hydride interference correction Strategies to lower hydride rate during measurement New working standards are needed to verify new methods
