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High Accuracy and High Precision Uranium Isotopic Analysis by DRC-ICP-MS Steve Beres July 2008
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Page 2 Origin of Uranium Uranium is created in the supernova explosions from large, dying stars. The Uranium on Earth today was created before the solar system was created. The radioactive decay of Uranium and Thorium has kept the Earth’s core hot and allowed life to still exist on our planet. Without Uranium and Thorium, our planet would be cold today and devoid of life – like Mars. Since the formation of the Earth, the radioactive decay of Uranium and Thorium has created most of the Pb we find today. The composition of Pb varies geographically – reflecting the original amount of U/Th in the rocks where the Pb was mined. Supernova SN1054 remnant in the Crab Nebula
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Page 3 The Element Uranium Uranium is the heaviest naturally occurring element has three naturally occurring isotopes – 238 U99.2745%½ life: 4.51e 9 years12 k Bq/g – 235 U0.7200%½ life: 7.04e 8 years78 k Bq/g – 234 U0.0055%½ life: 2.47e 5 years230 M Bq/g All radioactive – alpha emitters An important element in the nuclear fuel cycle Recycled Uranium (from nuclear power plants) also contains – 236 U½ life: 2.39e 7 years2.3 M Bq/g Note: Bq = Becquerel = 1 disintegration/sec
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Page 4 Uranium Decay Number of Neutrons Number of Protons Decay of 238 U into 206 Pb
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Page 5 Uranium Decay Number of Neutrons Number of Protons Decay of 235 U into 207 Pb
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Page 6 The isotopic composition of Pb varies Present when the Earth was createdProduced from decay of U and Th since the Earth was created The abundance of the four Pb isotopes vary geographically – makes Pb a good forensic tool
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Page 7 Uranium Enrichment & Depletion The most important Uranium isotope is 235 U It has the largest thermal neutron cross section [580 barns] –Most easily undergoes fission to release energy –Second only to Plutonium [742 barns], but much safer Uranium that has < 0.7200% 235 U is called depleted uranium Uranium that has > 0.7200% 235 U is called enriched uranium Important for Nuclear reactors – enrichment 3-5% Important for Nuclear weapons – enrichment 20-90% Note: 1 barn = 10 -24 cm 2
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Page 8 Uranium Enrichment During the Uranium enrichment process, the accuracy of the isotope ratios is critical Isotope ratios are typically measured using TIMS or ICP-MS Typical ratio precisions: (when m 1 /m 2 ~ 1) ICP-MS –Non-cell~ 0.2% RSD or 2000 ppmcost ~ $120k –DRC ~ 0.05% RSD or 500 ppmcost ~ $160k –Multi-collector ~ 0.005% RSD or 50 ppmcost ~ $500k TIMS ~ 0.0005% RSD or 5 ppmcost ~ $300k In general, sample prep and analysis is faster by ICP-MS than by TIMS Minutes/hours rather than hours/days
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Page 9 Isotope Ratio Analysis Precision is best when the signals are of similar intensity (abundance). The precision of the analysis can be improved by counting more ions – (increase sensitivity or longer analysis times) until the counting statistic limit is reached. Counting statistics limit – the theoretical best precision achievable based on the number ions measured. Using an ICP-MS with a high pressure cell, the ratio precisions can be improved 5-10x over non-cell ICP-MS instruments. On Dynamic Reaction Cell (DRC) instruments, the isotope ratios are within 30% of the counting statistics limit. (D.R. Bandura et. al., JAAS, 15, 921, 2000)
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Page 10 Uranium Enrichment/Depletion Analysis A recent prospect was looking for a cost effective way to verify the amount of 235 U depletion in a sample Required an isotope ratio accuracy < 0.1% (1000 ppm) for 235 U/ 238 U in depleted uranium (0.2 - 0.7% 235 U) Solution: ELAN DRC-e
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Page 11 Uranium Depletion Analysis A standard ELAN DRC-e was used - no options Cell gas – Argon @ 0.2 ml/min, RPq = 0.25 Analysis time 10 minutes per sample Sample - CRM U005A – 0.5% 235 U by weight ( 235 U/ 238 U = 1:200 ratio) Mass bias correction used for each sample Standard Sample Standard Sample
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Page 12 Uranium Depletion Analysis U500A ratio data for a single sample analysis (10 minutes) Certified ratio for U005A: 0.005090 ± 0.0000015 (as atom fraction) Note: Accuracy = {(measured-certified)/certified} DRC-e Results: Ratio Precision - 0.194% (1σ) (within 27% of Statistical Error) a.k.a counting statics limit Ratio Accuracy – 0.148%
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Page 13 Uranium Depletion Analysis U005A ratio data for a group of 10 samples Note: Accuracy = {(measured-certified)/certified} Ratio Precision - 0.066% (660 ppm) (1σ) (within 9% of Statistical Error) Ratio Accuracy – 0.054% (540 ppm)
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Page 14 Uranium Depletion Analysis Long term performance over multiple non-consecutive days Each group consists of 10 samples External precision is the RSD of the means Grand mean recovery - 100.014% External precision – 0.023% (1σ)
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Page 15 Uranium Enrichment/Depletion Analysis Quick look at all the isotopes in CRM U005A Data from a single 3 minute screening measurement: Even 236 U/ 238 U (1:90k) had better than 5% accuracy.
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Page 16 Uranium Analysis Summary For 200:1 ratio: Sample Precision < 0.3% (1σ) Sample Accuracy < 0.2% Group* precision < 0.15% (1σ) Group* accuracy < 0.05% 12 group precision < 0.025% (< 250 ppm) (1σ) 12 group accuracy < 0.015% (< 150 ppm) U005A grand mean recovery - 100.014% %RSD = 0.023% (1σ), 0.046% (2σ), *Each group consists of 10 samples The ELAN DRC-e can perform high precision and high accuracy Uranium analysis on isotopes that vary widely in abundance.
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