High Accuracy and High Precision Uranium Isotopic Analysis by DRC-ICP-MS

1. High Accuracy and High Precision Uranium Isotopic Analysis by DRC-ICP-MS Steve Beres July 2008

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. Supernova SN1054 remnant in the Crab Nebula • 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. Page 2

3. The Element Uranium • Uranium is the heaviest naturally occurring element • has three naturally occurring isotopes • 238U 99.2745% ½ life: 4.51e9 years 12 k Bq/g • 235U0.7200% ½ life: 7.04e8 years 78 k Bq/g • 234U0.0055% ½ life: 2.47e5 years 230 M Bq/g • All radioactive – alpha emitters • An important element in the nuclear fuel cycle • Recycled Uranium (from nuclear power plants) also contains • 236U ½ life: 2.39e7 years 2.3 M Bq/g Note: Bq = Becquerel = 1 disintegration/sec Page 3

4. Number of Protons Number of Neutrons Uranium Decay • Decay of 238U into 206Pb Page 4

5. Number of Protons Number of Neutrons Uranium Decay • Decay of 235U into 207Pb Page 5

6. The isotopic composition of Pb varies Present when the Earth was created Produced 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 Page 6

7. Uranium Enrichment & Depletion • The most important Uranium isotope is 235U • 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% 235U is called depleted uranium • Uranium that has > 0.7200% 235U is called enriched uranium • Important for Nuclear reactors – enrichment 3-5% • Important for Nuclear weapons – enrichment 20-90% Note: 1 barn = 10-24 cm2 Page 7

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 m1/m2 ~ 1) • ICP-MS • Non-cell ~ 0.2% RSD or 2000 ppm cost ~ $120k • DRC ~ 0.05% RSD or 500 ppm cost ~ $160k • Multi-collector ~ 0.005% RSD or 50 ppm cost ~ $500k • TIMS ~ 0.0005% RSD or 5 ppm cost ~ $300k • In general, sample prep and analysis is faster by ICP-MS than by TIMS • Minutes/hours rather than hours/days Page 8

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) Page 9

10. Uranium Enrichment/Depletion Analysis • A recent prospect was looking for a cost effective way to verify the amount of 235U depletion in a sample • Required an isotope ratio accuracy < 0.1% (1000 ppm) for 235U/238Uindepleted uranium (0.2 - 0.7% 235U) • Solution: ELAN DRC-e Page 10

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% 235Uby weight • (235U/238U = 1:200 ratio) • Mass bias correction used for each sample • Standard • Sample • Standard • Sample Page 11

12. Uranium Depletion Analysis • U500A ratio data for a single sample analysis (10 minutes) DRC-e Results: Ratio Precision - 0.194% (1σ) (within 27% of Statistical Error) a.k.a counting statics limit Ratio Accuracy – 0.148% Certified ratio for U005A: 0.005090 ± 0.0000015 (as atom fraction) Note: Accuracy = {(measured-certified)/certified} Page 12

13. Uranium Depletion Analysis • U005A ratio data for a group of 10 samples Ratio Precision - 0.066% (660 ppm) (1σ) (within 9% of Statistical Error) Ratio Accuracy – 0.054% (540 ppm) Note: Accuracy = {(measured-certified)/certified} Page 13

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) Page 14

15. Uranium Enrichment/Depletion Analysis • Quick look at all the isotopes in CRM U005A Data from a single 3 minute screening measurement: Even 236U/238U (1:90k) had better than 5% accuracy. Page 15

16. Uranium Analysis Summary • The ELAN DRC-e can perform high precision and high accuracy Uranium analysis on isotopes that vary widely in abundance. 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 Page 16