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

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

Page 3 The Element Uranium  Uranium is the heaviest naturally occurring element has three naturally occurring isotopes – 238 U %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

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

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

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

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 < % 235 U is called depleted uranium  Uranium that has > % 235 U is called enriched uranium Important for Nuclear reactors – enrichment 3-5% Important for Nuclear weapons – enrichment 20-90% Note: 1 barn = cm 2

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 ~ % 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

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)

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 ( % 235 U)  Solution: ELAN DRC-e

Page 11 Uranium Depletion Analysis A standard ELAN DRC-e was used - no options  Cell gas – 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

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

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

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 % External precision – 0.023% (1σ)

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.

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 % %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.