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

– 238U 99.2745% ½ life: 4.51e9 years 12 k Bq/g

– 235U 0.7200%½ life: 7.04e8 years 78 k Bq/g– 234U 0.0055%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/gNote: Bq = Becquerel = 1 disintegration/sec

Page 4

Uranium Decay

Number of Neutrons

Num

ber o

f Pro

tons

Decay of 238U into 206Pb

Page 5

Uranium Decay

Number of Neutrons

Num

ber o

f Pro

tons

Decay of 235U into 207Pb

Page 6

The isotopic composition of Pb varies

Nominal Pb Isotopic Distribution on Earth

1.4%

24.1% 22.1%

52.4%

0%

10%

20%

30%

40%

50%

60%

isotopic mass

Nat

ural

abu

ndan

ce

204 207206 208

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 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 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 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 235U depletion in a sample

Required an isotope ratio accuracy < 0.1% (1000 ppm) for 235U/238U in depleted uranium (0.2 - 0.7% 235U)

Solution: ELAN DRC-e

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% 235U by weight

• (235U/238U = 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: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%

time (min) 235U cps 238U cps 235/2381 9007 1776252 0.0050842 9060 1786479 0.0050843 8958 1772120 0.0050784 9016 1779813 0.0050955 9041 1780945 0.0051036 9073 1788929 0.005097 9010 1769245 0.0050948 9064 1786432 0.0050949 9055 1788526 0.00508410 9031 1779344 0.005087

mean 9031.3 1780808.6 0.005098%RSD 0.385 0.385 0.194

% dev fromStat Error % 0.153 Stat Error 27.02

Accuracy 0.148 %

Page 13

Uranium Depletion Analysis

U005A ratio data for a group of 10 samples

U005A analysis

0.005060

0.005070

0.005080

0.005090

0.005100

0.005110

0.005120

1 2 3 4 5 6 7 8 9 10

sample #

U235

/U23

8 ra

tio RSD = 0.066% (n=10)

Average = 0.005093

Certified value = 0.005090 ± 0.0000015

Accuracy = 0.054%

samplecorrected 235/238 %RSD

Accuracy %

Stat Error %

%Dev from Stat

Errorsample 1 0.005097 0.216 -0.132 0.1839 17.4sample 2 0.005088 0.190 0.037 0.1796 5.8sample 3 0.005089 0.195 0.024 0.1756 11.0sample 4 0.005097 0.283 -0.146 0.1730 63.6sample 5 0.005090 0.189 -0.006 0.1715 10.2sample 6 0.005096 0.155 -0.113 0.1706 -9.1sample 7 0.005093 0.164 -0.051 0.1703 -3.7sample 8 0.005095 0.172 -0.096 0.1701 1.1sample 9 0.005093 0.110 -0.059 0.1701 -35.3sample 10 0.005090 0.214 0.004 0.1698 26.0

mean 0.005093 -0.054 8.7%RSD 0.0665%

Note: Accuracy = {(measured-certified)/certified}

Ratio Precision - 0.066% (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

U005A External Precision

0.005060

0.005070

0.005080

0.005090

0.005100

0.005110

0.005120

0 2 4 6 8 10 12 14

Group

235/

238

ratio

Certified value = 0.005090 ± 0.0000015

Expt. mean = 0.0050907 ± 0.0000012

RSD = 0.023% (1σ, n=12)

Accuracy of mean= 0.014%

Each group consists of 10 samples

External precision is the RSD of the means

Grand mean recovery - 100.014%

External precision – 0.023% (1σ)

group ratio %RSD % accuracy1 0.0050923 0.120% -0.0452 0.0050897 0.108% 0.0063 0.0050895 0.071% 0.0104 0.0050926 0.134% -0.0515 0.0050907 0.085% -0.0136 0.0050900 0.102% -0.0017 0.0050898 0.098% 0.0038 0.0050907 0.085% -0.0139 0.0050900 0.102% -0.001

10 0.0050927 0.067% -0.05411 0.0050907 0.085% -0.01312 0.0050900 0.102% -0.001

mean 0.0050907 0.097% -0.014stdev 0.0000012%RSD 0.023%

Page 15

Uranium Enrichment/Depletion Analysis

Quick look at all the isotopes in CRM U005A

Data from a single 3 minute screening measurement:

Mass Intensity Corr. Ratio %RSD Measured Certified % Accuracy234 49.1 0.0000337 6.14 0.0000335 0.0000340 -1.407%235 6628.175 0.0050690 0.625 0.0050433 0.0050640 -0.410%236 16.4 0.0000113 9.738 0.0000113 0.0000118 -4.610%238 1293349.8 1.0000000 0.9949120 0.9948900 0.002%

Mass Intensity Corr. Ratio %RSD Measured Certified % Accuracy234 49.1 0.0000337 6.14 0.0000330 0.0000334 -1.320%235 6628.175 0.0050690 0.625 0.0049799 0.005 -0.403%236 16.4 0.0000113 9.738 0.0000112 0.0000117 -4.599%238 1293349.8 1.0000000 0.9949760 0.994955 0.002%

Mass Fraction

Atom Fraction

Even 236U/238U (1:90k) had better than 5% accuracy.

Page 16

Uranium Analysis Summary

U005A External Precision

0.005060

0.005070

0.005080

0.005090

0.005100

0.005110

0.005120

Group

235/

238

ratio

Certified value = 0.005090 ± 0.0000015

Expt. mean = 0.0050907 ± 0.0000012

Accuracy of mean = 0.014%

certifiedvalue

RSD = 0.023% (1σ, n=12) measured mean

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.