Prof. William E. (Liam) Kieser - PowerPoint Presentation

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Prof. William E. (Liam) Kieser

André E. Lalonde AMS Laboratory and Dept of Physics and Earth Sciences University of Ottawa

AMS Analysis of Uranium Isotopesand Trace Elements in UOC Samples

International Conference on Nuclear SecurityFebruary 2020

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Overview

Uranium Ore Concentrate (UOC) Project DescriptionAdvantages of Accelerator Mass Spectrometry (AMS)

How AMS WorksAMS Analysis of Trace Actinide Isotopes in UOCsAMS Analysis of Trace Medium Mass Isotopes in UOCsSummary and Outlook

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The Task:

307 Uranium Oxide Concentrate Samples from mines and mills around the world.

Find trace elements or even isotopes that could help distinguish one sample from another.If something looks interesting, develop new techniques to explore itThe Tools:For less abundant elements and isotopes, an Accelerator Mass SpectrometerAt the André E. Lalonde Accelerator Mass Spectrometry Lab:

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The A. E. Lalonde Accelerator

Mass Spectrometer

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The additional ion energy enables:Why add an accelerator to a mass spectrometer?

a) Destruction of molecular isobars and reduction of some atomic isobars

b) Noise-free single atom counting and some elemental identification

The Results

:

a)

Isotope ratios down to 10

-15

b)

Mass sensitivities in the femtogram range

Introduction to Accelerator

Mass

Spectrometry

Applications

:

Earth and Planetary Sciences, Environmental History and current monitoring, Bio-medical research, Archeological Science, Forensics

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Isobaric Interferences:

Atomic Isobars:

Several isobars can be eliminated if we use negative rather than positive ions:

e.g.

12

CH

2

,

13

CH,

7

Li

2

for

14

C analysis

Most molecules are broken apart removing their outer electrons – needs higher energies and interaction with a gas or foil

14

N for

14

C analysis

26

Mg for

26

Al analysis

129

Xe for

129

I analysis

Others can be attenuated using higher energy and interacting with gases or foils:

e.g.

10

B for

10

Be analysis

Molecular Isobars:

The accelerator enables both these techniques as well as providing noise-free, single atom counting in the detector.

Isobars are atoms of a different element or molecules that have nearly the same mass as the atoms to be analyzed – these must be eliminated for accurate measurement

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The Spectrometer – an overview

1. Negative ions produced from samples loaded in the Ion Source

2. Ions selected by mass and energy in the Low Energy Mass Spectrometer3. Ions accelerated towards the high voltage (3 MV) electrode in accelerator4. Ion charge is changed to positive and molecules are destroyed in argon filled canal5. Positive ions and molecular fragments accelerated to higher energies6. Analyte Ions sorted from fragments in the High Energy Mass Spectrometer7.

Analyte Ions counted and identified in the Faraday Cups and Gas Ionization Detector

1

2

3

4

5

6

7a

7b

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Development of the negative ion caesium sputter source in the 1970s

made AMS possible

Requirements:

Large ion current (at least 10s of μA, 100s good if possible)

to obtain sufficient counting statistics for low concentration of rare species with

a

large ratio to abundant species

Produce negative ions from a wide range of elements

Stable operation for a variety of sample matrices

Relatively low memory of previously analysed samples

SIMS Design

High Current Design

The Ion Source

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Caesium reservoir

Extraction Anode

Sample

(Target, Cathode)

Spherical Ionizer ~1200°C

~ -28 kV

~ -35 kV

Ion Source Schematic

– in cross section

Caesium reservoir neater

Cooling Channel

Analyte

Ions

0

10

20 mm

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Source Chamber

Focusing, Steering Electrodes

Faraday Cup

Ion Beam

The SO-110 high current ion source

Sample Insertion Actuator

200 Sample Carousel

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The SO-110 high current ion source head

Target (~-35 kV)

Caesium delivery tube

Ionizer Location

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– provides the

micro- environment for the conversion of CO

2 into negative carbon ions– one assembly must be prepared for each 14C measurement

For solid materials

– compress into a 1.3 mm

Φ

pellet in a replaceable

Al, Cu or

SS cylinder

For gases

The SO-110 – target assembly

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The Low Energy Mass Spectrometer –

Energy and Isotope Selection 1The electric analyzer selects ions within a defined energy range to eliminate momentum ambiguities in the following analyzing magnet

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The Low Energy Mass Spectrometer –

Energy and Isotope Selection 2Although the magnet selects ions by momentum – with the electric analyzer, this defines the mass to be injected.

Changing the magnetic field is too slow for isotope selection, so the momentum of the ions is locally changed by acceleration or deceleration.To do this, the vacuum box inside the magnet is insulated from the rest of the beam line and a voltage is applied to this box.IsotopeVoltageTemporary EnergyMomentumTimeCarbon-12

+2,927.5 V37.9275 keV30.1705 (u-keV)1/2100 µsCarbon-13+10.0 V35,010.0 keV30.1705 (u-keV)1/2100 µsCarbon-14-2,490.7 V

32.5093

keV

30.175 (u-keV)

1/2

2-3

ms

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The Low Energy Magnetic Analyzer

Upper yoke and coil removedInsulators

Vacuum BoxUpper Pole

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The Electron Stripping Canal

Ion Path

Argon inTurbopump

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The Accelerator Column

Accelerator Column Assembled

Column inside pressure vessel

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The High Energy Mass Spectrometer

Electric Analyzer ρ = 1.7mFaraday cup box (abundant isotopes)Analyzing Magnet ρ = 2 m

Switching Magnet (20° port used

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The Gas Ionization Detector

Detectors

Cathode GridTo measure up to 100,000 ions per secondeee

eAnode Grids75 nm thick SiN windowIsobutane gas – 5-25 mbar

Δ

E

1

E

f

~100V

~100V

Ion Beam

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Normally Uranium isotopes are measured with high precision by TIMS, but sample preparation is time-consuming and throughput can be slow

UOC samples can be prepared for an AMS ion source by mechanically mixing the powder with a fluorinating agent (e.g. PbF2) and pressing this mixture into the target holder.Accelerator Mass Spectrometry: Actinide Isotopes

The AMS system can be programmed to analyze several low-abundance isotopes in the gas ionization detector during one sequence (Slow Sequential Injection).

An actinides measurement was set up to measure

238

U current and counts of

236

U,

231

Pa,

230

Th and

226

Ra

Problem:

Just as in IRMS, AMS needs to measure to a standard. So far, no actinide standards exist

(but NRC is working on it)

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236

U / 238USample NumberSample Number231Pa / 238U

Accelerator Mass Spectrometry: Actinide Isotopes

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230

Th / 238USample NumberSample Number226Ra / 238U

Accelerator Mass Spectrometry: Actinide IsotopesX

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UOC samples can also be prepared for an AMS ion source without mechanically mixing the powder with a fluorinating agent. In this case an oxide beam is produced which, for actinides, would not be as efficient.

Accelerator Mass Spectrometry: Osmium / Iridium

A measurement was set up to look at the ratios 187Os/188Os and 191Ir/193Ir

Again, no standards are available for this material, so the data reported are simply based on the transmission of the instrument.

Unmixed UOC samples can be used to analyze trace isotopes of interest. Of a number of isotopes tried, osmium and iridium looked promising and gave appropriate particle currents.

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187Os/

188OsSample NumberSample NumberAccelerator Mass Spectrometry: Osmium / Iridium191Ir/193Ir

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Multi-Isotope Bar Code Summary of the Measurements

Measurements grouped by Mill-Mine origin; note that the range of concentrations or ratios exceeds the accuracy or the Direct AMS measurements.

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AMS analysis, using simple

samle preparation can provide actinide measurements which show noticeable differences, but require standard reference materials for reliable accuracySummaryWe are awaiting the results of the NRC global UOC calibration exercise, so that these measurements can be reported with greater accuracy

Even simpler sample preparation can provide other trace element measurements which also show noticeable differences, but again, there require standard reference materials for reliable accuracyRanges of isotope ratios and trace elemental concentrations can provide provenance information from the analysis of UOC samples.

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Investigators, Affiliations and Acknowledgements

Ian D. Clark, Liam Kieser, J

ack CornettXiao-Lei Zhao,, Gilles St-Jean,Norm St-Jean,,

A. E. Lalonde AMS Laboratory,University of Ottawa

Funding from:

Centre for Security Science

Canada Foundation for Innovation

Ontario Research Fund

uOttawa Advanced Research Complex

Home of the Jan

Veizer

Stable Isotope Laboratory

the André E. Lalonde AMS Laboratory

A. E. Litherland

IsoTrace Laboratory, University of Toronto

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