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A presentation in defense of the dissertation
entitled ANALYSIS OF FACTORS THAT AFFECT ION
BEAM CURRENTS FOR COSMOGENIC 10Be AND 26Al
ANALYSIS BY ACCELERATOR MASS SPECTROMETRY
(AMS)by Adam Lewis HuntIn Partial
Fulfillment of the Requirementsfor the Degree of
Doctor of PhilosophySpecializing in Chemistryat
the University of Vermont
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Outline of dissertation defense

• I. Introduction to the analysis of cosmogenic
  10Be and 26Al
• II. Investigation of factors which affect the
  sensitivity of accelerator mass spectrometry
  (AMS) for cosmogenic 10Be and 26Al isotope
  analysis
• III. Metal matrices to optimize ion beam currents
  for accelerator mass spectrometry
• IV. Investigation of metal matrix systems for
  cosmogenic 26Al analysis by accelerator mass
  spectrometry
• V. Closing remarks

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Strategy for isotope quantification
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Long-lived radioisotopes
Nuclide t1/2 Z (p) N (n)
9Be -----stable----- 4 5
10Be 1.51 Ma 4 6
26Al 0.70 Ma 13 13
27Al -----stable----- 13 14
14C 5.73 ka 6 8
36Cl 0.30 Ma 17 19
129I 15.7 Ma 53 76

• Pleistocene Age
• 1.806 Ma to 11 ka before now
• Latest period of glaciation

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10Be and 26Al production

• Principle sources
• Cosmogenesis
• Meteoric or garden variety (atmospheric)
• In situ terrestrial substrate (lt 2 m)
• Negative muon capture (gt 2 m)
• Radiogenesis
• Interstellar protons

• Principle mechanism
• 10Be (5.2 atoms g-1 a-1)
• 16O(n,4p3n)10Be
• 28Si(n,6p3n)210Be
• 26Al (30.1 atoms g-1 a-1)
• 28Si(n,p2n)26Al

Significance
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Dual analysis of cosmogenic nuclides
Nuclide activity is a function of

• (a) Constants (relatively)
• Nuclide half-life
• Nuclide production rate
• Substrate density
• Attenuation length for neutrons

• (b) Variables
• Exposure history
• Erosion history

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10Be- and 26Al-specific challenges
Challenge Solution (nominal)
Be is not native to quartz Add stable Be carrier
Al is native (feldspars) Acid leech and monitor Al
Chemical behavior Common extraction scheme
Stability of Be anion Accelerate BeO-
Be isobars (B) Minimize B exposure
Al isobars (Mg) Accelerate Al-
Al2O3 product current (mA)
AlO- 20-40
AlO2- 4-6
Al- 1
Data courtesy of Middleton, R. A Negative Ion
Cookbook, University of Pennsylvania
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Analysis of rare isotope abundances
Steps of the analytical method

• Conventional MS
• Formation of atomic and/or molecular ions
• Acceleration through electrostatic potential
  (kV)
• Separation of ions based on m/z
• Measurement of ions in detector

• Accelerator MS
• Formation of negative atomic and/or molecular
  ions
• Acceleration through electrostatic potential
  (kV)
• Acceleration to MeV energies
• Separation of ions based on m/z
• Measurement of ions in detector

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AMS block diagram
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Center for Accelerator Mass Spectrometry (CAMS)
at LLNL
(1)
(2)
(3)
(5)
(4)
Photographs courtesy of Lawrence Livermore
National Laboratories
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Principles of AMS operation

• Formation of negative atomic and/or molecular
  ions
• Cs sputter source
• Negative ions
• Source geometry
• Ion Sourcery
• Acceleration through electrostatic potential
  (kV)
• Injector magnet
• Low resolution filter
• Fast ion switching
• 3. Acceleration to MeV energies
• Tandem accelerator
• 10 MV terminal
• Electron stripper
• Molecular isobars

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Principles of AMS operation(continued)

• 4. Separation of ions based on m/z
• Magnetic analyzer (ME/q2)
• Electrostatic analyzer (E/q)
• Velocity analyzer (E/M)
• 5. Measurement of ions in detector
• Gas ionization detector
• Isobar-radionuclide pair with same E
• Stopping power (Z)
• Electron-ion pairs
• E vs. DE

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Figures of merit
Sensitivity

• (26Al)

1997
2007
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Figures of merit (continued)

• Precision
• External error (repgt3x)
• Internal error (Poisson)
• Throughput
• BeO cathode 10 min
• Al2O3 cathode 30 min

• Accuracy
• Systematic errors (uncertainty in)
• Production rate
• Latitude/longitude scaling
• Geomagnetic/solar modulation (temporal)
• Assigned constants

Significance
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Investigatory Aims

• Observations
• 10Be analysis is typically limited by B
• 26Al analysis is typically limited by ion beam
  currents
• Ultimate goals
• Improve cosmogenic 10Be and 26Al analysis with
  better wet analytical chemistry
• Improve precision for challenging samples
• Strategy to improve 10Be and 26Al AMS analyses
• Determine effect of sample composition on AMS ion
  source behavior
• Characterize quartz extraction chemistry
• Trace the fate of Be and Al
• Track the movement of impurities
• Identify problematic areas in the procedure
• Make blanks with better background ratios from
  beryl
• Produce AMS cathodes which generate sufficient
  ion beam current

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AMS ion beam currents (BeO-)
Value (mA) Mean13.1 Median13.4 Minimum1.6 Maxim
um25.1
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Elemental analysis
75th 25th
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Effect of elemental composition on BeO- ion beam
currents

• Notation
• parameter estimate
• standard error
• t ratio
• Probgtt.

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Concerning the extraction of BeO and Al2O3 from
quartz

• Overview
• Empirical procedure
• Time consuming
• Pre-treatment
• Acid-digestion
• Separation
• Hazardous
• Cleanliness (isotopic)

• Yield trace analysis
• Clean and characterize quartz
• Supplement native composition
• Aliquot during a standard extraction
• Elemental analysis by ICP-AES
• Matrix matched standards
• Dilute into linear dynamic range

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Separation methods Part I
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Separation methods Part II
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Anion exchange chromatography

• Parameters
• cv 20 mL
• Resin type AG X18
• Elution rate 1 drop/s
• (a) 8 M HCl
• (b) 1.2 M HCl

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pH selective precipitation

• Low pH (3.8-4.1)
• High pH (8.5)

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Cation exchange chromatography

• Parameters
• cv 10 mL
• resin AG 50W-X8
• Rate 1 drop/s
• (a) 0.5 M H2SO4
• (b) 1.2 M HCl
• (c) 3.0 M HCl
• (d) 6.0 M HCl

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Key steps in quartz extraction

• Dissolution by multi-acid digestion
• 2H3O TiF62- ltgt TiO2 6 HF
• Selective distillation of HF relative to HClO4
• Anion exchange
• Good for Fe but not good for Ti separation
• Precipitations
• Poor reproducibility
• Qualitative analysis
• Cation exchange
• Triple acid elution
• Boron removal
• Decent Ti separation

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The matrix effect

• Background
• Convention of mixing BeO in a metal matrix (e.g
  Ag or Nb) prior to AMS analysis to provide high
  ion beam currents
• Ion source design
• Experimental parameters
• Amount of metal mixing matrix
• Target packing with respect to depth
• Matrix composition
• Long term goal
• Understand mechanism of matrix enhancement and
  predict possible matrix for 26Al

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Matrix elemental properties

• Hypothesis
• Matrix effectiveness is dependent upon
• Ion source presentation
• Quantitative composition
• Physicochemical characteristics of matrix

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Experimental design

• Sample preparation
• Measuring volumetric curette
• Mixing BeO with metal
• Ratio serial dilution with mole fraction
  (cmatrix 0.50 to 0.95)
• Packing tamped into targets
• Instrumental analysis
• Stable BeO- beam (10 min) instantaneous and
  integrated current measurements
• Usual matrices Ag, Nb
• Novel matrices Mo, Ta, V, W, Os

1 mm
2.5 mm
Meyhoefer curette
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Stability for NbBeO cathodes
t0-660 s t30-330 s
i (µA) 13990 6380
rsd 3.59 2.76
(post Ag)
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BeO- beam currents
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Effect of analyte matrix ratio
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Control of cathode presentation depth

• Experimental design
• Depth is defined as space above target surface
• Sample composition is an equimolar mixture of
  matrix and oxide
• Depth is measured with a micrometer
• Measure integrated currents for a typical
  analysis period

depth
CAMS target
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Implications of depth effect

• Implications
• BeO in Nb has no depth effect
• BeO in Ag has a significant depth effect
• currents for samples in Ag matrix can be improved
  (with limited practical value)
• Nb and Ag exhibit a different response
• Is the Nb enhancement related to the depth
  effect?

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More BeO ion beam currents
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Matrix enhancement, part 2
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AMS counting efficiency for BeO
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Correlation to matrix properties
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Interpretation of matrix effects

• Observations
• Nb is not a magic powder all of the tested
  matrices provide some level of BeO signal
  enhancements
• Low e.a. is important (and low k and f)
• The mixing ratio effect is important for
  optimization
• The presentation depth effect may be important
• Practical recommendations
• Effect of matrix mole ratio
• Optimal cNb between 0.5 and 0.65 (41 to 71 bm)
• Achieve high beam currents with less BeO
• Effect of presentation depth
• In a Nb matrix, packing depth is not significant

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Al2O3 ion beam currents
cmatrix
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More Al2O3 ion beam currents
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Correlation to matrix properties
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Elemental Al
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Correlation to matrix properties
R20.70
R295
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Future work for Al

• Correlate cathode composition with current for Al
• Separation of elemental Al
• Better characterization of ionization
• Analysis of cathodes post-AMS analysis (physical
  and/or chemical)

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Acknowledgments

• Committee members
• Petrucci group
• Bierman group
• LLNL CAMS group
• DOD-EPSCoR

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Effect of analyte matrix ratio
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Titanium in the ion source
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osmium beam currents elemental properties
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AMS counting efficiency for Al2O3
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Matrix enhancement
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Initial digestion

• Sample contains
• native elements
• Be (and sometimes Al carrier)

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Anion exchange separation

• Separate major interferences
• soluble ferric and titanic ions

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Selective precipitation (low pH)
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A brief history of ultra-radiation

• Observation Radioactive species (a,b,c)
• Hypothesis They come from Earth
• Experiment Electrometry at different altitudes
• Results They come from Space

Austria (1912)