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Outline of dissertation defense

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Outline of dissertation defense

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  1. 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)”byAdam Lewis HuntIn Partial Fulfillment of the Requirementsfor the Degree of Doctor of PhilosophySpecializing in Chemistryat the University of Vermont

  2. 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

  3. Strategy for isotope quantification

  4. Long-lived radioisotopes Pleistocene Age • 1.806 Ma to 11 ka before now • Latest period of glaciation

  5. Principle sources Cosmogenesis Meteoric or “garden variety” (atmospheric) In situ terrestrial substrate (< 2 m) Negative muon capture (> 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 10Be and 26Al production Significance

  6. (b) Variables Exposure history Erosion history 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

  7. 10Be- and 26Al-specific challenges Data courtesy of Middleton, R. A Negative Ion Cookbook, University of Pennsylvania

  8. 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 Analysis of rare isotope abundances Steps of the analytical method

  9. AMS block diagram

  10. Center for Accelerator Mass Spectrometry (CAMS) at LLNL (2) (1) (3) (5) (4) Photographs courtesy of Lawrence Livermore National Laboratories

  11. 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 Principles of AMS operation

  12. 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

  13. Figures of merit Sensitivity (26Al) 1997 2007

  14. Accuracy Systematic errors (uncertainty in…) Production rate Latitude/longitude scaling Geomagnetic/solar modulation (temporal) Assigned constants Precision External error (rep>3x) Internal error (Poisson) Throughput BeO cathode ~ 10 min Al2O3 cathode ~ 30 min Figures of merit(continued) Significance

  15. 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

  16. AMS ion beam currents (BeO-) Value (mA) Mean=13.1 Median=13.4 Minimum=1.6 Maximum=25.1

  17. Elemental analysis 75th % 25th %

  18. Effect of elemental composition on BeO- ion beam currents Notation • parameter estimate • standard error • t ratio • Prob>t.

  19. 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 Concerning the extraction of BeO and Al2O3 from quartz

  20. Separation methods: Part I

  21. Separation methods: Part II

  22. 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

  23. pH selective precipitation Low pH (3.8-4.1) High pH (~8.5)

  24. 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

  25. Key steps in quartz extraction Dissolution by multi-acid digestion • 2H3O+ + [TiF6]2- <=> 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

  26. 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

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

  28. 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 2.5 mm 1 mm Meyhoefer curette

  29. Stability for Nb:BeO cathodes (post Ag)

  30. BeO- beam currents

  31. Effect of analyte: matrix ratio

  32. 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

  33. 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?

  34. More BeO ion beam currents

  35. Matrix enhancement, part 2

  36. AMS counting efficiency for BeO

  37. Correlation to matrix properties

  38. 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 (4:1 to 7:1 bm) • Achieve high beam currents with less BeO • Effect of presentation depth • In a Nb matrix, packing depth is not significant

  39. Al2O3 ion beam currents cmatrix

  40. More Al2O3 ion beam currents

  41. Correlation to matrix properties

  42. Elemental Al

  43. Correlation to matrix properties R2=0.70 R2=95

  44. 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)

  45. Acknowledgments • Committee members • Petrucci group • Bierman group • LLNL CAMS group • DOD-EPSCoR

  46. Effect of analyte: matrix ratio

  47. Titanium in the ion source

  48. osmium beam currents & elemental properties

  49. AMS counting efficiency for Al2O3

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