A presentation in defense of the dissertation entitled “ANALYSIS OF FACTORS THAT AFFECT ION BEAM CURRENTS FOR COSMOGENIC 10 Be AND 26 Al ANALYSIS BY ACCELERATOR MASS SPECTROMETRY (AMS)” by Adam Lewis Hunt In Partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy Specializing in Chemistry at the University of Vermont

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

Strategy for isotope quantification

Long-lived radioisotopes Nuclidet 1/2 Z (p)N (n) 9 Be-----stable Be1.51 Ma46 26 Al0.70 Ma13 27 Al-----stable C5.73 ka68 36 Cl0.30 Ma I15.7 Ma5376 Pleistocene Age Ma to 11 ka before now Latest period of glaciation

10 Be and 26 Al production Principle sources Cosmogenesis –Meteoric or “garden variety” (atmospheric) –In situ terrestrial substrate (< 2 m) Negative muon capture (> 2 m) Radiogenesis Interstellar protons Principle mechanism 10 Be (5.2 atoms g -1 a -1 ) – 16 O(n,4p3n) 10 Be – 28 Si(n,6p3n)2 10 Be 26 Al (30.1 atoms g -1 a -1 ) – 28 Si(n,p2n) 26 Al Significance

Dual analysis of cosmogenic nuclides (a) Constants (relatively) Nuclide half-life Nuclide production rate Substrate density Attenuation length for neutrons (b) Variables Exposure history Erosion history Nuclide activity is a function of:

Al 2 O 3 product current (  A) AlO AlO Al - 1 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 - 10 Be- and 26 Al-specific challenges Data courtesy of Middleton, R. A Negative Ion Cookbook, University of Pennsylvania

Analysis of rare isotope abundances Conventional MS 1.Formation of atomic and/or molecular ions 2.Acceleration through electrostatic potential (~kV) 3.Separation of ions based on m/z 4.Measurement of ions in detector Accelerator MS 1.Formation of negative atomic and/or molecular ions 2.Acceleration through electrostatic potential (~kV) 3.Acceleration to MeV energies 4.Separation of ions based on m/z 5.Measurement of ions in detector Steps of the analytical method

AMS block diagram

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

Principles of AMS operation 1.Formation of negative atomic and/or molecular ions Cs + sputter source Negative ions Source geometry “Ion Sourcery” 2.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 (continued) 4. Separation of ions based on m/z Magnetic analyzer (ME/q 2 ) 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.  E

Figures of merit ( 26 Al) Sensitivity

Figures of merit (continued) 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 Al 2 O 3 cathode ~ 30 min Significance

Investigatory Aims Observations 10 Be analysis is typically limited by B 26 Al analysis is typically limited by ion beam currents Ultimate goals Improve cosmogenic 10 Be and 26 Al analysis with better wet & analytical chemistry Improve precision for challenging samples Strategy to improve 10 Be and 26 Al 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

AMS ion beam currents (BeO - ) Value (  A) Mean=13.1 Median=13.4 Minimum=1.6 Maximum=25.1

Elemental analysis 75th % 25 th %

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

Concerning the extraction of BeO and Al 2 O 3 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

Separation methods: Part I

Separation methods: Part II

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

pH selective precipitation Low pH ( ) High pH (~8.5)

Cation exchange chromatography Parameters cv: 10 mL resin: AG 50W-X8 Rate: 1 drop/s (a) 0.5 M H 2 SO 4 (b) 1.2 M HCl (c) 3.0 M HCl (d) 6.0 M HCl

Key steps in quartz extraction Dissolution by multi-acid digestion 2H 3 O + + [TiF 6 ] 2- TiO HF Selective distillation of HF relative to HClO 4 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

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 26 Al

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

Experimental design Sample preparation Measuring: volumetric curette Mixing: BeO with metal Ratio: serial dilution with mole fraction (  matrix = 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 mm2.5 mm Meyhoefer curette

Stability for Nb:BeO cathodes t=0-660 st= s i (μA) rsd3.59%2.76% (post Ag)

BeO - beam currents

Effect of analyte: matrix ratio

Control of cathode presentation depth depth CAMS target 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

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?

More BeO ion beam currents

Matrix enhancement, part 2

AMS counting efficiency for BeO

Correlation to matrix properties

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  ) The mixing ratio effect is important for optimization The presentation depth effect may be important Practical recommendations Effect of matrix mole ratio  Optimal  Nb 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 Interpretation of matrix effects

 matrix Al 2 O 3 ion beam currents

More Al 2 O 3 ion beam currents

Correlation to matrix properties

Elemental Al

Correlation to matrix properties R 2 =0.70 R 2 =95

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)

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

Effect of analyte: matrix ratio

Titanium in the ion source

osmium beam currents & elemental properties

AMS counting efficiency for Al 2 O 3

Matrix enhancement

Initial digestion Sample contains –native elements –Be (and sometimes Al carrier)

Anion exchange separation Separate major interferences –soluble ferric and titanic ions

Selective precipitation (low pH)

Victor Hess (1936) Austria (1912) Observation: Radioactive species (a,b,c) Hypothesis: They come from Earth Experiment: Electrometry at different altitudes Results: They come from Space A brief history of “ultra-radiation”