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 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 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
Strategy for isotope quantification
Long-lived radioisotopes
Nuclide
t1/2
Z (p)
N (n)
9Be
-----stable----1.51 Ma
4
4
5
6
36Cl
0.70 Ma
-----stable----5.73 ka
0.30 Ma
13
13
6
17
13
14
8
19
129I
15.7 Ma
53
76
10Be
26Al
27Al
14C
Pleistocene Age
• 1.806 Ma to 11 ka before now
• Latest period of glaciation
Significance
10Be
and 26Al 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
•
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
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
10Be-
and 26Al-specific challenges
Challenge
Solution (nominal)
Be is not native to
quartz
Al is native
(feldspars)
Add stable Be
carrier
Acid leech and
monitor [Al]
Common extraction
scheme
Stability of Be anion Accelerate BeOChemical behavior
Be isobars (B)
Al isobars (Mg)
Minimize B
exposure
Accelerate Al-
Data courtesy of Middleton, R. A Negative Ion Cookbook, University of Pennsylvania
Al2O3
current
product
(mA)
AlO-
20-40
AlO2-
4-6
Al-
1
Analysis of rare isotope abundances
Steps of the analytical method
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
AMS block diagram
Center for Accelerator Mass
Spectrometry (CAMS) at LLNL
(3)
(2)
(1)
(5)
(4)
Photographs courtesy of Lawrence Livermore National Laboratories
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/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
Figures of merit
10
Cosmogenic Be abundance
Sensitivity
1E-13
1E-14
Old Blanks
1E-15
(26Al)
1E-16
1997
New Blanks
0
50
100
Blank #
150
2007
Figures of merit
Significance
(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)
 Poisson
1

n
 Poisson
1
 3% 
1100
Throughput
• BeO cathode ~ 10 min
• Al2O3 cathode ~ 30 min
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
ion beam current (mA)
AMS ion beam currents (BeO-)
25
20
Value (mA)
Mean=13.1
Median=13.4
Minimum=1.6
Maximum=25.1
15
10
5
0
0
10
20 30 40 50
sample (n=64)
60
Elemental analysis
Al
Fe
concentration in quartz (ppm)
400
Ti
Ca
Mg
Na
K
(a) native quartz
300
200
100
75th %
400
25th %
(b) BeO cathodes
300
200
100
0
Al
Be
Fe
Ti
Mn
Ca
Mg
Theoretical ion beam current (mA)
Effect of elemental composition on
BeO- ion beam currents
25
20
mg
a
Al -0.02
Be 0.03
Ca 0.03
Fe -0.05
Mg 0.03
Mn -0.02
Ti -0.02
15
10
5
0
0
b
0.01
0.01
0.05
0.12
0.12
0.06
0.01
c
-4.15
2.77
0.70
-0.46
0.24
-0.25
-4.03
5
10 15 20 25 30
Predicted ion beam current (mA)
d
0.00
0.01
0.49
0.64
0.81
0.80
0.00
Notation
• parameter
estimate
• standard
error
• t ratio
• Prob>t.
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
Separation methods: Part I
Separation methods: Part II
Fraction of net mass of element recovered
Anion exchange chromatography
(a)
(b)
0.75 Al
0.50
0.25
Parameters
• cv: 20 mL
• Resin type: AG X18
• Elution rate: 1 drop/s
• (a) 8 M HCl
• (b) 1.2 M HCl
0.75 Be
0.50
0.25
0.75 Fe
0.50
0.25
0.75 Ti
0.50
0.25
0.00
0
1
2
Column volumes eluted
3
pH selective precipitation
Low pH (3.8-4.1)
High pH (~8.5)
Cation exchange chromatography
Fraction of net mass of element recovered
(a)
(b)
(c) (d)
0.75 Be
0.50
0.25
0.75 Al
0.50
0.25
0.75 Mg
0.50
0.25
0.75 Ca
0.50
0.25
0.75 B
0.50
0.25
0.75 Ti
0.50
0.25
0.00
0
5
10
15
Column volumes eluted
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
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
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
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 (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
Stability for Nb:BeO cathodes
(post Ag)
i (μA)
rsd
t=0-660 s
13990
3.59%
t=30-330 s
6380
2.76%
BeO- beam currents
Effect of analyte: matrix ratio
mass BeO (mg)
mass BeO (mg)
1.08
0.79
0.48
0.17
0.94
0.71
0.48
0.28
0.09
10
4
10
4
10
3
10
3
10
1.0
2
cAg
cNb
10
2
0.5
0.6
ion current (mA)
ion current (mA)
1.35
0.7
0.8
0.9
1.0
0.5
0.6
0.7
0.8
0.9
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
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
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
Al2O3 ion beam currents
cmatrix
More Al2O3 ion beam currents
Correlation to matrix properties
Elemental Al
Correlation to matrix properties
R2=0.70
R2=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
5
10
4
10
3
1.24
cMo
ion current (mA)
0.5
10
5
10
4
10
3
0.15
ion current (mA)
10
mass BeO (mg)
0.97
0.7
0.43
0.6
0.7
0.8
0.9
mass BeO (mg)
1.33 1.05 0.74
1.59
0.6
0.7
0.8
5
10
4
10
3
0.4
cV
0.5
10
0.9
1.0
10
5
10
4
10
3
mass BeO (mg)
0.54 0.36
0.2
0.74
0.07
cTa
0.5
1.0
ion current (mA)
ion current (mA)
Effect of analyte: matrix ratio
0.6
0.7
0.8
0.9
mass BeO (mg)
1.03 0.75 0.45
1.3
1.0
0.16
cW
0.5
0.6
0.7
0.8
0.9
1.0
Titanium in the ion source
ion beam current ( m A)
TiO2 impurity
3.00
2.50
2.00
1.50
1.00
0.50
0.00
0
500
1000
time (s)
10% TiO2 impurity
pure Al2O3
1500
osmium beam currents &
elemental properties
25
10
Os
9
Os
8
Ag
Nb
Os
7
6
15
1:1
10
2.3:1
5
9:1
value
ion current (mA)
20
5
4
3
2
1
0
0
100 200 300 400 500 600
time (s)
0
e.a. (ev)
k (W/cmK) f (eV)
r (n ) DHvap (J/g)
AMS counting efficiency for Al2O3
Matrix enhancement
Initial digestion
• Sample contains
– native elements
– Be (and sometimes Al carrier)
Anion exchange separation
• Separate major interferences
– soluble ferric and titanic ions
Mass ratio (supernatant/precipitate)
Selective precipitation (low pH)
10
2
10
1
10
0
10
-1
10
-2
Be
Mg
Ca
Al
Ti
Fe
10
-3
3.0
3.5
4.0
pH
4.5
A brief history of “ultra-radiation”
Victor Hess
(1936)
• Observation: Radioactive species
(a,b,c)
• Hypothesis: They come from Earth
• Experiment: Electrometry at
different altitudes
• Results: They come from Space
Austria (1912)