1. Labor of Ion Beam Physics, ETHZ
Radiocarbon AMS at 45 keV
Martin Seiler
Labor of Ion Beam Physics, ETHZ
PhD seminar 2011

2. Overview Introduction μCADAS – 45 keV Radiocarbon MS
Background analysis
Measured samples

3. Radiocarbon 14C Other isotopes decay in seconds 14C -> 14N + β- + ν
Introduction
Radiocarbon 14C
Other isotopes decay in seconds
14C -> 14N + β- + ν
T1/2(14C)=5730 years
Almost the same chemical behavior
Isotope
12C
13C
14C*
Natural Abundance
0.99
0.01
10-12

4. Radiocarbon Applications
Introduction
Radiocarbon Applications
Dating (archaeology)
Geology
Environmental research
Biology / pharmacy

5. Introduction
Cultural heritage

6. AMS AMS = Accelerator mass spectrometry Measurement of isotope ratios
Introduction
AMS
AMS = Accelerator mass spectrometry
Measurement of isotope ratios
Detection limit of rare isotopes ~10-15
Acceleration voltage: 200 kV – 6 MV
Several filter elements for mass separation

7. Timeline of Radiocarbon AMS
Introduction
Timeline of Radiocarbon AMS
MP-Tandem
AMS System
Rochester
EN-Tandem AMS Systems:
ETH, Oxford, Lower Hutt, Utrecht, Erlangen,….
FN-Tandem
AMS System
MacMaster University
HVEE-Tandetron (Purser)
AMS Systems:
Woods Hole, Groningen, Kiel,…
IONEX (Ken Purser)
Arizona, Oxford, Gif-sur.-Yvette,….
Purser-AMS System
ETH-“MICADAS” AMS Systems
Zurich, Davis, Mannheim, Debrecen, Seville,….
200 kV PS (vacuum insulated)
ETH-μCADAS
ETH-“Tandy”(Compact)-AMS Systems:
Zurich, Georgia, Poznan, Irvine…NEC 500 kV Pelletron ?
SSAMS Systems (NEC)
Lund, ANU, SUERC,… 250 kV HV-deck
45 keV

8. Stripping Gas stripper Areal density d on beam axis
Introduction
Stripping
Gas stripper
Areal density d on beam axis
Common stripper gases: nitrogen, argon
stripper gas

9. Introduction to AMS Ion beam in gas: Introduction Ion charge: -11+ 2+
Stripper gas atom 14C electron

10. Stripping Gas stripper Changing charge state of ions
Introduction
Stripping
Gas stripper
Changing charge state of ions
Dissociating molecules
Angular straggling
Common stripper gases: nitrogen, argon
stripper gas

11. Helium Stripping (80-140 keV)
Introduction
Helium Stripping ( keV)
Molecular dissociation:
Cross section σ=4*10-16 cm2
Required areal density ~0.5 μg/cm2
Charge state yield:
> 50 % in 1+
Up to 70 % in 1+ at lower energies
Reference: T. Schulze-König et al., Nucl. Instr. Meth. B 269 (2011) 34

12. μCADAS
MICADAS
μCADAS-Design
2.5 m
Remove
Accelerator!
2 m
3 m

13. μCADAS μCADAS Analyzing magnet Injection magnet with beam switching
Stripper
Electrostatic analyzer
Detector
Ion source
45 kV

14. μCADAS
First Measurement
14C identified with enriched sample (100-times modern)
14C/12C=10-10
Blank level:
14C/12C=10-12
Not caused by molecules
13C
14C

15. Theory about Background
Background analysis
Theory about Background
Neutralization zone

16. Checking the Theory Check in the injection magnet
Background analysis
Checking the Theory
Check in the injection magnet
Apply an offset voltage at the vacuum vessel of the magnet
Energy change of neutralized particles
Separation with the ESA

17. Energy spectrum at different offset voltages
Background analysis
ESA Scans
Energy spectrum at different offset voltages
12C[H2] 13C[H] 13C C
neutralized:
13C[H] C,13C
events in the detector ?
14C
particle energy / keV

18. Negative Offset Voltage
Background analysis
Negative Offset Voltage
Lower energy for neutralized particles
12C, 13C, 13C[H], 12C[H2]: m < 14 u
Improved separation in the analyzing magnet
May pass with charge changing

19. Background analysis
ESA Scan at -9 kV
Blank: 14C/12C ~ 5*10-15

20. Background analysis
Background analysis
13C
14C
13C (scattered)

21. Measurement Program Sequential measurement Several passes
Measured Samples
Measurement Program
Sequential measurement
Several passes
Blank correction
Normalization (oxalic acid 2, 134 pMC)
Total measurement time ~1800 s/ sample
Sample list of magazine C110114C
Position
Label
Comment
Type 1 X
--- 2
OX100-1
OX100 3
OX12-1
OX12 4 5 6 7 8 9
OX2-1
Neu
OXA2 10
BL-1 BL 11
C3-1
129.4 pMC C3 12
OLDBL 13
C3-2 14
OX2-2 15
BL-2 16 C4
<0.44 pMC (BL) 17 C6
150.6 pMC 18 C7
49.53 pMC 19
OX2-3 20
BL-3 21
C8-1
15.03 pMC C8 22
C8-2 23
OX2-4

22. Measured Samples Measured Samples
Messzeit pro Probe etwa 30 min (18 cycles), Strom HE: 16 uA C4 zu nahe am Blank

23. Summary Molecular dissociation possible at 45 keV
Scattered particles can be separated
Blank measured at an age of 40‘000 years

24. Thank you for your attention Questions?
End
Thank you for your attention Questions?

25. Storage
Unused Slides
Additional Material

26. SSAMS Overview

27. SSAMS

28. Reference: T. Schulze-König et al., Nucl. Instr. Meth. B 269 (2011) 34
Einführung
Cross Sections
Reference: T. Schulze-König et al., Nucl. Instr. Meth. B 269 (2011) 34

29. Measured Samples Background reduction
Messzeit pro Probe etwa 30 min (18 cycles), Strom HE: 16 uA

30. Adapting the Offset Voltage
Background reduction
Adapting the Offset Voltage
Moving the peaks by changing the offset voltage
Aim:
No peaks in the 14C-window
Achieved background:
14C/12C = 10-14

31. Separation of the Background
Background reduction
Separation of the Background
Positive offset voltage
Higher energies for neutralized particles
12C, 13C, 13C[H], 12C[H2]: m < 14
Higher energy changes momentum towards the value of 14C
Worse separation in the magnet

32. Energy spectrum at different offset voltages
Background reduction
Highest Energy
Energy spectrum at different offset voltages
12C (p/q)
events in the detector
particle energy / keV
~8 keV