Labor of Ion Beam Physics, ETHZ Radiocarbon AMS at 45 keV Martin Seiler Labor of Ion Beam Physics, ETHZ PhD seminar 2011 29.08.2011

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

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

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

Introduction Cultural heritage

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

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

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

Introduction to AMS Ion beam in gas: Introduction Ion charge: -1 1+ 2+ Stripper gas atom 14C electron

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

Helium Stripping (80-140 keV) Introduction Helium Stripping (80-140 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

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

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

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

Theory about Background Background analysis Theory about Background Neutralization zone

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

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

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

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

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

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

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

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

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

Storage Unused Slides Additional Material

SSAMS Overview

SSAMS

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

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

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

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

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