Measuring 14 C concentrations with AMS AMS Accelerator

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Measuring 14 C concentrations with AMS ( AMS Accelerator Mass Spectrometry )

Measuring 14 C concentrations with AMS ( AMS Accelerator Mass Spectrometry )

What is special about 14 C ? • Produced in upper atmosphere • Mixed

What is special about 14 C ? • Produced in upper atmosphere • Mixed effectivly in the atmosphere • Uniform concentration in biosphere • Radioactive => possibility of dating (t 1/2 = 5730 yrs) • Difficulty: concentration low modern 14 C/C = 10 -12

Basic Considerations 14 C is a radionuclide, why not counting the radioactive decay?

Basic Considerations 14 C is a radionuclide, why not counting the radioactive decay?

Basic Considerations 14 C is a radionuclide, why not counting the radioactive decay? sample

Basic Considerations 14 C is a radionuclide, why not counting the radioactive decay? sample with 1 mg C => 5. 0 * 1019 C atoms modern sample, i. e. 14 C/C = 10 -12 => 5. 0 *107 14 C atoms half life 5730 yrs => decay probability 3. 9 * 10 -12 s-1 Þ for the 1 mg modern sample low statistical error 50000 counts 0. 7 decays / h => 4 years

In mass spectrometry (MS): • the sample is atomized and ionized, • the ions

In mass spectrometry (MS): • the sample is atomized and ionized, • the ions mass separated (M/q), and • the intensities measured. => MS waits not for decay !!! 10 µA current = 6. 2 * 1013 ions/s => 62 14 C ions/s object could be ion source or aperture (slit, hole)

Limitation of MS § isobaric ions 14 C+ = 14 N+ § higher charged

Limitation of MS § isobaric ions 14 C+ = 14 N+ § higher charged ions 28 Si 2+ § molecular ions 12 CH + 2 § resolution (tailing) § 14 C 10 -5 level intensity ( current measurement require 1 p. A = 6. 2*106 e-/s) § background events due to scattering or charge exchange note: single ones of these problems can be overcome, but not all of them simultaneously

Example of a mass spectra negative ions Þ no 14 N- Þ no 28

Example of a mass spectra negative ions Þ no 14 N- Þ no 28 Si 2 - Fig. R. Beukens, Radiocarbon after four decades, Springer-Verlag, 1992

Limitation of MS solution of AMS § isobaric ions 14 C+ = 14 N+

Limitation of MS solution of AMS § isobaric ions 14 C+ = 14 N+ neg. ions § higher charged ions 28 Si 2+ neg. ions § molecular ions § 12 CH + 2 resolution (tailing) 10 -5 level § 14 C intensity background event due to scattering or charge exchange

In the sum e- are stripped off => name device „stripper“ 2 tasks: supression

In the sum e- are stripped off => name device „stripper“ 2 tasks: supression of molecules (no 3+ molecule) gaining more energy (next slide) note: 1µg/cm 2 = 5. 6 x 10 -3 mbar m Fig. from M. Kiisk et al. NIM A 481 (2002) 1 Ions fly normally in vacuum (HV, 10 -6 mbar). One interaction of ions with matter is charge exchange.

Second aim of the stripper: gaining energy C- large energy helpful for detector (see

Second aim of the stripper: gaining energy C- large energy helpful for detector (see later) C 3+

Limitation of MS solution of AMS § isobaric ions 14 C+ = 14 N+

Limitation of MS solution of AMS § isobaric ions 14 C+ = 14 N+ neg. ions § higher charged ions 28 Si 2+ neg. ions § molecular ions 12 CH + 2 § resolution (tailing) 10 -5 level stripper several magnets, ESA, high energy § 14 C intensity § background event due to scattering or charge exchange single ion counting identification of nuclide by (ΔE, E) measurement

detector has splitted anode ΔE & ER signals è identification of nuclid (Z &

detector has splitted anode ΔE & ER signals è identification of nuclid (Z & A) if energy is high enough, together with e. g. analyzing magnet

Interaction of ion with matter: energy loss • in the stripper small, • important

Interaction of ion with matter: energy loss • in the stripper small, • important in the detector in ionisation chamber signal ~

Interaction of ion with matter: energy loss • in the stripper small, • important

Interaction of ion with matter: energy loss • in the stripper small, • important in the detector in ionisation chamber signal ~ Þidentification of ion in (ΔE, Eres) measurements

Note: 1 D- spectrum of Eres would be sufficient

Note: 1 D- spectrum of Eres would be sufficient

Limitation of MS solution of AMS § isobaric ions 14 C+ = 14 N+

Limitation of MS solution of AMS § isobaric ions 14 C+ = 14 N+ neg. ions § higher charged ions 28 Si 2+ neg. ions § molecular ions 12 CH + 2 § resolution (tailing) 10 -5 level stripper several magnets, ESA, high energy § 14 C intensity § background event due to scattering or charge exchange single ion counting identification of nuclide by (ΔE, E) measurement

dedicated 14 C-AMS set-up this scheme and the terminal voltage correspond to the Jena

dedicated 14 C-AMS set-up this scheme and the terminal voltage correspond to the Jena AMS facility

not the only solution „thick“ stripper reduces molecules 3· 1016 instead of 4· 1015

not the only solution „thick“ stripper reduces molecules 3· 1016 instead of 4· 1015 atoms/cm² è no 3+ charge state required è not 2. 5 MV but 250 -500 k. V are sufficient è „new“ generation of „small“ instruments (since 2000)

Example of the new machines: MICADAS less components => less maintenance similar background, higher

Example of the new machines: MICADAS less components => less maintenance similar background, higher efficiency ground floor is 3 x 2. 5 m² in Jena starting in 2017

Scheme of the period with parallel installation and operation

Scheme of the period with parallel installation and operation

Thank you for your attention. Welcome to the tour through the Jena AMS facility

Thank you for your attention. Welcome to the tour through the Jena AMS facility ! Stay outside the blue floor !

Technical Specifications General: Model 4130 -Tandetron, High Voltage Engineering Europa (HVEE) Ion Sources: 2

Technical Specifications General: Model 4130 -Tandetron, High Voltage Engineering Europa (HVEE) Ion Sources: 2 sources, both Cs-ion-sputter sources Model 846 with 59 samples load capacity (only solid samples) Model SO 110 with 200 samples load capacity (solid and gaseous) Recombinators: two four-magnet recombinators (one for each source) chopper wheel on mass 12 position Accelerator: terminal voltage 2. 5 MV (in operation), 3 MV (nominal) parallel-fed Cockroft Walton generator High-Energy Beamline: 110° analyzing magnet for 12, 13, 14 separation electrostatic analyser 90° analyzing magnet 14 C ionization detector

Scheme of the ion source

Scheme of the ion source