Measuring 14 C concentrations with AMS AMS Accelerator
- Slides: 23
Measuring 14 C concentrations with AMS ( AMS Accelerator Mass Spectrometry )
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? 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 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 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 Si 2 - Fig. R. Beukens, Radiocarbon after four decades, Springer-Verlag, 1992
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 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 later) C 3+
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 & A) if energy is high enough, together with e. g. analyzing magnet
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 in the detector in ionisation chamber signal ~ Þidentification of ion in (ΔE, Eres) measurements
Note: 1 D- spectrum of Eres would be sufficient
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 AMS facility
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 efficiency ground floor is 3 x 2. 5 m² in Jena starting in 2017
Scheme of the period with parallel installation and operation
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 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
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