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

What is special about 14C ? Produced in upper atmosphere Mixed effectivly in the atmosphere Uniform concentration in biosphere Radioactive => possibility of dating (t1/2 = 5730 yrs) Difficulty: concentration low modern 14C/C = 10-12

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

Basic Considerations 14C is a radionuclide, why not counting the radioactive decay? sample with 1 mg C => 5.0 * 1019 C atoms modern sample, i.e. 14C/C = 10-12 => 5.0 *107 14C atoms half life 5730 yrs => decay probability 3.9 * 10-12 s-1 for the 1 mg modern sample 0.7 decays / h low statistical error 50000 counts => 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 14C ions/s object could be ion source or aperture (slit, hole)

Limitation of MS isobaric ions 14C+ = 14N+ higher charged ions 28Si2+ molecular ions 12CH2+ resolution (tailing) 10-5 level 14C intensity ( current measurement require 1 pA = 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 no 14N- no 28Si2- Fig. R.Beukens, Radiocarbon after four decades, Springer-Verlag, 1992

Limitation solution of MS of AMS isobaric ions 14C+ = 14N+ neg. ions higher charged ions 28Si2+ neg. ions molecular ions 12CH2+ resolution (tailing) 10-5 level 14C intensity 14C intensity background event due to scattering or charge exchange

Ions fly normally in vacuum (HV, 10-6 mbar). One interaction of ions with matter is charge exchange. Fig. from M. Kiisk et al. NIM A481 (2002) 1 note: 1µg/cm2 = 5.6 x 10-3 mbar m In the sum e- are stripped off => name device „stripper“ 2 tasks: supression of molecules (no 3+ molecule) gaining more energy (next slide)

C- C3+ Second aim of the stripper: gaining energy large energy helpful for detector (see later)

Limitation solution of MS of AMS isobaric ions 14C+ = 14N+ neg. ions higher charged ions 28Si2+ neg. ions molecular ions 12CH2+ stripper resolution (tailing) 10-5 level several magnets, ESA, high energy 14C intensity single ion counting background event due to identification of nuclide scattering or charge exchange by (ΔE, E) measurement

detector has splitted anode  ΔE & ER signals 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 ~ identification of ion in (ΔE, Eres) measurements

Note: 1D- spectrum of Eres would be sufficient

Limitation solution of MS of AMS isobaric ions 14C+ = 14N+ neg. ions higher charged ions 28Si2+ neg. ions molecular ions 12CH2+ stripper resolution (tailing) 10-5 level several magnets, ESA, high energy 14C intensity single ion counting background event due to identification of nuclide scattering or charge exchange by (ΔE, E) measurement

dedicated 14C-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 no 3+ charge state required not 2.5 MV but 250-500 kV are sufficient „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 SO110 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 14C ionization detector

Scheme of the ion source