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

2. 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

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

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

5. 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 => C ions/s
object could be
ion source or aperture (slit, hole)

6. Limitation of MS isobaric ions 14C+ = 14N+ higher charged ions 28Si2+
molecular ions CH2+
resolution (tailing) 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

7. Example of a mass spectra
negative ions
no 14N-
no 28Si2-
Fig. R.Beukens, Radiocarbon after four decades, Springer-Verlag, 1992

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

9. 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)

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

11. Limitation solution of MS of AMS
isobaric ions C+ = 14N neg. ions
higher charged ions 28Si neg. ions
molecular ions CH stripper
resolution (tailing) 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

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

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

14. 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

15. Note: 1D- spectrum of Eres would be sufficient

16. Limitation solution of MS of AMS
isobaric ions C+ = 14N neg. ions
higher charged ions 28Si neg. ions
molecular ions CH stripper
resolution (tailing) 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

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

18. not the only solution
„thick“ stripper reduces molecules ·1016 instead of 4·1015 atoms/cm²
no 3+ charge state required
not 2.5 MV but kV are sufficient
„new“ generation of „small“ instruments (since 2000)

19. 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

20. Scheme of the period with parallel installation and operation

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

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

23. Scheme of the ion source