C - AMS. Introduction Identification of 14 C. Remarks for users

Transcription

1 C - AMS Introduction Identification of C C concentrations Remarks for users AMS Accelerator Mass Spectrometry a method for measuring very small isotopic ratios very small => radioisotopes

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

3 Basic Considerations C is a radionuclide, why not counting the radioactive decay? sample with 1 mg C => 5.0 * C atoms modern sample, i.e. C/C = => 5.0 *10 7 C atoms half life 5730 yrs => decay probability 3.9 * s -1 for the 1 mg modern sample 0.7 decays / h low statistical error counts => 4 years mass spectrometry (MS) does not wait for decay!!! 10 µa current = 6.2 * ions/s => 62 C ions/s

4 Limitation of classical MS isobaric ions C + = N + higher charged ions 28 Si 2+ molecular ions 12 CH + 2 resolution (tailing) 10-5 level C intensity other background note: single ones of these problems can be overcome, but not all of them simultaneously

5 C - AMS Introduction Identification of C C concentrations Remarks for users

6 Limitation of classical MS isobaric ions C + = N + higher charged ions 28 Si 2+ molecular ions 12 CH + 2 resolution (tailing) 10-5 level C intensity other background isobaric ions higher charged ions molecular ions other background note: single ones of these problems can be overcome, C intensity but not all of them simultaneously

7 Example of a mass spectra isobaric ions negative ions no N - no 28 Si 2- higher charged ions molecular ions other background C intensity Fig. R.Beukens, Radiocarbon after four decades, Springer-Verlag, 1992

8 Molecular ions ions fly in vacuum (10-6 mbar) but hit matter at the stripper isobaric ions Fig. from P. Person et al., NIM A500 (2003) 55 higher charged ions molecular ions other background C intensity

9 Example C with 2.4 MeV in argon Fig. from M. Kiisk et al. NIM A481 (2002) 1 isobaric ions higher charged ions molecular ions other background C intensity

10 12 C charge states as function of energy (equilibrium thickness) isobaric ions higher charged ions molecular ions molecular ions in 3+ charge state break off no background contribution used at the Jena AMS system other background C intensity

11 Insertion: energy of ions Highest efficiency for 12 C 3+ is at 2.5 MeV 1 ev = J 1 MeV = J isobaric ions higher charged ions molecular ions other background C intensity

12 Insertion: energy of ions Highest efficiency for 12 C 3+ is at 2.5 MeV 1 ev = J 1 MeV = J isobaric ions higher charged ions molecular ions other background C intensity

13 Insertion: energy of ions isobaric ions higher charged ions molecular ions Second aim of the stripper: gaining energy other background C intensity

14 Second way: molecules are also destroyed by impacts with stripper atoms/molecules thickness > equilibrium. thickness for q side effects: energy straggling angular straggling so why? isobaric ions higher charged ions molecular ions a) charge 3+ b) thick stripper other background C intensity

15 isobaric ions higher charged ions molecular ions a) charge 3+ no molecules no 3+ charge state not 2.5 MeV AMS systems with terminal voltages of 500 or 250 kv!!! (less costes, less space, less ion optical elements) b) thick stripper other background C intensity

16 Other backgrounds the start solved up to 10-5 higher energy reduces tails of peaks isobaric ions higher charged ions molecular ions but other background C intensity

17 example for background: isobaric ions higher charged ions molecular ions other background two unlikely processes C intensity background before last magnet

18 Detector energy loss of ions in matter depends on the ion & its energy isobaric ions higher charged ions molecular ions other background C intensity

19 Detector ionisation chamber signal ~ energy loss isobaric ions more precise: signal ~ energy loss integral higher charged ions molecular ions other background C intensity

20 Detector identification of ions in (ΔE, E res ) measurements isobaric ions higher charged ions molecular ions other background C intensity

21 Detector isobaric ions higher charged ions molecular ions other background single ion counting C intensity no problem C intensity

22 C - AMS Introduction Identification of C C concentrations Remarks for users

23 C concentrations lesson learned: identification of C wanted C concentrations = C / 12 C trick: measurement of the 12 C current like weighing paper-clips instead of counting them

24 C concentrations lesson learned: identification of C wanted C concentrations = C / 12 C trick: measurement of the 12 C current C concentration Cevents I C ()d t t 12 sensitivity of AMS 10µA (q = 1) ~ 6.2 * ions/s = 2.2 * ions/h

25 classical Set-up of AMS 1-3+ counting 12 C by current measurement (1µA = 6.2*10 12 e/s) but different positions of 12 C and C measurements

26 dedicated C-AMS set-up this scheme and the terminal voltage corresponds to the Jena AMS facility

27 C - AMS Introduction Identification of C C concentrations Remarks for users

28 Remarks for the user o A C result is not a number! Its a value and an uncertainty!!! o For normal requirements the AMS system do not matter. o Chemical pretreatment more important. o What is normal? feature uncertainty of modern samples background of processed sample sample mass normal 0.5 pmc 0.4 pmc 1 mg very good 0.25 pmc 0.2 pmc 10 µg

29 0.5 pmc 100 pmc = measurement time (knowing I) N N % N N N N Same measurement time, same currents, other concentration C S N S C S 100 pmc uncertainty

30 curve uncertainty Black pMC Red pMC

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

32 Technical Specifications General: Ion Sources: Model 4130-Tandetron, High Voltage Engineering Europa (HVEE) 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, separation electrostatic analyser 90 analyzing magnet C ionization detector

33 W. Kutschera, NIM B50 (1990) 252 A.E. Litherland, NIM 186 (1981) 463

34 application of energy loss detection E de dx dx Bethe-Bloch equation: de dx 2 Z A E