Low energy accelerators – Compact AMS systems

1. Low energy accelerators – Compact AMS systems José María López Gutiérrez Universidad de Sevilla Centro Nacional de Aceleradores

2. Overview • A bit of history • First applications of (today) low-energy accelerators • What to do with the “old” accelerators? • Accelerator Mass Spectrometry • Decay Counting or counting atoms (AMS) • Key physics points in AMS • Accelerators • Stripping • Detectors • Problems at low energies • Where are the limits? • Challenges

3. Energies in the atomic and subatomic world J. Holmes, USPAS, January 2009

4. A bit of history • 1906: Rutherford bombards mica sheet with natural alphas and develops the theory of atomic scattering. Natural alpha particles of 1911 Rutherford publishes theory of atomic structure. • 1919: Rutherford induces a nuclear reaction with natural alphas. • ... Rutherford believes he needs a source of many MeV to continue research on the nucleus. This is far beyond the electrostatic machines then existing, but ... • 1928: Cockcroft & Walton start designing an 800 kV generator encouraged by Rutherford. • 1932: Generator reaches 700 kV and Cockcroft & Walton split lithium atom with only 400 keV protons. They received the Nobel Prize in 1951. • 1932: Van de Graaf invents a 1.5 MV accelerator for nuclear physics research. • Some years later, Van de Graaf type accelerators increase their potential to more than 10 MV and also Tandem accelerators are invented.

5. First applications of (today) low-energy accelerators • Nuclear physics: • Nuclear reactions • Nuclear energy levels • Excited levels lifetimes • Decay schemes

6. What to do with the “old” accelerators? • The energies that could be reached by the accelerators used before the 1950’s were too low for the proposed nuclear physics experiments. • New applications had to be found in order to give use to them: • Ion Beam Analysis techniques: PIXE, PIGE, RBS… • AMS • Nuclear Physics • …

7. Accelerator mass spectrometry A technique going for every time smaller accelerators

8. AMS-Pioneers Rochester A.E. Litherland K.H. Purser H.E. Gove R.P. Beukens R.P. Clover W.E. Sondheim R.B. Liebert C.L. Bennet Discovery of AMS in 1977 McMaster D.E. Nelson, R.G. Korteling, W.R. Stott. The Rochester MP Tandem accelerator (12 MV)

9. Decay Counting Nobel Prize in Chemistry 1960 • How many atoms we need for a good measurement? • N: Number of atoms • A: Activity • : Decay constant • Reasonable assumptions: • Measurement time: 106 s (12 days) • Minimum count rate: 0.01 cps • Detection efficiency: 100 % Willard F. Libby

10. Counting atoms (AMS) • With AMS the number of atoms is counted!! • N: Number of atoms • tot: Overall efficiency • T: Transmission • Typical values: • Negative ion yield ion: 0.5-30% • Instrument transmission T: 10-50% • Detection efficiency det : 100 %Total efficiency few %independent of half-life At least 4 orders of magnitude better!!!

11. Magnetic Analyzer (ME/q2) Ion source Electrostatic deflector (E/q) Magnetic deflector (ME/q2) Traditional AMS system Tandem Accelerator E = (1+q) eV EM/q2 M E,q0 E/q M/q Detection systems (E, dE/dx, v…)

12. Traditional AMS system Undercertainconditions, molecules are broken in theaccelerator stripper The use of highenergiesmakesitpossibleto use nuclear properties (likestoppingpower) to reduce interferences at the detector

13. Interferences

14. Key physics points in AMS • Sputtering ion source • Sripping process • Coulomb explosion at high AMS energies • Interactions with residual stripping gas  ambiguities on E/q and M/q • Beam analysis and transmission • Focusing • Detection system • Isobar discrimination • Similar masses and energies discrimination

15. Sputtering ion source • High efficiency, good stability, low dispersion, low memory effects. • Typical extraction energy: tens of keV • Charge state: -1 • Non-stable negative ions: • 14N- • 129Xe- • … Sample Ion beam Acceleration 10 kV Lens Ionizer Heater Cs reservoir

16. Tandem accelerators Cockcroft-Walton Higher stability Lower terminal voltages (up to 6 MV) Van de Graaf

17. Tandem accelerators VERA AMS 3 MV facility, Vienna, Austria Leibniz AMS 3 MV facility, Kiel, GER

18. Stripping • Electron-loss • Break-up of molecules • Energy straggling • Angular straggling

19. Stripping Golden rule of molecular destruction: high efficiency for charge state 3 No surviving molecules TV  2.5 MV Bonani et al. (1990) Minimum gas pressure needed for stable distribution Higher charge states result from stripping at higher energies

20. Detection system • Best option  Gas Ionization Chamber • Able to give information on total energy and energy loss. • Bethe-Bloch formula: • For heavy ions  qef instead of Zp: Eres (36Cl)  Eres (36S) E (36Cl)  E (36S)

21. Traditional 3-6 MV AMS systems Leibniz AMS 3 MV facility, Kiel, GER HZDR 6 MV Tandetron AMS facility, Rossendorf, GER ≈ 10 -15 m 20 -25 m VERA AMS 3 MV facility, Vienna, Austria

22. What if we go to smaller energies??? • Advantages: • Smaller facilities • Lower cost • Less (or no) specialized personnel needed • Conditions: • High transmission at the stripper • Good sensitivity • High reproducibility

23. Several problems arise… • Charge states 3 after stripping  very low probability • Lower charge states after stripping: “Surviving” molecules?? 330 kV [Jacob et al., 2000]

24. Several problems arise… Energy dependence of angular straggling • Lower energies • Higher angular straggling  Low beam transmission (stripping channel acceptance) • Higher energy dispersion in the beam  Difficult ion beam transmission and worst separation at the detector 2 µg /cm2stripper gas (Ar) Transmitted beamintensities

25. Several problems arise… • Possible separation at the detector? • Relevant nuclear stopping • Energy losses and dispersion at the detector window • Influence of electronic noise, etc.

26. Stripping Process Injected negative mass 14 ions q=1-, 0, 1+, 2+, 3+,.. 14C- 1 13Cq 13CHq 12Cq 14Cq 13CH- 108 12CH2q Hq 12CH2- 109 Destruction of molecular ions in q=1+ • Electron-loss • Electron capture • Break-up of molecules • Energy straggling • Angular straggling σ: dissociation cross section Charge state distribution

27. Charge state yield of 14C ions in Ar gas Compact AMS 0.2 - 1 MV Traditional AMS 2.5 - 9 MV Multiple ion gas collisions Coulomb disintegration 0 1+ 2+ 3+ 4+

28. Angular straggling • Different stripper channel design: • Shorter • Wider • Higher pumping capacity

29. Energy straggling • Design of achromatic optics Electrostatic deflector Magnetic deflector

30. gas detector electrodes "innards" 5 cm Use of specialized gas ionization chambers CREMAT preamp modules mounted directly on the anodes (Electronic noise (protons): 16 keV) E-Eres anodes CF 100 Frisch-grid Ions Cathode

31. Compact AMSSystems (1 MV- 500KV) ≈ 6 m KECK AMS facility, Irvine, USA AMSfacility, Seville, Spain ≈ 3.5 m ≈ 3 m ≈ 5 m ≈ 4.5 m 1 MV Tandetron accelerator Tandy AMS facility, Zurich, CH ≈ 6 m

32. Where are the limits? Cross sections of molecule destruction in Ar Energy dependence of angular straggling 2 µg /cm2stripper gas (Ar) • Cross sections are comparable to molecular sizes • Only weak energy dependence • @ 230 keV cross sections are about 10 % lower Transmitted beamintensities Molecular species Deal with ion beams of large divergence New concepts can be applied at stripping energies below 250 keV!!

33. Inside view of vacuum insulated acceleration system 1 m acceleration section HE acceleration section LE Stripper gas flow q=1- q=1+ Vacuum pumps

34. Compact lab-sized instrument • Designed for operator safety • No open high voltages • Easy to operate • Easy to tune • Fully automated 200-250kV- AMS systems 6.5 m 3.0 m 2.5 m 5.4 m BernMICADAS, Universtity of Bern SSAMS - High Voltage platform (open air)

36. 1 0 0 V e 1 0 M / y g r e 1 n e n o i C 4 0 . 1 1 0 . 0 1 1 9 7 5 1 9 8 0 1 9 8 5 1 9 9 0 1 9 9 5 2 0 0 0 2 0 0 5 2 0 1 0 2 0 1 5 Y e a r / A D Moore’s Law of radiocarbon AMS MP-Tandem AMS System Rochester EN-Tandem AMS Systems: ETH, Oxford, Lower Hutt, Utrecht, Erlangen,…. HVEE-Tandetron (Purser) AMS Systems: Woods Hole, Groningen, Kiel,… FN-Tandem AMS System McMaster University IONEX (Ken Purser) Arizona, Oxford, Gif-sur-Yvette,…. ETH-“MICADAS” AMS Systems Zurich, Davis, Mannheim, Debrecen, Seville,…. 200 kV PS (vacuum insulated) ETH-“Tandy”(Compact)-AMS Systems: Zurich, Georgia, Poznan, Irvine… NEC 500 kV Pelletron ? SSAMS Systems (NEC) Lund, ANU, SUERC,… 250 kV HV-deck

37. Physical properties of molecule dissociation Nitrogen stripper gas

38. Physical properties of molecule dissociation He stripper gas He areal density of ≈ 0.5μg / cm2 should be sufficient to get rid of molecules

39. Ion Scattering Beam losses due to small angle scattering Angular acceptance of stripper: max= 30 mrad 

40. Ion Scattering Beam losses due to small angle scattering Angular acceptance of stripper: max= 30 mrad 

41. ETH radiocarbon MS (μCADAS)

42. Challenges (there’s a lot of work to do!) Development of new detectors Stripping • New stripping gasses as He • Optimization of vacuum out of the stripping channels • Reduction of electronic noise through new designs • Modified detection techniques • Ion sources Sample preparation • Reduction of memory effects and cross contamination • Selection of specific chemical compounds  Combination with other techniques • Reduction of background (isobars, neighbours, molecules…) • Small samples • Liquid and gaseous samples

43. Acknowledgements Thank you very much to Hans-Arno Synal (ETH-PSI, Switzerland) Elena Chamizo (CNA) for providing me of ideas, graphics and pictures

44. Thanks for your attention!