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

2. OPAC School. London, July 10th 2014 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. OPAC School. London, July 10th 2014 Energies in the atomic and subatomic world J. Holmes, USPAS, January 2009

4. OPAC School. London, July 10th 2014 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. OPAC School. London, July 10th 2014 First applications of (today) low-energy accelerators Nuclear physics: ▫Nuclear reactions ▫Nuclear energy levels ▫Excited levels lifetimes ▫Decay schemes

6. OPAC School. London, July 10th 2014 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 … What to do with the “old” accelerators?

7. OPAC School. London, July 10th 2014 A technique going for every time smaller accelerators

8. OPAC School. London, July 10th 2014 Discovery of AMS in 1977 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 The Rochester MP Tandem accelerator (12 MV) McMaster D.E. Nelson, R.G. Korteling, W.R. Stott.

9. OPAC School. London, July 10th 2014 How many atoms we need for a good measurement? –N:Number of atoms –A:Activity – :Decay constant Reasonable assumptions: –Measurement time: 10 6 s (12 days) –Minimum count rate:0.01 cps –Detection efficiency:100 % Decay Counting Willard F. Libby Nobel Prize in Chemistry 1960

10. OPAC School. London, July 10th 2014 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 efficiencyfew % independent of half-life Counting atoms (AMS) At least 4 orders of magnitude better!!!

11. OPAC School. London, July 10th 2014 Traditional AMS system Tandem Accelerator E = (1+q) eV Detection systems (E, dE/dx, v…) Ion source E,q 0 Magnetic deflector (ME/q 2 ) M Magnetic Analyzer (ME/q 2 ) EM/q 2 Electrostatic deflector (E/q) E/q M/q

12. OPAC School. London, July 10th 2014 Traditional AMS system The use of high energies makes it possible to use nuclear properties (like stopping power) to reduce interferences at the detector Under certain conditions, molecules are broken in the accelerator stripper

13. OPAC School. London, July 10th 2014 Interferences MS Interferences RadioisotopeT 1/2 (years) Isotopic abundance in environmental samples Analyzed ion IsobarsMolecules E/q and M/q ambiguities 10 Be1.51·10 610 Be/ 9 Be=10 -11 -10 -510 Be +10 B +9 Be 1 H +20 Ne 2+ 14 C5730 14 C/ 12 C=10 -14 -10 -1114 C +14 N + 12 C 1 H 2 +, 13 C 1 H + 28 Si 2+ 32 Si172 32 Si/ 28 Si=10 -15 -10 -1232 Si +32 S +31 P 1 H +64 Ni 2+ 36 Cl3·10 536 Cl/ 35 Cl=10 -15 -10 -836 Cl +36 S +35 Cl 1 H +72 Ge 2+ 41 Ca1.03·10 541 Ca/ 40 Ca=10 -14 -10 -1141 Ca +41 K +40 Ca 1 H +82 Se 2+ 129 I1.57·10 7129 I/ 127 I=10 -12 -10 -7129 I +129 Xe + 127 I 1 H 2 +, 128 Te 1 H + ------ 239 Pu2411010 6 atoms 239 Pu + ------ 238 U 1 H + ------ 240 Pu6564 y10 6 atoms 240 Pu + ------ 238 U 1 H 2 + ------

14. OPAC School. London, July 10th 2014 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. OPAC School. London, July 10th 2014 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: ▫ 14 N - ▫ 129 Xe - ▫… Lens Ion beam Cs reservoir Heater Ionizer Sample Acceleration 10 kV

16. OPAC School. London, July 10th 2014 Tandem accelerators Van de Graaf Cockcroft-Walton Higher stability Lower terminal voltages (up to 6 MV) Higher stability Lower terminal voltages (up to 6 MV)

17. OPAC School. London, July 10th 2014 Tandem accelerators Leibniz AMS 3 MV facility, Kiel, GER VERA AMS 3 MV facility, Vienna, Austria

18. OPAC School. London, July 10th 2014 Stripping Electron-loss Break-up of molecules Energy straggling Angular straggling

19. OPAC School. London, July 10th 2014 Minimum gas pressure needed for stable distribution Higher charge states result from stripping at higher energies Stripping Golden rule of molecular destruction: high efficiency for charge state  3 No surviving molecules TV  2.5 MV Bonani et al. (1990)

20. OPAC School. London, July 10th 2014 Detection system Best option  Gas Ionization Chamber ▫Able to give information on total energy and energy loss. Bethe-Bloch formula: For heavy ions  q ef instead of Z p :  E ( 36 Cl)   E ( 36 S) E res ( 36 Cl)  E res ( 36 S)

21. OPAC School. London, July 10th 2014 Traditional 3-6 MV AMS systems Leibniz AMS 3 MV facility, Kiel, GER ≈ 10 -15 m HZDR 6 MV Tandetron AMS facility, Rossendorf, GER VERA AMS 3 MV facility, Vienna, Austria 20 -25 m

22. OPAC School. London, July 10th 2014 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. OPAC School. London, July 10th 2014 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. OPAC School. London, July 10th 2014 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 Several problems arise… Energy dependence of angular straggling Transmitted beam intensities 2 µg /cm 2 stripper gas (Ar) V T (MV) d stripper (µg/cm 2 ) E 0 (MeV) E f (MeV) ΔE (keV) ΔE/ E f (%)q ΔE/( E f +qV T ) (%) 14 C30.232.9980.440.01530.004 14 C0.62 0.5952.260.3810.2

25. OPAC School. London, July 10th 2014 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. OPAC School. London, July 10th 2014 Stripping Process Electron-loss Electron capture Break-up of molecules Energy straggling Angular straggling Charge state distribution 14 C - 13 CH - 12 CH 2 - 1 10 8 10 9 14 C q 13 C q 12 C q 13 CH q 12 CH 2 q HqHq q=1 -, 0, 1 +, 2 +, 3 +,.. σ: dissociation cross section Injected negative mass 14 ions Destruction of molecular ions in q=1 +

27. OPAC School. London, July 10th 2014 Charge state yield of 14 C ions in Ar gas 01+2+ 3+4+ Traditional AMS 2.5 - 9 MV Multiple ion gas collisions Coulomb disintegration Compact AMS 0.2 - 1 MV

28. OPAC School. London, July 10th 2014 Angular straggling Different stripper channel design: ▫Shorter ▫Wider ▫Higher pumping capacity

29. OPAC School. London, July 10th 2014 Energy straggling Design of achromatic optics Electrostatic deflectorMagnetic deflector

30. OPAC School. London, July 10th 2014 Use of specialized gas ionization chambers 5 cm gas detector electrodes "innards" CREMAT preamp modules mounted directly on the anodes (Electronic noise (protons): 16 keV)  E-E res anodes Frisch-grid Cathode CF 100 Ions

31. OPAC School. London, July 10th 2014 Compact AMS Systems (1 MV- 500KV) AMS facility, Seville, Spain 1 MV Tandetron accelerator ≈ 4.5 m KECK AMS facility, Irvine, USA ≈ 3 m ≈ 6 m ≈ 5 m Tandy AMS facility, Zurich, CH ≈ 6 m ≈ 3.5 m

32. OPAC School. London, July 10th 2014 Where are the limits? Cross sections of molecule destruction in Ar Molecular species Energy dependence of angular straggling Transmitted beam intensities Cross sections are comparable to molecular sizes Only weak energy dependence @ 230 keV cross sections are about 10 % lower New concepts can be applied at stripping energies below 250 keV!! Deal with ion beams of large divergence 2 µg /cm 2 stripper gas (Ar)

33. OPAC School. London, July 10th 2014 Inside view of vacuum insulated acceleration system q=1 + acceleration section LE acceleratio n section HE q=1 - Vacuum pumps Stripper gas flow 1 m

34. OPAC School. London, July 10th 2014 200-250kV- AMS systems 5.4 m 6.5 m SSAMS - High Voltage platform (open air) 2.5 m 3.0 m BernMICADAS, Universtity of Bern Compact lab-sized instrument –Designed for operator safety –No open high voltages –Easy to operate –Easy to tune –Fully automated

35. OPAC School. London, July 10th 2014

36. Moore’s Law of radiocarbon AMS MP-Tandem AMS System Rochester EN-Tandem AMS Systems: ETH, Oxford, Lower Hutt, Utrecht, Erlangen,…. IONEX (Ken Purser) Arizona, Oxford, Gif-sur-Yvette,…. HVEE-Tandetron (Purser) AMS Systems: Woods Hole, Groningen, Kiel,… ETH-“Tandy”(Compact)-AMS Systems: Zurich, Georgia, Poznan, Irvine… NEC 500 kV Pelletron ETH-“MICADAS” AMS Systems Zurich, Davis, Mannheim, Debrecen, Seville,…. 200 kV PS (vacuum insulated) SSAMS Systems (NEC) Lund, ANU, SUERC,… 250 kV HV-deck FN-Tandem AMS System McMaster University ?

37. OPAC School. London, July 10th 2014 Nitrogen stripper gas Physical properties of molecule dissociation

38. OPAC School. London, July 10th 2014 He stripper gas He areal density of ≈ 0.5μg / cm 2 should be sufficient to get rid of molecules Physical properties of molecule dissociation

39. OPAC School. London, July 10th 2014  Angular acceptance of stripper:  max = 30 mrad Ion Scattering Beam losses due to small angle scattering

40. OPAC School. London, July 10th 2014 Ion Scattering Beam losses due to small angle scattering  Angular acceptance of stripper:  max = 30 mrad

41. OPAC School. London, July 10th 2014 ETH radiocarbon MS (μCADAS)

42. OPAC School. London, July 10th 2014 Stripping New stripping gasses as He Optimization of vacuum out of the stripping channels Ion sources Reduction of memory effects and cross contamination Selection of specific chemical compounds  Combination with other techniques Sample preparation Reduction of background (isobars, neighbours, molecules…) Small samples Liquid and gaseous samples Development of new detectors Challenges (there’s a lot of work to do!) Reduction of electronic noise through new designs Modified detection techniques

43. OPAC School. London, July 10th 2014 Acknowledgements Thank you very much to Hans-Arno Synal (ETH-PSI, Switzerland) Elena Chamizo (CNA) for providing me of ideas, graphics and pictures

44. OPAC School. London, July 10th 2014