Low energy accelerators

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Low energy accelerators Compact AMS systems

• José María López Gutiérrez
• Universidad de Sevilla
• Centro Nacional de Aceleradores

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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

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

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First applications of (today) low-energy
accelerators

• Nuclear physics
• Nuclear reactions
• Nuclear energy levels
• Excited levels lifetimes
• Decay schemes

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What to do with the old accelerators?

• The energies that could be reached by the
  accelerators used before the 1950s 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

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Accelerator mass spectrometry

• A technique going for every time smaller
  accelerators

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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
McMaster D.E. Nelson, R.G. Korteling, W.R.
Stott.
The Rochester MP Tandem accelerator (12 MV)
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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
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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!!!
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Traditional AMS system
Tandem Accelerator E (1q) eV
EM/q2
M
E,q0
E/q M/q
Detection systems (E, dE/dx, v)
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Traditional AMS system
Under certain conditions, molecules are broken in
the accelerator stripper
The use of high energies makes it possible to use
nuclear properties (like stopping power) to
reduce interferences at the detector
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Interferences
      MS Interferences MS Interferences MS Interferences MS Interferences
Radioisotope T1/2 (years) Isotopic abundance in environmental samples Analyzed ion Isobars Molecules E/q and M/q ambiguities
10Be 1.51106 10Be/9Be10-11-10-5 10Be 10B 9Be1H 20Ne2
14C 5730 14C/12C10-14-10-11 14C 14N 12C1H2, 13C1H 28Si2
32Si 172 32Si/28Si10-15-10-12 32Si 32S 31P1H 64Ni2
36Cl 3105 36Cl/35Cl10-15-10-8 36Cl 36S 35Cl1H 72Ge2
41Ca 1.03105 41Ca/40Ca10-14-10-11 41Ca 41K 40Ca1H 82Se2
129I 1.57107 129I/127I10-12-10-7 129I 129Xe 127I1H2, 128Te1H ------
239Pu 24110 106 atoms 239Pu ------ 238U1H ------
240Pu 6564 y 106 atoms 240Pu ------ 238U1H2 ------
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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

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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-

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Tandem accelerators
Cockcroft-Walton
Higher stability Lower terminal voltages (up to 6
MV)
Van de Graaf
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Tandem accelerators
VERA AMS 3 MV facility, Vienna, Austria
Leibniz AMS 3 MV facility, Kiel, GER
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Stripping

• Electron-loss
• Break-up of molecules
• Energy straggling
• Angular straggling

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Stripping
Bonani et al. (1990)
Minimum gas pressure needed for stable
distribution Higher charge states result from
stripping at higher energies
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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)
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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
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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

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Several problems arise

• Charge states ?3 after stripping ? very low
  probability
• Lower charge states after stripping Surviving
  molecules??

330 kV
Jacob et al., 2000
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Several problems arise

• 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

VT (MV) dstripper (µg/cm2) E0 (MeV) Ef (MeV) ?E (keV) ?E/ Ef () q ?E/( EfqVT) ()
14C 3 0.2 3 2.998 0.44 0.015 3 0.004
14C 0.6 2 0.6 0.595 2.26 0.38 1 0.2
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Several problems arise

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

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Stripping Process
Injected negative mass 14 ions
q1-, 0, 1, 2, 3,..
14C-
1
13Cq
13CHq
14Cq
12Cq
13CH-
108
12CH2q
Hq
12CH2-
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Destruction of molecular ions in q1

• Electron-loss
• Electron capture
• Break-up of molecules
• Energy straggling
• Angular straggling

s dissociation cross section
Charge state distribution
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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
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Angular straggling

• Different stripper channel design
• Shorter
• Wider
• Higher pumping capacity

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Energy straggling

• Design of achromatic optics

Electrostatic deflector
Magnetic deflector
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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
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Compact AMS Systems (1 MV- 500KV)

6 m
AMS facility, Seville, Spain
KECK AMS facility, Irvine, USA
3.5 m
3 m
4.5 m
1 MV Tandetron accelerator
Tandy AMS facility, Zurich, CH
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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
• _at_ 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!!
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Inside view of vacuum insulated acceleration
system
acceleration section HE
acceleration section LE
Stripper gas flow
q1-
q1
Vacuum pumps
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200-250kV- AMS systems

• Compact lab-sized instrument
• Designed for operator safety
• No open high voltages
• Easy to operate
• Easy to tune
• Fully automated

6.5 m
5.4 m
BernMICADAS, Universtity of Bern
SSAMS - High Voltage platform (open air)
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Moores 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
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Physical properties of molecule dissociation
Nitrogen stripper gas
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Physical properties of molecule dissociation
He stripper gas
He areal density of 0.5µg / cm2 should be
sufficient to get rid of molecules
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Ion Scattering
Beam losses due to small angle scattering
Angular acceptance of stripper ?max 30 mrad
?
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Ion Scattering
Beam losses due to small angle scattering
Angular acceptance of stripper ?max 30 mrad
?
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ETH radiocarbon MS (µCADAS)
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Challenges (theres a lot of work to do!)

• Stripping

• Development of new detectors

• 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

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