Annular Gas Ionization Detector for Heavy Ion Beam Analysis

1
Annular Gas Ionization Detector for Heavy Ion
Beam Analysis

• M. Mallepell
• Ion Beam Physics
• PSI and ETH Zürich

2
Overview

• Introduction on Rutherford Backscattering
  Spectrometry

• Heavy Ion Backscattering Spectrometry (HIBS)

• Development Annular Gas Ionization Detector
• Design and construction
• Measured performance

• Conclusion

Marc Mallepell, PhD Seminar 2007
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Rutherford Backscattering Spectrometry
Determination of the involved target masses
Energy measurement of MeV ions backscattered from
target surface
Depth distribution of the target elements
Marc Mallepell, PhD Seminar 2007
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RBS important involved processes
Marc Mallepell, PhD Seminar 2007
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Typical RBS Spectra
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Measurement of heavy target elements
Reason Mass resolution is not good enough for
light projectiles
Marc Mallepell, PhD Seminar 2007
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Mass resolution
1.0
Mass resolution can be significantly improved by
using heavy projectiles
RBS
He
0.8
O
0.6
Si
Gain in mass resolution
k kinematic factor
Cu
2 MeV 4He
15 MeV 28Si
0.4
0.2
0.0
0
20
40
60
80
100
140
180
target mass Mt / u
PhD Seminar, Marc Mallepell, 2007
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Standard Detectors Si

• Cannot be used for HIBS
• Radiation damage due to impact of heavy
  projectiles
• Relatively bad energy resolution for elements
  heavier than 7Li

Marc Mallepell, PhD Seminar 2007
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Improved Energy resolution of GID

• Recent developments at our Laboratory
• SiN entrance windows
• Low noise CoolFET preamps

Vast improvement of GID resolution at low energies
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Comparison of energy resolution
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GID design criteria

• Minimize contributions to the energy resolution
• kinematic broadening
• electronic noise (especially for light particles)
• Large solid angle
• Use and handling
• Integrable in an already existing analysis system

Marc Mallepell, PhD Seminar 2007
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Development of GID for HIBS

• Energy resolution
• 1) kinematic broadening

Contribution of kinematic broadening to the total
energy resolution is minimized at large
scattering angles
6 MeV 7Li on 28Si
60
120
160
50
40
Mechanically achievable scattering angle of
176.5 and window width of 6 mm
170
Energy / keV
30
Total energy resolution of GID
20
176.5
10
Annular GID
180
Contribution 1.3
0
0
5
10
15
20
25
30
Width of entrance windows / mm
Marc Mallepell, PhD Seminar 2007
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Development of GID for HIBS

• Energy resolution
• 2) Small electronic noise

Capacitance of the detector should be as small as
possible
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Annular GID for HIBS
Primary ion beam
Backscattered projectiles
detector signal output to preamp
Frisch grid
voltage supply
anode
SiN entrance windows
cathode
voltage supply
Detector Gas Isobutane at 20 mbar Solid angle
2.6 msr Circular array of 75 nm silicon nitride
entrance windows (6x8 mm) Detector capacitance
42 pF
Inline Gas Ionization Detector with a mean
scattering angle of f176.5 ?f1
Marc Mallepell, PhD Seminar 2007
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Annular GID for HIBS
Marc Mallepell, PhD Seminar 2007
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Measured detector performance
100
40
PIPS Hinrichsen
7Li Annular GID
6Li PIPS Hinrichsen
80
30
Annular GID
60
Resolution (FWHM) / keV
Resolution (FWHM) / keV
20
40
10
20
0
0
0
0.5
1
1.5
2
2.5
3
0
2
4
6
8
Energy / MeV
Energy / MeV
300
250
35Cl PIPS Hinrichsen
200
Resolution (FWHM) / keV
150
28Si Annular GID
100
50
0
0
2
4
6
8
Energy / MeV
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Detector performance GaAs
4 MeV 4He RBS with Si-Detector (14 keV)
15 MeV 28Si HIBS with new Annular GID (60 keV)
69Ga
71Ga
75As
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Copper isotopes with HIBS
2 MeV 4He RBS with Si-Detector (14 keV)
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Can the new Annular GID be used for standard 2
MeV 4He RBS?
O
2 MeV 4He beam
Al2O3
Al
Al substrate
Detector
Al
YES GID equals Si-detector under standard
He-RBS conditions!
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Conclusion

• A new annular Gas Ionization Detector has been
  developed for HIBS
• Better mass resolution for heavy projectiles
• Easy to handle and use
• No radiation damage
• Performance
• Outperforms standard implanted silicon detectors
  for all ions heavier than Li
• Comparable energy resolution for standard 4He RBS
  conditions
• Annular design allows simple integration into
  existing analysis beam line

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• Thank you for your attention !

Marc Mallepell, PhD Seminar 2007