Imaging Nuclear Reactions

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Imaging Nuclear Reactions

• Zhon Butcher
• 2006 REU Program
• Cyclotron Institute
• Mentor Dr. Robert Tribble

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Applications of Nuclear Imaging

• Space Telescopes Cosmic radiation
  identification and direction of origin.
• Imaging reactions in the nuclear physics
  laboratory.

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How Imaging Works in the Lab

• Several detectors are placed around the reaction
  site covering a given solid angle.
• Detectors determine particle identity and
  position.
• The resulting image gives a picture of the
  reactions that took place in the chamber.

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Particle Identification

• Telescopes Front detector and rear detector.
• Front detector picks up energy loss as the
  particle passes through.
• Rear detector picks up residual energy.
• Particle identification determined by

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Methods for Position Determination

• Many small detectors coupled with a large amount
  of electronics (clustering).
• Resistive strip detectors.
• Double sided strip detectors.
• Resistive sheets.

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1-D Position Sensitive Detector
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Resistive Strip Detectors

• Consist of many resistive strips placed alongside
  one another.
• Good resolution in the X direction, poor
  resolution in the Y direction (or vice versa
  depending on orientation).

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PSSDs
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Double Sided Strip Detectors

• Two sheets of strips placed one in front of the
  other so the strips form a grid.
• Results in better position resolution
• Washington University team had detectors with 32
  strips in each direction.
• 64 strips per detector x 4 detectors 256
  channels for position reconstruction

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Double sided PSSDs
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Resistive Sheets

• A single resistive sheet spans the entire active
  area of the detector.
• Advantages
• Fewer signals to process.
• Less electronic equipment.
• Detector Types
• Duo-lateral Generates two signals from each face
  of the detector, two from the front and two from
  the back.
• Tetra-lateral Generates five signals, one from
  each corner of the resistive side, and one signal
  from the back.

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Tetra Lateral Detectors
Particle impinging position calculated by
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Signal Processing
ADC
Preamplifier
Spectroscopy Amplifier
Preamplifier
Spectroscopy Amplifier
Detector
Computer
Preamplifier
Spectroscopy Amplifier
Preamplifier
Spectroscopy Amplifier
Gate Generator
Discriminator
Preamplifier
Timing Amplifier
Rear signal
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How Silicon Detectors Work
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Current Through Semiconductor
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Doped Semiconductor

• What is doping?
• Doping is the integration of impurities into the
  lattice structure of the semiconductor.
• This allows extra electron and hole energy levels
  which will increase the conductivity of the
  semiconductor.

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Experiment

• To characterize the Micron Semiconductors
  tetra-lateral detectors in terms of energy and
  position resolution as well as non-linearity in
  position reconstruction.
• Three tetra-lateral type PSDs were investigated.
  One 200 mm and one 400 mm thick detectors with a
  resistive strip around the active area, and one
  200 mm without a resistive strip.
• Optimal strip resistance is approx. 1/10th the
  resistance of the detector active area.

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Setup

• The detectors were placed in a vacuum chamber
  with a radioactive source. (241Am and 228Th were
  used)
• The distance between the source and the detector
  was approx 25cm for 241Am and 10cm for 228Th

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Calibration Masks

• Two masks were used to cover the detectors.

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Position Reconstruction 200mm
Position reconstruction of impinging alpha
particles for the 200 mm thick detector with and
without a resistive strip.
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Position Reconstruction 400 mm
Position reconstruction of impinging alpha
particles with and without a mask for the 400 mm
thick detector with a resistive strip.
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Energy Resolution

• Energy Spectrum of alpha decay from 228Th with
  400mm detector

Energy Resolution Approx 10
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Results

• The position resolution was determined to be
  around 3-4 mm and energy resolution of 8 for
  both the 400 mm and 200 mm thick detectors with
  the resistive strip.
• The resistive strip has a major contribution in
  reducing the position reconstruction distortion.

For more information see T.Doke et.al. NIM A261
(1987) 605
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Conclusion

• The position resolution for the tetra-lateral
  PSDs strongly depends on the resistivity of the
  resistive sheet, electrode termination resistors,
  the filter components of the preamplifiers, and
  the shaping times of the amplifiers.
• The measurements done were employing the use of
  Indiana University preamplifiers and CAEN
  amplifiers (3 ms shaping time). Further
  investigation of these dependencies is ongoing.

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Acknowledgements

• Special thanks to
• Dr. Robert Tribble
• Dr. Livius Trache
• Dr. Adriana Banu
• Matthew McCleskey