NEEP 541 Radiation Interactions

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NEEP 541Radiation Interactions

• Fall 2003
• Jake Blanchard

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Outline

• Elementary Particles
• Electrons
• Ions
• Neutrons
• Photons
• Radiation in Space
• Photon Interactions
• Charge Transfer

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Elementary Particles

• Photons
• no mass
• no charge

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Elementary Particles

• Electrons
• Light
• negatively charged
• Rest mass0.51 MeV
• Protons
• Mass about 1837 times that of an electron
• Positive charge
• Neutron
• Slightly heavier than proton
• No charge

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Neutrons

• Flux is neutrons/area/time
• Equivalent to beam intensity

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Fission Energy Spectrum
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Neutrons

• Collisions are elastic or inelastic
• Neutrons can be captured, form compound nucleus
  then nucleus emits neutrons, protons, gammas, or
  fission products
• These products are recoils

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Recoil and PKA spectra
recoils
PKAs
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Gammas

• Mean gamma energies are on the order of 100 eV
• Negligible effect on displacements

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Electrons

• Charge implies short range, but low mass implies
  longer range
• Most energy transfer is to other electrons in
  target
• Collisions are coulomb
• Must be relativistic to cause displacements (gt
  0.2 MeV)
• Low temperature irradiation leads to homogeneous
  vacancy and interstitial distribution over about
  1 mm

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Positive Ions

• Light ions He, H, D (gt 1 MeV)
• Heavy ions (lt1 MeV)
• Fission products (100 MeV)
• Inelastic losses low at low energy
• Electrons dominate at high energy
• Damage is narrower because range is smaller

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Radiation in Space

• Energies from keV to TeV
• Particles are trapped by Earths magnetic field
  or pass through solar system
• Trapped radiation is broad spectrum of charged
  particles (radiation belts)
• Cosmic Rays are low fluxes of heavy ions beyond
  TeV
• Solar Flares produce protons with energies to
  hundreds of MeV
• Space is also pervaded by plasma of electrons and
  protons with energies around 100 keV

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Energies
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Belts
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Plasma Conditions
Equilibrium Potentials at Increasing Altitude Equilibrium Potentials at Increasing Altitude
Ionosphere a few tenths of a volt negative
Magnetosphere normally, a few volts positive in eclipse, may become highly negative
Solar Wind a few volts positive
Interstellar Space a few volts positive or negative
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More Plasma
Parameter Plasmasphere Plasma sheet Solar wind
Plasma density, cm-3 10 - 1000 1 6
Electron mean kinetic energy, eV 1 1000 15
Ion mean kinetic energy, eV 1 6000 10
Electron random current density, µA/m² 0.25 - 25 0.85 0.62
Ion random current density, µA/m² 0.006 - 0.6 0.05 0.012
Electron Debye length, m 2.5 - 0.25 240 12
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Van Allen Belts
Inner Zone (lt 2.5RE) Outer Zone (gt 2.5RE)
Proton flux dominates About 10 times higher electron flux in outer zone than inner zone
Electron energies lt 5 MeV Electron energies around 7 MeV
Electron and proton fluxes peak at 1.5RE to 2.0RE Electron flux peaks at about 5RE
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Cosmic Rays

• 85 protons, 14 alpha particles
• 1 nuclides with Zgt4 (ions of Zgt26 are rare)
• energies from 0 to over 10 GeV
• Most heavy ions are H, He, C, and O with peak
  energies around 1 GeV.
• Cosmic rays have low flux but higher energy

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Solar Flares

• heaviest doses at solar maximum (10-12 yrs)
• Flares produce heavy ions and protons (90,
  remainder is He, heavy ions, electrons)
• Heavy ion fluxes from solar flares are generally
  less than galactic background but can be 4 times
  greater
• Hheavy ion spectrum is less energetic than
  galactic cosmic ray spectrum
• Solar protons are energetic (10 MeV to 1 GeV)
• Protons from a single flare produce fluences up
  to 2x1010 p/cm2

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Radiation Interactions

• What happens when energetic particles interact?
• Elastic Scattering
• Inelastic Scattering
• Bremsstrahlung Radiation
• Accelerating charged particles emit radiation
• Amplitude of radiation is proportional to
  acceleration

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Photon Interactions

• Photoelectric Effect low energy
• Photon interacts with target electrons and is
  absorbed
• Electron is ejected
• Electron energy is photon energy minus binding
  energy
• Binding energy reappears as x-rays

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Photon Interactions

• Compton Effect
• Elastic scattering between photon and target
  electron
• Electrons emitted in spectrum with average about
  2/3 of maximum

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Photon Interactions

• Pair production
• Threshold energy is 1 MeV
• Photon disappears and electron-positron pair is
  formed

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Attenuation Coefficients

• Measure of absorption of beam impinging on
  absorbing medium
• Units are inverse length
• Total attenuation coefficient is sum of
  contributions from three photon interaction
  effects

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Attenuation of Photons in Air
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Attenuation of Photons in Lead
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Attenuation of Photons in Al
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Charge Transfer

• Radiation interactions result in the transfer of
  charge from one location to another
• Electrons from photon interactions tend to be
  emitted with a forward bias, so there is a net
  charge transfer in the forward direction
• For 1.3 MeV photons on Si, current is on the
  order of 10-21 C/photon (Compton scattering)

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Forward Electron Current