NEEP 541Radiation Interactions

Fall 2003Jake Blanchard

Outline Elementary Particles

Electrons Ions Neutrons Photons

Radiation in Space Photon Interactions Charge Transfer

Elementary Particles Photons

no mass no charge

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sergh

c

Ep

chhE

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27

Elementary Particles Electrons

Light negatively charged Rest mass=0.51 MeV

Protons Mass about 1837 times that of an electron Positive charge

Neutron Slightly heavier than proton No charge

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

0

)( dEE

Fission Energy Spectrum

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”

Recoil and PKA spectra

recoils

PKAs

Gammas Mean gamma energies are on the

order of 100 eV Negligible effect on displacements

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 (> ~0.2 MeV) Low temperature irradiation leads to

homogeneous vacancy and interstitial distribution over about 1 mm

Positive Ions Light ions = He, H, D (> 1 MeV) Heavy ions (<1 MeV) Fission products (~100 MeV) Inelastic losses low at low energy Electrons dominate at high energy Damage is narrower because

range is smaller

Radiation in Space Energies from keV to TeV Particles are trapped by Earth’s 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

Energies

Belts

Plasma Conditions

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

More PlasmaParameter 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

Van Allen Belts

Inner Zone (< 2.5RE) Outer Zone (> 2.5RE)

Proton flux dominates About 10 times higher electron

flux in outer zone than inner zone

Electron energies < 5 MeV

Electron energies around 7 MeV

Electron and proton fluxes peak at 1.5RE to

2.0RE Electron flux peaks at about 5RE

Cosmic Rays

85% protons, 14% alpha particles 1% nuclides with Z>4 (ions of Z>26 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

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

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

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

Photon Interactions Compton Effect

Elastic scattering between photon and target electron

Electrons emitted in spectrum with average about 2/3 of maximum

Photon Interactions Pair production

Threshold energy is 1 MeV Photon disappears and electron-

positron pair is formed

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

Attenuation of Photons in Air

Attenuation of Photons in Lead

Attenuation of Photons in Al

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)

Forward Electron Current