NEEP 541 Radiation Interactions Fall 2003 Jake Blanchard

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

Elementary Particles Photons no mass no charge

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

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 Magnetosp here: 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 Plasma ParameterPlasmasphere Plasma sheet Solar wind Plasma density, cm Electron mean kinetic energy, eV Ion mean kinetic energy, eV Electron random current density, µA/m² Ion random current density, µA/m² Electron Debye length, m

Van Allen Belts Inner Zone (< 2.5R E )Outer Zone (> 2.5R E ) 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 1.5R E to 2.0R E Electron flux peaks at about 5R E

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 ~2x10 10 p/cm 2

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 C/photon (Compton scattering)

Forward Electron Current