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Lesson 15

Lesson 15. Interaction of Radiation with Matter Charged Particles. Basic Ideas. For charged particles and photons, the interaction is between the radiation and the atomic electrons . Nine orders of magnitude more probable than interaction with nucleus.

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Lesson 15

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  1. Lesson 15 Interaction of Radiation with Matter Charged Particles

  2. Basic Ideas • For charged particles and photons, the interaction is between the radiation and the atomic electrons. Nine orders of magnitude more probable than interaction with nucleus. • For neutrons, the story is different and interactions are with nuclei.

  3. Basic Ideas • Divide subject (and lectures) into interactions of charged particles (alpha, electrons, heavy ions) and uncharged particles (photons, neutrons).. Another way of saying this is short range particles and long range particles

  4. Areal Density • One does not usually speak of thickness in terms of a linear dimensions (nm, microns, cm, meters, etc) but in terms of areal density (mg/cm2, g/cm2, etc.) • The conversion is trivial, i.e., Linear thickness = areal density/density • Original motivation for this unit is operational.

  5. Heavy Charged Particles(particles like protons or heavier) • Straight line tracks in matter • Concept of range

  6. Stopping power • Stopping power = Formally

  7. Bethe-Bloch equation Converting to a differential Substituting

  8. Evaluating bmin and bmax When b=min, get max Etransfer Note the expressions are relativistic Similarly

  9. Back to the Bethe-Bloch equation Substituting Making this qm correct

  10. How to use this Z<13 Z13 See p. 503 of LSM for example

  11. General conclusions for E/A < 10 MeV/A Bragg peak

  12. dE/dx for compounds, mixtures Note this relation is an approximate relation

  13. Straggling

  14. Straggling

  15. Ranges

  16. Practical approaches to ranges and dE/dx

  17. Practical approaches to ranges and dE/dx • SRIM (http://www.srim.org) • OSU variant (Range)-See class website • ORNL--STOPX

  18. Ranges in air • Ranges of -particles in air

  19. Interaction of electrons with matter • In addition to electron-electron interactions, have the possibility of radiative processes such as bremsstrahlung

  20. Formalism Sradiative important only for Z=80-90 and E=10-100 MeV

  21. Practical aspects • Concept of “range” is problematic

  22. “Range” of -particles • When monoenergetic electrons interact with matter, there is a distribution of stopping distances • -particles emitted in -decay have a range of energies, from 0 to Emax • Get exponential attenuation Nt = N0e-t

  23. Practical aspects of beta-counting • Backscattering • Bremsstrahlung • Cerenkov radiation

  24. Backscattering

  25. Self Absorption

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