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  1. 1. RADIOBIOLOGY Prof.Dr.Tarek Elnimr L 8 Presented to the Biology Departments in Faculty of Sciences on February 15 , 2009
  2. 2. Interactions of radiation with Matter
  3. 3. Interaction with beta Electrons excited or kicked off. ionization Energy dissipated as heat. As Z of material increases, so does bremsstrahlung. Note that range is different from path .
  4. 4. Interaction with gamma Photon travels until it hits something, either an electron or a nucleus. Several types of interactions have been observed.
  5. 5. Interaction of gamma with matter <ul><li>Photoelectric effect </li></ul><ul><ul><li>Photon hits electron, all of energy is transmitted, electron is ejected. </li></ul></ul><ul><ul><li>Most likely with low energy photons, high Z material </li></ul></ul>
  6. 6. Gamma interaction-2 <ul><li>Compton scattering </li></ul><ul><ul><li>Not all energy transmitted to electron. </li></ul></ul><ul><ul><li>Electron ejected, secondary photon emitted </li></ul></ul><ul><ul><li>With low energy photons, independent of Z </li></ul></ul>
  7. 7. Raleigh scattering and nuclear magnetic resonance <ul><li>Both involve impact of gamma on nucleus </li></ul><ul><ul><li>Raleigh: gamma is deflected (elastic collision), keeps going. </li></ul></ul><ul><ul><li>occurs when particles are very small compared to the wavelength of the radiation. (10 -15 vs 10 -10 ) </li></ul></ul><ul><ul><li>NMR: absorbed, emitted in a new direction </li></ul></ul>
  8. 8. Pair production and annihilation Two gamma collide, convert to a positron and a negatron. Complete energy to matter conversion These two betas collide, converting to 2 gammas with equal energy of 511 kev. Complete matter to energy conversion. student/images/26f14.jpg
  9. 9. Summary of interactions <ul><li>Alpha </li></ul><ul><ul><li>Penetrates short distance into matter, giving up its energy by ionizing matter and releasing heat. </li></ul></ul><ul><li>Beta </li></ul><ul><ul><li>Bounces around, giving up energy by ionizing matter and dissipating kinetic energy as heat. </li></ul></ul><ul><li>Gamma </li></ul><ul><ul><li>Penetrates, colliding with electrons </li></ul></ul><ul><ul><ul><li>Photoelectric effect, Compton scattering </li></ul></ul></ul><ul><ul><li>Collides with nuclei (Raleigh scattering, NMR) </li></ul></ul><ul><ul><li>Collides with another gamma </li></ul></ul>
  10. 10. About interactions <ul><li>Radiation is moving energy </li></ul><ul><ul><li>All types have kinetic energy </li></ul></ul><ul><ul><li>Alpha and beta particles have charge </li></ul></ul><ul><li>Energy cannot be created or destroyed </li></ul><ul><ul><li>Energy is transferred </li></ul></ul><ul><li>Dose is a measure of how much energy is deposited in an “absorber” </li></ul><ul><ul><li>Absorber could be inanimate or could be flesh </li></ul></ul><ul><ul><li>Energy left as heat, electrical potential, etc. </li></ul></ul>
  11. 11. Bragg Effect <ul><li>As particles (alpha, beta) slow down, ionizations increase near the end of their paths. </li></ul><ul><ul><li>Proton anti-cancer therapy relies on this. </li></ul></ul>
  12. 12. About Dose <ul><li>Linear Energy Transfer </li></ul><ul><ul><li>Average energy deposited in absorber per unit distance traveled by charged particle. </li></ul></ul><ul><li>RAD: radiation absorbed dose </li></ul><ul><ul><li>The amount of energy absorbed per unit of absorbing material. (new units: Gray) </li></ul></ul><ul><li>RBE: Relative Biological Effectiveness </li></ul><ul><ul><li>Depends directly on the LET, a quality factor “Q” used in determining the effect of LET on the absorbed dose, i.e. how much damage. </li></ul></ul>
  13. 13. More on dose <ul><li>REM: roentgen equivalent man </li></ul><ul><ul><li>Effective dose resulting from the RAD and the RBE </li></ul></ul><ul><ul><li>REM = Q x dose (in RAD) </li></ul></ul><ul><ul><li>Q is a measure of RBE as determined from LET. </li></ul></ul><ul><ul><li>New unit is sieverts </li></ul></ul><ul><ul><li>Slowly moving, greatly ionizing alpha particles have a much higher LET, so Q will be >1, and the energy absorbed will have a bigger biological effect (if absorbed by living tissue) </li></ul></ul>
  14. 14. More on calculating REM LET (keV per µm) Q example 3.5 and less 1 X-rays, β ,  7 2 neutrons 23 5 53 10 175 and over 20 alpha
  15. 15. Comparing old, SI units Rad = 100 ergs/gram; Rem = rad x Q; 1 Gray = 100 Rads, 1 j/kg; 1 Sievert = 100 rem; Old SI Radioactive material curies becquerels Deposited energy Rads Grays Dose to humans Rems Sieverts Units of energy in air Roentgens none
  16. 16. Radiation Safety Rules of Thumb <ul><li>Alpha particles up to 7.5 MeV are stopped in the dead layer of normal skin. </li></ul><ul><li>Beta particles will penetrate about 4 meters in air per MeV of energy. </li></ul><ul><li>Beta particles will penetrate about 0.5 cm in soft tissue per MeV of energy. </li></ul><ul><li>Beta particles up to 70 KeV are stopped in the dead layer of normal human skin. </li></ul>