Dr. Mohammed Alnafea Radiation Interaction with matter

Types of Ionizing Radiation Absorption of energy from radiation in biological material may lead to excitation or to ionization. Excitation – electron from atom or molecule raised to a higher energy level but not ejected. Ionization – ejection of one or more electrons. IONIZING RADIATION 2 5th lecture RAD /03/2010

Photon Interactions Three ways to deposit photon energy Photoelectric effect Compton Scattering Pair production Low h  High h  E 3 5th lecture RAD /03/2010

4 5th lecture RAD /03/2010

Photoelectric Effect photon interacts with an inner bound electron photon interacts with an inner bound electron photon disappears after interaction (pure absorption) photon disappears after interaction (pure absorption) e-ke-k 5 5th lecture RAD /03/2010

Photoelectric Effect In the photo electric effect, the incident photon disappears after interaction with inner bound electron. The photon is replaced by an electron ejected from the atom with kinetic energy E k = h  - E b (where E b is the electron binding energy). Characteristic radiations and Auger electrons are emitted as cascading electrons replace the ejected photoelectron. 6 5th lecture RAD /03/2010

Compton Scatter Photon interacts with a “free” electron Photon interacts with a “free” electron Photon survives interaction, leaves with reduced energy Photon survives interaction, leaves with reduced energy Compton e -   7 5th lecture RAD 311 Compton scattering of an incident photon, with photon scattered at and angle . The Compton electron ejected at an angle  with respect to the direction of the incident photon. 22/03/2010

Pair Production Incident gamma ray Electron Positron 0.511keV 8 5th lecture RAD 311 Pair production interaction of a high-energy photon near a nucleus. Annihilation photons are produced when the positron and a nearby electron react and annihilate each other. 22/03/2010

Iso Effect Curve 9 5th lecture RAD 31122/03/2010

Iso Effect Curve 5th lecture RAD The iso effect curve shows which photon interaction is likely to occur based on the incident photon energy and the mass of the target atom. The photoelectric effect has the highest probability with low energy photon and a high atomic number absorber. For the intermediate energy photons Compton scattering is the most frequent interaction. For the intermediate energy photons Compton scattering is the most frequent interaction. At higher photon energies, pair production is the predominant interaction. At higher photon energies, pair production is the predominant interaction. 22/03/2010

Particle Radiation Alpha Alpha Beta (  - or  + ) Beta (  - or  + ) Electrons Electrons Neutrons Neutrons Protons Protons Mesons Mesons 11 5th lecture RAD /03/2010

Water and Radiobiology Water is important in Radiobiology. Water is important in Radiobiology. A lot of biological structures are mainly composed of water. A lot of biological structures are mainly composed of water. Bacterial cell is 75% water by weight, and mammalian cell are 80% water by weight. Bacterial cell is 75% water by weight, and mammalian cell are 80% water by weight. “80% of initial energy loss is in water” “80% of initial energy loss is in water” 12 5th lecture RAD 31122/03/2010

Free Radicals… “A free radical is any species capable of independent existence that contains one or more unpaired electrons. An unpaired electron is one that occupies an orbital by itself”. 13 5th lecture RAD /03/2010

Aqueous Radical Formation Ionization of water Further reactions… 14 5th lecture RAD 31122/03/2010

Important Radicals Hydroxyl Radical Solvated Electron Hydrogen Radical 15 5th lecture RAD 31122/03/2010

Resultant Radicals In the end, you have five different radicals that may interact and disrupt the cell 16 5th lecture RAD 31122/03/2010

G values G values represent the number of molecules altered per 100eV. These values vary based on the type of free radical. Number of molecules damaged 100eV of Energy Absorbed G = 17 5th lecture RAD 31122/03/2010

Different G values for different radicals PRODUCTG value H*H* 0.6 H2H2 0.4 H2O2H2O2 0.7 HO Solvated e OH * th lecture RAD /03/2010

Radiation damage to DNA Strand Breakage  Single Strand Breaks  Double Strand Breaks Damage without Strand Breakage  Sugar Damage  Additions, breaking C-C bond  Base Damage  Radical Additions, Cross links, Ring openings 19 5th lecture RAD /03/2010

Direct and Indirect Direct Effect: the radiation energy is deposited into the DNA. Indirect Effect: the radiation energy causes radicals to be produced- which in turn causes damage. 20 5th lecture RAD /03/2010

Low LET: Ratio of direct to indirect effects is estimated to be 30% direct to 70% indirect for. High LET: Direct effect dominates 21 5th lecture RAD /03/2010

22 5th lecture RAD /03/2010

23 5th lecture RAD /03/2010