CHAPTER 2 RADIATION INTERACTIONS WITH MATTER HDR 112 RADIATION BIOLOGY AND RADIATION PROTECTION MR KAMARUL AMIN BIN ABDULLAH

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1 HDR 112 RADIATION BIOLOGY AND RADIATION PROTECTION CHAPTER 2 RADIATION INTERACTIONS WITH MATTER PREPARED BY: MR KAMARUL AMIN BIN ABDULLAH SCHOOL OF MEDICAL IMAGING FACULTY OF HEALTH SCIENCE

2 Interactions & Processes which lead to Radiation Injury Slide 2 of 52

3 LEARNING OUTCOMES At the end of the lesson, the student should be able to:- Explain the interaction and processes which lead to radiation injury Describe Photoelectric Effect Describe Compton Scattering Describe Linear Energy Transfer (LET) Describe Relative Biological Effects (RBE) Slide 3 of 52

4 Particle Interactions Energetic charged particles (e.g. electron, proton) interact with matter by electrical forces and lose kinetic energy via:- Excitation Ionization Radiative losses ~ 70% of charged particle energy deposition leads to nonionizing excitation. Slide 4 of 52

5 Slide 5 of 52

6 Specific Ionization Number of primary and secondary ion pairs produced per unit length of charged particle s path is called specific ionization. Expressed in ion pairs (I.P.)/mm Increases with electrical charge of particle. Decreases with incident particle velocity. Slide 6 of 52

7 Specific Ionization for 7.69 MeV alpha particle from polonium 214 Slide 7 of 52

8 Charged Particle Tracks Electrons follow tortuous paths in matter as the result of multiple scattering events. Ionization track is sparse and non-uniform. Larger mass of heavy charged particle results in dense and usually linear ionization track. Path length is actual distance particle travels; range is actual depth of penetration in matter. Slide 8 of 52

9 Path Lengths vs. Ranges Slide 9 of 52

10 Linear Energy Transfer Amount of energy deposited per unit path length is called the linear energy transfer (LET). Expressed in units of ev/cm LET of a charged particle is proportional to the square of the charge and inversely proportional to its kinetic energy. High LET radiations (alpha particles, protons, etc.) are more damaging to tissue than low LET radiations (electrons, gamma and x-rays). Slide 10 of 52

11 Bremsstrahlung Slide 11 of 52

12 Bremsstrahlung Probability of bremsstrahlung production per atom is proportional to the square of Z of the absorber. Energy emission via bremsstrahlung varies inversely with the square of the mass of the incident particle. Protons and alpha particles produce less than onemillionth the amount of bremsstrahlung radiation as electrons of the same energy. Slide 12 of 52

13 Bremsstrahlung Ratio of electron energy loss by bremsstrahlung production to that lost by excitation and ionization = EZ/820 E = kinetic energy of incident electron in MeV Z = atomic number of the absorber Bremsstrahlung x-ray production accounts for ~1% of energy loss when 100 kev electrons collide with a tungsten (Z = 74) target in an x-ray tube. Slide 13 of 52

14 Neutron Interactions Neutrons are uncharged particles. They do not interact with electrons. Do not directly cause excitation or ionization. They do interact with atomic nuclei, sometimes liberating charged particles or nuclear fragments that can directly cause excitation or ionization. Neutrons may also be captured by atomic nuclei. Retention of the neutron converts the atom to a different nuclide (stable or radioactive). Slide 14 of 52

15 Neutron Interaction Slide 15 of 52

16 X- and Gamma-Ray Interactions Rayleigh scattering Compton scattering Photoelectric absorption Pair production Slide 16 of 52

17 Rayleigh Scattering Incident photon interacts with and excites the total atom as opposed to individual electrons. Occurs mainly with very low energy diagnostic x-rays, as used in mammography (15 to 30 kev). Less than 5% of interactions in soft tissue above 70 kev; at most only 12% at ~30 kev. Slide 17 of 52

18 Rayleigh Scattering Slide 18 of 52

19 Compton Scattering Predominant interaction in the diagnostic energy range with soft tissue. Most likely to occur between photons and outer ( valence ) shell electrons. Electron ejected from the atom; photon scattered with reduction in energy. Binding energy comparatively small and can be ignored. Slide 19 of 52

20 Slide 20 of 52

21 Compton Scatter Probabilities As incident photon energy increases, scattered photons and electrons are scattered more toward the forward direction. These photons are much more likely to be detected by the image receptor, reducing image contrast. Probability of interaction increases as incident photon energy increases; probability also depends on electron density. Number of electrons/gram fairly constant in tissue; probability of Compton scatter/unit mass independent of Z Slide 21 of 52

22 Relative Compton Scatter Probabilities Slide 22 of 52

23 Compton Scattering Laws of conservation of energy and momentum place limits on both scattering angle and energy transfer. Maximal energy transfer to the Compton electron occurs with a 180-degree photon backscatter. Scattering angle for ejected electron cannot exceed 90 degrees. Energy of the scattered electron is usually absorbed near the scattering site. Slide 23 of 52

24 Compton Scattering Incident photon energy must be substantially greater than the electron s binding energy before a Compton interaction is likely to take place. Probability of a Compton interaction increases with increasing incident photon energy. Probability also depends on electron density (number of electrons/g density) With exception of hydrogen, total number of electrons/g fairly constant in tissue Probability of Compton scatter per unit mass nearly independent of Z Slide 24 of 52

25 Photoelectric Absorption All of the incident photon energy is transferred to an electron, which is ejected from the atom. Kinetic energy of ejected photoelectron (E c ) is equal to incident photon energy (E 0 ) minus the binding energy of the orbital electron (E b ) E c = E o - E b Slide 25 of 52

26 Photoelectric Absorption (Iodine- 131 ) Slide 26 of 52

27 Photoelectric Absorption Incident photon energy must be greater than or equal to the binding energy of the ejected photon. Atom is ionized, with an inner shell vacancy. Electron cascade from outer to inner shells Characteristic x-rays or Auger electrons Probability of characteristic x-ray emission decreases as Z decreases Does not occur frequently for diagnostic energy photon interactions in soft tissue Slide 27 of 52

28 Photoelectric Absorption Although probability of photoelectric effect decreases with increasing photon energy, there is an exception. Graph of probability of photoelectric effect, as a function of photon energy, exhibits sharp discontinuities called absorption edges. Photon energy corresponding to an absorption edge is the binding energy of electrons in a particular shell or subshell. Slide 28 of 52

29 Photoelectric Mass Attenuation Coefficients Slide 29 of 52

30 Photoelectric Absorption At photon energies below 50 kev, photoelectric effect plays an important role in imaging soft tissue. Process can be used to amplify differences in attenuation between tissues with slightly different atomic numbers, improving image contrast. Photoelectric process predominates when lower energy photons interact with high Z materials (screen phosphors, radiographic contrast agents, bone). Slide 30 of 52

31 Percentage of Compton and Photoelectric Contributions Slide 31 of 52

32 Pair Production Can only occur when the energy of the photon exceeds 1.02 MeV. Photon interacts with electric field of the nucleus; energy transformed into an electron-positron pair. No consequence in diagnostic x-ray imaging because of high energies required. Slide 32 of 52

33 Pair Production Slide 33 of 52

34 TYPES OF RADIATION INJURY Slide 34 of 52

35 LEARNING OUTCOMES At the end of the lesson, the student should be able to:- Explain types of radiation injury. Describe Direct Action of radiation. Describe Indirect Action of radiation. Slide 35 of 52

36 The Effects of Radiation on the Cell at the Molecular Level When radiation interacts with target atoms, energy is deposited, resulting in ionization or excitation. The absorption of energy from ionizing radiation produces damage to molecules by direct and indirect actions. Slide 36 of 52

37 For direct action, damage occurs as a result of ionization of atoms on key molecules in the biologic system. This causes inactivation or functional alteration of the molecule. Indirect action involves the production of reactive free radicals whose toxic damage on the key molecule results in a biologic effect. Slide 37 of 52

38 Direct Action Direct ionization of atoms in molecules is a result of absorption of energy by photoelectric and Compton interactions. Ionization occurs at all radiation qualities but is the predominant cause of damage in reactions involving high LET radiations. Absorption of energy sufficient to remove an electron can result in bond breaks. Ionizing radiation+rh R - + H + Slide 38 of 52

39 Indirect Action These are effects mediated by free radicals. A free radical is an electrically neutral atom with an unshared electron in the orbital position. The radical is electrophilic and highly reactive. Since the predominant molecule in biological systems is water, it is usually the intermediary of the radical formation and propagation. Slide 39 of 52

40 Indirect Action- Radiolysis of Water Free radicals readily recombine to electronic and orbital neutrality. However, when many exist, as in high radiation fluence, orbital neutrality can be achieved by: 1. Hydrogen radical dimerization (H 2 ) 2. The formation of toxic hydrogen peroxide (H 2 O 2 ). 3. The radical can also be transferred to an organic molecule in the cell. H-O-H H + + OH - H-O-H H 0 +OH 0 (ionization) (free radicals) Slide 40 of 52

41 Indirect Action H 0 + OH 0 HOH (recombination) H 0 + H 0 H 2 (dimer) OH 0 + OH 0 H 2 O 2 (peroxide dimer) OH 0 + RH R 0 + HOH (Radical transfer) The presence of dissolved oxygen can modify the reaction by enabling the creation of other free radical species with greater stability and lifetimes H 0 +O 2 HO 20 (hydroperoxy free radical) R 0 +O 2 RO 0 2 (organic peroxy free radical) Slide 41 of 52

42 Indirect Action - The Lifetimes of Free Radicals The lifetimes of simple free radicals (H 0 or OH 0 ) are very short, on the order of sec. While generally highly reactive they do not exist long enough to migrate from the site of formation to the cell nucleus. Slide 42 of 52

43 Indirect Action - The Lifetimes of Free Radicals However, the oxygen derived species such as hydroperoxy free radical does not readily recombine into neutral forms. These more stable forms have a lifetime long enough to migrate to the nucleus where serious damage can occur. Slide 43 of 52

44 Indirect Action- Free Radicals The transfer of the free radical to a biologic molecule can be sufficiently damaging to cause bond breakage or inactivation of key functions The organic peroxy free radical can transfer the radical form molecule to molecule causing damage at each encounter. Thus a cumulative effect can occur, greater than a single ionization or broken bond. Slide 44 of 52

45 End of Chapter 2 Slide 45 of 52

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