Interaction of radiation with matter

Interaction of radiation with matter in radiation oncology

1. Interaction of Radiation with Matter 1

2. INTRODUCTION Objective: To understand the important interaction processes between radiation and matter. Two basic entities need to be discussed before dealing with interaction of radiation with matter: RADIATION MATTER 2

3. RADIATION Definition : “It is a form of energy which can be emitted and propagated through the space or a medium by different modes of energy transfer and deposition.” 3

4. ELECTROMAGNETIC RADIATION E and H are the peak amplitudes of electric and magnetic fields respectively. ELECTROMAGNETIC RADIATION Electromagnetic Radiation consists of oscillating electric and magnetic fields which are right angles to each other and to the direction of the energy propagation Electromagnetic radiations are: Radio waves Infrared radiations Visible-light Ultra violet radiations X-rays Gamma rays Cosmic rays 4

5. ELECTROMAGNETIC RADIATION Electromagnetic radiation: Wave model Quantum model W ave : Continuous flow of energy c = υ λ c- velocity, υ - frequency, λ- wavelength Photon : Packages of energy (quanta ) E= h υ E- energy , h – P lanck’s constant(6.63x10 -³³js), υ - frequency E= h c/ λ 5

6. Common Properties Electromagnetic waves travel in straight line. Travel at speed of light ( c ) in a vacuum (or air) c= 3x 10 10 cm/s or 3x10 8 m/s Transfer energy from place to place in quanta Intensity is reduced ( attenuation ) while passing through a medium because of absorption and scattering processes. In free space, all obey inverse square law i.e , “ Intensity from a point source is inversely proportional to square of distance at which intensity is measured.” ELECTROMAGNETIC RADIATION 6

7. Ionizing ionizes [strips electrons from] atoms Non-Ionizing many other modes of interaction Electro Magnetic Spectrum 7

8. ELECTROMAGNETIC SPECTRUM E= h c/ λ or E= h υ 8

9. CLASSIFICATION OF RADIATION Depending upon ionization property : radiation can be classified as 1.Ionizing : removes electrons from atoms Directly ionizing (Alpha and Beta) Indirectly ionizing (Gamma, X-rays & neutrons) 2.Non-ionizing : can't remove electrons from atoms infrared, visible, microwaves, radar, radio waves, lasers 9

10. MATTER Matter can simply be described in terms of the atomic number of the constituent elements Matter is composed of elements ATOMS : a particle of an element An atom consists of a positively charged nucleus surrounded by a cloud of negatively charged electrons An atom is specified by the formula A Z X, where X is the symbol for the element, A is the mass number (number of protons + neutrons), Z is the atomic number (number of protons). 10

11. THE ATOMIC STRUCTURE Central nucleus (protons and neutrons) surrounded by orbiting electron Dimension : Atom 10 -10 m Nucleus 10 -15 m 11

12. PLANETORY MODEL OF ATOM NIELS BOHR – in 1913 Hydrogen atom – extended to multi-electron atom Energy levels- K,L ,M…….. so forth from nucleus No. of electron 2 x n 2 n – integer specific to each shell ( principal quantum no. ) Energy : released - electron moves to closer orbit : required - to move to higher orbit 12

13. Fig : Bohr’s model of the atom Fig : Energy level diagram According to Niels Bohr , electrons revolve in specific orbits around the nucleus. These orbits are named as K,L,M etc; K being innermost orbit. These electron orbits are synonymous with energy levels. Higher the atomic number, greater is this binding energy. 13

14. The process by which one or more electrons are removed from an atom and a neutral atom acquires a positive or negative charge IONIZATION electron is stripped from atom - - - - The neutral atom gains a + charge = an ion + + Alpha Particle 14

15. IONIZATION Formation of a charged and reactive atom - - - - The neutral absorber atom acquires a positive charge Beta particle - Colliding coulombic fields Ejected electron 15

16. Excitation It is a process by which the orbital electrons of an atom are raised to a higher energy level. Inner shell electrons are imparted sufficient energy to “jump up” to higher energy level. Electron then immediately “jumps down” to its original shell to fill the vacancy and in this process , excess energy is shed as electromagnetic radiation This EM Radiation is characteristic of a given element ( difference in shell energy levels ) and is called characteristic radiation 16

17. INTERACTION OF RADIATION WITH MATTER INTRODUCTION Study of these interactions is medically important because 1. Mechanism of cell injury caused by radiation exposure. 2. Radiation protection: shielding requirements. 3. Detection of radiation. 17

18. Contd….. Unaffected by the chemical or physical state of the elements. During the passage of radiation through matter, radiation is either absorbed or scattered. Effect of radiation on matter depends on amount of energy absorbed by the matter. The specific mechanisms which are responsible for either absorption or scattering vary with type of radiation. 18

19. INTERACTION OF RADIATION WITH MATTER contd… When photon passes through matter , it may be 1 . Transmitted unchanged 2. Deflected from its original path to a new direction with unchanged energy 3 . Deflected and lose some energy 4 . Disappear altogether Ref: Fundamental Physics of Radiology by W. J. Meredith & Messey 19

20. Process Definition Attenuation Reduction in intensity of radiation by the matter. Attenuation may occur due to scattering and absorption Absorption The taking up of the energy from the beam by the irradiated material. It is absorbed energy, which is important in producing the radiobiological effects in material or soft tissues. Scattering Refers to a change in the direction of the photons and it contributes to both attenuation and absorption Transmission Any photon, which does not suffer the above processes is transmitted. 20

21. When radiation passes through any material, a reduction in the intensity of the beam occurs, This is known as attenuation . Attenuation occurs exponentially , i.e. a given fraction of the photons is removed for a given thickness of the attenuating material. Attenuation Fig : Semilog plot showing exponential attenuation of a monoenergetic photon beam. 21

22. ATTENUATION The greater the thickness of material , the greater the attenuation. The greater the atomic number and/or the density of the material, the greater the attenuation produced by any given thickness. The greater the photon energy , the smaller the attenuation produced by a given thickness of a particular material. Ref: Fundamental Physics of Radiology by W. J. Meredith & Messey 22

23. Half-value-layer (HVL) Half-value-layer (HVL)- The thickness of the absorber material required to decrease (attenuate) the intensity of a monoenergetic photon-beam to half of its original value. This shows the quality or the penetrating power of an x-ray beam. 2 nd HVL 1 st HVL First layer will decrease the intensity of the incident beam to half of its original value, which gets further reduced to its 1/4 th by the next layer of the material. 23

24. Linear attenuation coefficient ( μ ) : The fractional reduction (in any monoenergetic photon-beam) for any given material per unit thickness. μ : is the probability of the photon being removed by a given material. μ = 0.693 / HVL The linear attenuation coefficient depends upon the density of the material. As compression of a layer of material to one half of the thickness will not affect its attenuation. To circumvent this problem, the mass attenuation coefficient is used which is defined as: Mass attenuation coefficient = μ / ρ Attenuation Coefficients 24

25. Attenuation of a photon beam by an absorbing material is caused by five major types of interactions : Processes causing Attenuation 25

26. Basically FIVE main processes which describe the interaction of x-ray and gamma ray through matter. These are : 1. Elastic or Classical or Unmodified or Thomson or Rayleigh or Coherent scattering. 2. In-elastic or Compton or Modified or Incoherent scattering. 3. Photoelectric effect. 4. Pair production. 5. Photo disintegration 26

27. High Speed Electrons Photon 27

28. CLASSICAL/ELASTIC SCATTERING Also known as coherent scattering, unmodified, Thomson, classical, Rayleigh scattering Explained by considering radiation as a waves rather than photon Radiation Interaction with bound electron causes oscillation of electron which reradiates energy and scatters x rays with same frequency in all directions Radiation is deflected with same frequency (energy) at small angles. The scattered photon has the same wavelength as the incident photon. No energy is changed in to electronic motion No energy is absorbed in the medium Ref: Fundamental Physics of Radiology by W. J. Meredith & Messey 28

29. classical scattering or Rayleigh scattering or coherent scattering Diagram illustrating the process of coherent scattering. The scattered photon has the same wavelength as the incident photon. No energy is transferred .( No attenuation and No absorption) Ref: Fundamental Physics of Radiology by W. J. Meredith & Messey 29

30. CLASSICAL/ELASTIC SCATTERING Contd… This process involves bound electron, coherent scattering occurs more in high atomic number materials and with low energy radiations. MASS ATTENUATION CO-EFFICIENT- Measure of the probability of this process, Directly proportional to Z 2 . Inversely proportional to radiation energy (E) This process is of academic interest in radiotherapy It is important in X-ray crystallography: to know about the structure of materials Ref: Fundamental Physics of Radiology by W. J. Meredith & Messey 30

31. 2. COMPTON SCATTERING Also known as incoherent scattering, modified scattering Compton process involves transfer of a part of the energy of the incoming photon to a “free electron”. Electron receives some energy and ejected at an angle  and photon with reduced energy (increased wavelength) scattered at an angle  . Since the Compton process involves these free electrons, the process is independent of the atomic number of the medium in which the interaction takes place. Ref: Fundamental Physics of Radiology by W. J. Meredith & Messey 31

32. COMPTON SCATTERING - - - Incoming photon Collides with electron - - - - Electron is ejected from atom - Scattered Photon 32

33. If the angle by which the electron is ejected is θ and the angle by which the photon is scattered is Φ , then the following formula describes the change in the wavelength ( δλ )of the photon: λ 2 – λ 1 = δλ = 0.024 ( 1- cos θ ) Å COMPTON SCATTERING contd… Ref: Fundamental Physics of Radiology by W. J. Meredith & Messey 33

34. COMPTON SCATTERING contd… 1. Direct hit : Photon makes a direct hit with the electron, Photon scattered backward (  = 180 degrees) Scattered photon left with minimum energy Electron travel forward (  =0degrees) Electron receive maximum energy (E max) . 2. Grazing hit : Photon makes a grazing hit with the electron Photon scattered forward direction ( = 0 degrees) Electron at right angle(  = 90 degrees) 3. 90-degree photon scatter Photon is scattered at right angles to its original direction (φ = 90 degrees). 34

35. COMPTON SCATTERING contd… salient features… 1. The fraction of the energy absorbed in a collision increases with increase in energy of the incident photon 2. The fraction of the energy scattered is large for low energy photons and very low for high energy photon. 3. Compton effect decreases with increase in energy of the incident photon Ref: Fundamental Physics of Radiology by W. J. Meredith & Messey 35

36. COMPTON SCATTERING contd… 4. In soft tissue the range of 100 keV to 10 MeV, Compton absorption is much more prominent than photoelectric or pair production process . As the energy of the incident photon energy increased, the electron will be ejected in more in forward direction and will carry the maximum energy. 75% of radiation damage is caused by ejected electron from the orbit and 25% of radiation damage is caused by free radical. 36

37. 3. Photoelectric effect In this process, the photon imparts all its energy to an orbital electron of an atom of the medium and causes ejection of electron ( photoelectron ) from its orbit. Vacancy created in shell filled by outer electron with emission of characteristic x ray Absorption of x ray internally produces mono energetic AUGER electron. A photon of energy h ν will release an electron with kinetic energy KE = h ν – E B Where E B is the binding energy of the electron for that particular orbit. 37

38. PHOTOELECTRIC EFFECT 38

39. Photoelectric process involves bound electron . The probability of ejection of an electron is maximum when the photon energy is just higher than the binding energy of the electron. (Photon energy h  > electron binding energy E B) The mass photoelectric attenuation co-efficient Directly proportional to Z 3 . Inversely proportional to E 3 where, Z - Atomic no. E - Photon energy The probability of interaction decreases as h  increases. High Z materials are strong X Ray absorber As the photon energy increases there is greater probability for photoelectron to be ejected in the forward direction. Photoelectric effect is predominant up to 60keV. PHOTOELECTRIC EFFECT CONTD…. 39

40. As the graph on the right shows, there are discontinuities in the attenuation coefficient at specific photon energies. The absorption edges , correspond to the binding energies of the electrons in different shells. PHOTOELECTRIC EFFECT 40

41. PAIR PRODUCTION Pair production is the conversion of a photon into a pair of positive and negative charges and this interaction occur in the nuclear field. Since the creation of the pair requires a minimum energy of 1.02 MeV (which is twice the rest mass energy according to the mass energy relationship E = mC2 ), photon must possesses at least 1.02 MeV . Energy in excess of 1.02 MeV is shared equally in the form of kinetic energy by the pair formed. 41

42. Contd…. The positron having lost its kinetic energy combined with an electron giving rise to annihilation radiation normally in the form of two photons each with 0.51 MeV moving in opposite directions. The attenuation coefficient varies with Z 2 per atom. This process increases with increase in energy of the incoming photon. 42

43. PAIR PRODUCTION 43

44. Annihilation radiation 44

45. PRINCIPAL MODES OF INTERACTION 45

46. Photon Energy (MeV) Relative Number of Interactions (%) P.E. ( τ / ρ ) Compton ( σ / ρ ) Pair Prod. ( π / ρ )   0.01 95 5 0.026 50 50 0.060 7 93 0.150 100 4.00 94 6 10.00 77 23 24.00 50 50 100.00 16 84 Data from Johns HE, Cunningham JR. The physics of radiology. 3rd ed. Springfield, IL: Charles C Thomas, 1969. Relative Importance OF P.E. ( τ ) , Compton ( σ ) And Pair production ( π ) processes in Water 46

47. This reaction occurs when the photon has energy greater than the binding energy of the nucleus itself. In this case, it enters the nucleus and ejects a particle from it. The photon disappears altogether, and any energy possesses in excess of that needed to remove the particle becomes the kinetic energy of escape of that particle. In most cases, this process results in the emission of neutrons by the nuclei. This has a threshold of 10.86 MeV . Now a days, the main use of this reaction is for energy calibration of machines producing high energy photons. Photo Disintegration Reaction 47

48. CLINICAL IMPORTANCE CLASSICAL SCATTERING Academic interest in radiotherapy Important in X-ray crystallography Photoelectric effect Diagnostic radiology procedures Compton effect Radio therapy procedures Pair production PET scan 48

49. INTERACTION OF PARTICLES WITH MATTER Charged particles (electrons, protons, α particles) interact principally by: Ionization Excitation. 49

50. ELECTRONS Interaction can be Elastic Inelastic Elastic collisions occur with either atomic electrons or with atomic nuclei - characterized by change in only direction with no loss of energy Inelastic collisions occur with - atomic electrons results in ionization and excitation of atoms -atomic nuclei results in production of BREMSSTRAHLUNG x rays ( braking radiation ) 50

51. Contd… Electrons ejected by ionization can acquire sufficient energy to cause ionization and ejected electrons are called SECONDARY ELECTRONS or DELTA RAYS. Typical energy loss in tissue for a therapeutic electron beam , averaged over its entire range is about 2 MeV/cm of water . As electrons are very lighter than atomic nuclei, electron can loss large fraction of energy in a single process and deflected by very large angles . Even if the electron is monoenergetic when on first impinging , large variation occur among moving electrons( range straggling ) 51

52. Electrons contd..... Probability of bremsstrahlung production per atom is proportional to the square of Z of the absorber Energy emission via bremsstrahlung varies inversely with square of mass of incident particle 52

53. PROTONS AND HEAVY IONS Protons traverse straight path, slowing down continuously by interactions with atomic electrons and atomic nuclei . Results in depth dose characteristic - shows constant absorbed dose value over most of the beam range until near the end sharp increase in the dose occur (BRAGG PEAK) The RBE of proton beam is similar to photon and electron beams. Because of the Bragg peak effect and minimal scattering, protons and heavier charged particle have the ability to concentrate dose inside the target volume and minimize dose to surrounding normal tissues. The rate of energy loss or stopping power of charged particles is proportional to the square of the particle charge inversely proportional to the square of its velocity 53

54. BRAGG PEAK Depth dose distribution for various heavy particle beams with modulated Bragg peak at a depth of 10 cm and normalized at the peak center. (From Raju MR. Heavy Particle Radiotherapy. New York: Academic Press, 1980.) 54

55. NEUTRONS Like x-rays and γ rays, neutrons are indirectly ionizing . Neutrons interact basically by two processes: (a) recoiling protons from hydrogen and recoiling heavy nuclei from other elements (b) nuclear disintegrations. Exponentially attenuated by matter. Interactions are primarily nuclear and include elastic scattering . Neutron interactions result in recoil protons and charged nuclear fragments which have relatively low energy. 55

56. Lead is an efficient absorber of x-rays but not of neutrons. The most efficient absorber of neutrons is a hydrogenous material such as water, paraffin wax, and polyethylene 56

57. Conclusion Rayleigh Scattering Compton effect Photoelectric effect Pair production Photon interaction With bound electrons With free electrons With whole atom (bound electron) With nuclear Coulomb field Mode of photon interaction Photon scattered Photon scattered Photon disappears Photon Disappears Energy dependence Decreases with energy Increases with Energy Threshold No No No 2 mec² Linear attenuation coefficient Particles released None Compton (recoil) electron Photoelectron Electron– positron pair 57

58. Conclusion Mass coefficient dependence on Z Independent Subsequent effect None Characteristic X ray, Auger effect Characteristic X ray, Auger effect Annihilation radiation Significant energy region for water <20 keV 20 keV– 10 MeV <20 keV >10 MeV 58

59. Thank you for your attention Any Questions? 59