Interaction of photons with matter

Radiation oncology , Interactions of Photons with matter

1. Interaction of Photons with Matter Dr Ankita Singh JR, RT&RM IMS-BHU

2. Radiation The term radiation applies to the emission and propagation of energy through space or a material medium. Types Electromagnetic Particulate

3. Electromagnetic radiation A . Wave model

4. B. Quantum model Considered electromagnetic radiation as particle The amount of energy carried by such a packet of energy or Photon ,

5. Eletromagnetic spectrum

6. Radiation ionizing radiation Non ionizing radiation Directly ionising Indirectly ionising Particulate Electron Proton Neutron Alpha particle Heavy charge particle EM waves X rays Gamma rays Ultraviolet Infrared Visible light Microwaves Radio waves

7. Interactions of photons

8. A. Excitation & Deexcitation with subsequent release of EM radiation B. Ionisation & the production of Delta rays

9. Excitation & Ionization Excitation is the transfer of some of the incident particle's energy to electrons in the absorbing material, promoting them to electron orbitals farther from the nucleus (i.e., higher energy levels). In excitation, the energy transferred to an electron does not exceed its binding energy. the electron will return to a lower energy level, with the emission of the excitation energy in the form of electromagnetic radiation If the transferred energy exceeds the binding energy of the electron, ionization occurs, whereby the electron is ejected from the atom . The result of ionization is an ion pair consisting of the ejected electron and the positively charged atom. Sometimes the ejected electrons possess sufficient energy to produce further ionizations called secondary ionization. These electrons are called delta rays

10. Photon Beam Attenuation 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.

11. The reduction in the number of photons ( dN ) is proportional to the number of incident photons (N) and to the thickness of the absorber (dx) . • Where μ is called Proportionality constant This equation can be written in terms of intensity If thickness x is expressed as a length, then μ is called the “linear attenuation coefficient ”

12. The greater the thickness of material , the greater the attenuation. For a given thickness, the greater the atomic number and/or the density of the material, the greater the attenuation. The greater the photon energy , the smaller the attenuation produced by a given thickness of a particular material .

13. 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 reflects the quality or the penetrating power of an x-ray beam. From the equation , I (x) =I e  x

14. ATTENUATION CO-EFFICIENTS This coefficients depends on the energy of the photons and the nature of material. • Since the attenuation produced by a thickness x depends on the number of electrons present in that thickness, μ depends on the density of the material. Mass attenuation coefficient: Attenuation coefficient per unit density ρ is called mass attenuation coefficient.  /  (cm 2 /g) Electronic attenuation coefficient: The absorber thickness can also be expressed in units of electrons/cm 2 . (  /  ) ( 1/N O ) (cm 2 /electron ) where N0 is the number of electrons per gram

15. Energy transfer coefficient : The fraction of photon energy transferred into kinetic energy of charged particles per unit thickness of absorber . - where Etr is the average energy transferred into kinetic energy of charged particles per interaction. - The mass energy transfer coefficient is given by  tr/  . The energy absorption coefficient  en : T he product of energy transfer coefficient and (1 - g) where g is the fraction of the energy of secondary charged particles that is lost to bremsstrahlung in the material.  en =  tr (1 - g) The mass energy absorption coefficient is given  en /  .

16. 1. can penetrate the section of matter without interacting. 2. It can interact with the matter and be completely absorbed by depositing its energy. 3. It can interact and be scattered or deflected from its original direction and deposit part of its energy .

17. Scattering Scattering refers to an interaction resulting in the deflection of a particle or photon from its original trajectory , Elastic scattering Inelastic Scattering The total kinetic energy of the colliding particles is unchanged Attenuation without absorption When scattering occurs with a loss of kinetic energy i.e., the total kinetic energy of the scattered particles is less than that of the particles before the interaction

18. Coherent scattering/classical/ Rayleigh scattering EM waves passing near the electron Setting it into oscillation Irradiates the energy at the same frequency as the incident EM wave .

19. Scattered X rays have the same wavelength as the incident beam. No energy is changed into electronic motion & no energy is absorbed into medium This process involves bound electron, coherent scattering occurs more in high atomic number materials and with low energy radiations . Important in X-ray crystallography: to know about the structure of materials.

20. Photoelectric effect The process in which a photon is absorbed by an atom, and as a result one of its orbital electrons is ejected is called ‘Photoelectric effect’ . In this process, the entire energy ( hν ) of the photon is first absorbed by the atom and then essentially all of it is transferred to the atomic electron. The kinetic energy of the ejected electron (called the photoelectron) is equal to hν - EB. where EB is the binding energy of the electron

21. The ionized atom regains electrical neutrality by rearrangement of the other orbital electrons. The electrons that undergo the these rearrangements surrender some of the energy in form of a photon known as the characteristic radiation of the atom.

22. Absorption of the characteristic radiation internally in the atom may result in emission of Auger electrons . These electrons are monoenergetic in nature Dominant interaction at energies of 10- 26 KeV The probability of the photoelectric effect occurring is strongly dependent on the atomic number of the material traversed and on the energy of the incident photon Probability ~ Z 3 /E 3 The mass photoelectric attenuation coefficient ( τ/ρ) is directly proportional to the cube of the atomic number and inversely proportional to the cube of the radiation energy. τ/ρ = k Z3/ E3

23. Absorption edges The sudden increase in attenuation of the radiation occur at photon energies equal to the binding energies of the different shells due to increased probability of PE absorption.

24. Graph of mass photoelectric attenuation coefficients plotted against photon energy, & for different materials. • The graph for lead has discontinuities at about 15 and 88keV. • These are absorption edges, & correspond to the binding energies of L & K shells. • A photon with energy less than 15 keV does not have enough energy to eject an L electron. • Thus, below 15 keV , the interaction is limited to the M or higher-shell electrons.

25. Clinical application- In diagnostic imaging PE absorption – white areas on the radiograph Transmitted X rays - grey areas on radiograph 1. As it provides clear differentiation between tissues with different atomic number ( Eg - bone ,muscle, fat) amplifies differences in Xray absorption due to differences in Z. 2. Z dependence is also exploited when using contrast materials such as Barium for the greater appreciation of structures that would otherwise not visible clearly. 3. Benefit of photoelectric absorption in x-ray transmission imaging is that there are no nonprimary photons to degrade the image.( radiograph – too black) Therapeutic radiology The low energy beams produced by orthovoltage machines cause high absorption of Xray energy in Bone as a result of Z 3 dependence

26. The probability of photoelectric interaction is proportional to 1/E 3 explains, why image contrast decreases when higher x-ray energies are used in the imaging process KV imaging is better soft tissue visibility and contrast that MV imaging

27. 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 ”. Predominant at 100 KeV - 1 MeV 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.

28. If the angle by which the electron is ejected is θ and the angle by which the photon is scattered is Φ, then theformula describes the change in the wavelength ( δλ ) of the photon λ2 – λ1 = δλ = 0.024 ( 1- cos θ ) Å

29. The Compton process can be analysed in terms of collison between 2 particles , a Photon & an Electron By applying Laws of conservation of Energy & Momentum, hν0 , hν ', and E are the energies of the incident photon, Scattered photon, and electron, respectively, α = hν0 /m0 c 2 , where m0 c 2 is the rest energy of the electron (0.511 MeV) .

30. Direct Hit If a photon makes a direct hit with the electron, the electron will travel forward (θ = 0 degrees) and the scattered photon will travel backward (φ = 180 degrees) after the collision. • In such a collision, the electron will receive maximum energy Emax and the scattered photon will be left with minimum energy h ν I min . • Emax and h ν I min can be calculated by substituting cos φ = cos 180 o = -1

31. Application Interaction with low Energy incident Photon Interaction with High Energy Incident Photon Compton scattered Photon have approx. the same energy as the original photons, only small part is imparted to the electron. Scattered photon carry away only a small fraction of initial energy Compton effect causes a large amount of energy absorption as compared to tat with low energy photons.

32. Average proportion of Photon energy transmitted to secondary electron during Compton process Energy transmitted to the secondary electrons increases with increase in energy of incident photon

33. Grazing Hit If a photon makes a grazing hit with the electron, the electron will be emitted at right angles (θ = 90 degrees) and the scattered photon will go in the forward direction (φ = 0 degrees). By substituting cos φ = cos 0 o = 1 Substituting these above values in the equations we get , Emax = 0 h ν ' = h ν0

34. 90-Degree Photon Scatter If a photon is scattered at right angles to its original direction (φ = 90 degrees) • Emax and hν ' can be calculated from acquired equations by substituting cos φ = cos 90 = 0 • The angle of the electron emission in this case will depend on α.

35. Dependence of Compton effect on E & Z Compton effect decreases with increase in Energy Independent of Atomic number Depends only on Electron density i . e number of electron per gram Number of electron per gram decreases slowly but systemically with atomic number Most materials except Hydrogen have approx. same electron density  c  nearly same for all material.

36. Clinical application The probability of the Compton interaction depends on the density of electrons in a material, which varies as Z/A. This ratio is almost constant for elements except hydrogen So the Compton effect can be considered to be independent of the atomic number of the material the photons pass through and is dependent only on the electron density. Medical imaging with megavoltage photons leads to poorer contrast than imaging with kilovoltage photon beams . 2. A benefit for radiotherapy to tumors as a dependence on atomic number would lead to higher absorbed dose being delivered to bone than soft tissue .

37. As the incident photon energy increases, a higher proportion of its energy is transferred to the electron. have implications for radiotherapy and radiation dosimetry. For kilovoltage photon beams, electrons set in motion through Compton interactions & deposit their energy very close to the site of interaction, 2. For megavoltage photons, these interactions produce high energy secondary electrons which will travel a significant distance. observed skin-sparing effect of absorbed dose deposition in tissue by megavoltage photon beams, as electrons set in motion near the skin surface deposit their energy over a significant depth .

38. Pair production

39. The threshold energy for the pair production process is 1.02 MeV. The photon energy in excess of this threshold is shared between the particles as kinetic energy. The total kinetic energy available for the electron-positron pair is given by ( hν = 1.02) MeV. The particles tend to be emitted in the forward direction relative to the incident photon. The pair production process is an example of an event in which energy is converted into mass, as predicted by Einstein's equation E = mc2 The reverse process, namely the conversion of mass into energy, takes place when a positron combines with an electron to produce two photons, called the annihilation radiation.

40. Variation of pair production with E & Z Pair production results from interaction with electromagnetic field of nucleus, probability increases rapidly with atomic number Pair atomic attenuation coefficient a П α Z 2 Pair electronic attenuation coefficient П  α Z Pair mass attenuation coefficient П  α Z

41. The likelihood of pair production increases as the logarithm of the incident photon energy , above the threshold energy. For energies upto 20 MeV , curves are coincident for all materials indicating , a П α Z 2 For Higher energies , the curve for high Z materials fall below the low Z materials because of screening of nuclear charge by orbital electron

42. In water (and soft tissue), pair-production only becomes significant at photon energies above approximately 10 MeV so accounts for very little of the absorbed dose to a patient undergoing radiotherapy. Mass attenuation coefficients, showing the relative contributions from the photoelectric effect, Compton effect and pair- production in water (effective Z = 7)

43. Annihilation Radiation Two photons of energy 0.51 MeV are produced when positron generated in Pair Production combines with electron after many interactions These photons are called as “Annihilation photons” . Due to momentum conservation of energy the direction of propagation these photons becomes opposite

44. Photodisintegration 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.

45. An example of such a reaction is provided by the nucleus of 63 Cu bombarded with a photon beam: The above reaction has a definite threshold, 10.86 MeV Because of the production of neutrons , it is important to consider for neutron shielding in RT bunker where Energy of Photon is above 10 MeV

46. Relative importance of Various types of Interactions The Total mass attenuation coefficient (  ) is the sum of four individual coefficients for these processes: Where., •  - Total mass attenuation co-efficient •  coh   - Coherent scattering •    - Photoelectric effect •  c  - Compton effect •  - Pair production

47. The mass attenuation coefficient is large for low energies and high-atomic number media because of the predominance of photoelectric interactions under these conditions. The attenuation coefficient decreases rapidly with energy until the photon energy far exceeds the electron-binding energies and the Compton effect becomes the predominant mode of interaction. In the Compton range of energies, the  of lead and water do not differ greatly , since this type of interaction is independent of atomic number. The coefficient, however, decreases with energy until pair production begins to become important. The dominance of pair production occurs at energies much greater than the threshold energy of 1.02 MeV .