Interactions-of-Radiation-With-Matter ( Dr Dipanjan ).pptx

Ppt on interactions of radiation with matter

1. Interactions of Radiation With Matter Dr DIPANJAN NANDI JUNIOR RESIDENT

2. RADIATION Is energy emitted from a source either as waves or particles .

3. IONIZATION Ionization is a process by which a neutral atom acquires a positive or a negative charge. Ionizing radiation can strip electrons from atoms as they travel through media. An atom from which an electron has been removed is a positive ion. In some cases a stripped electron may subsequently combine with a neutral atom to form a negative ion, usually free electron is called an negative ion.

4. Directly ionizing radiation : Charged particles produce large amount of ionization in its energy loss to the medium. Eg. , electrons, protons, a-particles Indirectly ionizing radiation : Neutral particles themselves produce very little ion pairs. Instead, they eject directly ionizing particles from the medium. Eg. , photons, neutrons

5. Basic Concepts Of Interaction Three possible occurrences when x-ray or gamma photons in the primary beam pass through matter:

6. PHOTON – BEAM INTERACTIONS

7. Photon Beam Attenuation A narrow beam of mono-energetic photons is incident on an absorber of variable thickness. A detector is placed at a fixed distance from the source and sufficiently farther away from the absorber so that only the primary photon are measured by the detector. Any photon scattered by the absorber is not supposed to be measured in this arrangement. Thus, if a photon interacts with an atom, it is either completely absorbed or scattered away from the detector.

8. 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). dN Ndx dN = - μ Ndx Where μ is called Proportionality constant . The equation can also be written in terms of intensity {l} : dI = - μ Idx = - μ dx On integration we get, I(x) = If thickness ‘x’ is expressed as a length, then μ is called the “linear attenuation coefficient”.  

9. Attenuation Of A Photon Beam

10. Half value layer(HVL) The thickness of an absorber required to attenuate the intensity of a monoenergetic photon-beam to half its original value is as the half-value-layer(HVL). The half-value-layer is also an expression of the quality or the penetrating power of an x-ray beam. Can never be reduced to zero. From the equation, I(x) = it can be shown that HVL =  

11. The Five Interactions Of X and Gamma Rays With Matter Photoelectric effect Compton scatter Coherent scatter Pair production Photodisintegration

12. PHOTOELECTRIC EFFECT Emission of electrons (called photoelectrons) from a surface when light is shined on it. FIRST observed by Alexander Edmond Becquerel in 1839. Heinrich Hertz published the first thorough investigation in 1887 Albert Einstein was awarded the Nobel Prize n Physics in 1921 for his theory of the Photoelectric Effect.  

13. Photoelectric Effect All of the energy of the incoming photon is totally transferred to the atom The incoming photon interacts with an orbital electron in an inner shell – usually K The orbital electron is dislodged To dislodge the electron, the energy of the incoming photon must be equal to, or greater than the electron’s energy

14. Photoelectric Effect The incoming photon gives up all its energy, and ceases to exist The ejected electron is now a photoelectron This photoelectron energy = energy of the incoming photon- the binding energy of the electron shell This photoelectron can interact with other atoms until all its energy is spent These interactions result in increased patient dose, contributing to biological damage

15. Photoelectric Effect A vacancy now exists in the inner shell To fill this gap, an electron from an outer shell drops down to fill the gap Once the gap is filled, the electron releases its energy in the form of a characteristic xray The energy released as a result of outer electron filling the vacancy is given to another electron in the higher shell which is then ejected , producing AUGER electrons

16. The Byproducts of the Photoelectric Effect Photoelectrons Characteristic xrays

17. The Probability of Occurrence Depends on the following: The energy of the incident photon The atomic number of the irradiated object It increases as the photon energy decreases, and the atomic number of the irradiated object increases The mass photoelectric attenuation coefficient ( ) is directly proportional to the cube of the atomic number and inversely proportional to the cube of the radiation energy; In water or soft tissue This type of interaction is prevalent in the diagnostic kVp range – 10-25keV(30-75kVp)  

18. Graph of mass photoelectric attenuation coefficients plotted against photon energy, and for different materials reveal several important and interesting features. The graph for lead has discontinuities at about 15 and 88keV. These are called absorption edges , and correspond to the binding energies of L and 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.

19. Implications of Photoelectric effect The photoelectric effect has several important implications in practical radiology: Diagnostic radiology: Photoelectric interaction of low energy is almost 4 times greater in bones than in a equal mass of soft tissue. In soft tissue binding energy of K-shell is too low, elements like iodine, Barium are ideal absorbers of x-rays in diagnostic energy range. For this reason they are widely used as contrast agent in diagnostic radiology. Therapeutic radiology: The low-energy beams in superficial & orthovoltage irradiation causes excessive absorption of energy in bone.

20. Compton Scattering An incoming photon is partially absorbed in an outer shell electron The electron absorbs enough energy to break the binding energy, and is ejected The ejected electron is now a Compton electron Not much energy is needed to eject an electron from an outer shell The incoming photon, continues on a different path with less energy as scattered radiation

21. Byproducts Of Compton Scatter Compton scattered electron causes projectile damage in the tissue. Possesses kinetic energy and is capable of ionizing atoms. The atom becomes a free radical, causing biological damage in the tissue Scattered x-ray photon with lower energy Continues on its way, but in a different direction It can interact with other atoms, either by photoelectric or Compton scattering It may emerge from the patient as scatter

22. Probability Of Compton Scatter Occurring Probability of a Compton interaction is inversely proportional to energy of the incoming photon. In water More probable at kVp ranges of 10-150. and decreases further with increase in energy. Most dominant interaction in tissues at treatment energies(30keV-24MeV). That includes all xray beams in radiation therapy. It is independent of atomic number, so at treatment energies, bone and soft-tissue interfaces are barely distinguishable (= poor contrast)

23. Implications of Compton scattering Photons interact by Compton interaction more readily in materials with high concentration of Hydrogen. Fat has greater concentration of Hydrogen and absorbs more energy by Compton interaction. A volume of bone attenuates more photons by Compton interaction since the number of electrons present in a volume of bone is greater then the same volume of fat. In soft tissue, Compton effect is most important interaction of X and photons. Hence Compton scattering is more predominant in 'Therapeutic radiology'.  

24. Coherent Scattering Also called Classical/ Rayleigh / Unmodified Scattering The electromagnetic radiation cause oscillation of electrons which reradiates the energy at the same frequency as the incident electromagnetic wave The scattered xrays have the same wavelength as the incident beam No energy is changed into electronic motion and no energy is absorbed in the medium The only effect is scattering of photon at small angles

25. Coherent Scatter It is only probable in high atomic number materials and with photons of low energy ( <10 keV )

26. PAIR PRODUCTION The photon may interact with matter through the mechanism of pair production, If the energy of the photon is greater than 1.02 MeV. In this process ,the photon interacts strongly with the electromagnetic field of an atomic nucleus and gives up all its energy in the process of creating a pair consisting of a negative electron ( ) and a positive electron( ). As the rest mass energy of the electron is equivalent to 0.51 MeV, a minimum energy of 1.02 MeV is required to create the pair of electrons. Thus, the threshold energy is 1.02 MeV.  

27. Pair Production An incoming photon of 1.02 MeV or greater interacts with the nucleus of an atom The incoming photon disappears The transformation of energy results in the formation of two particles Negatron Possesses negative charge Positron Possesses a positive charge

28. Thus, 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 (h - 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 = m 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.  

29. 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.

30. si multaneous detection of gamma ray photons in two detectors places the source on a line between those detectors (PET SCAN: where radioisotopes used for positron emission).

31. Pair Production The probability of pair production increases rapidly with atomic number Significant pair production can be seen in blocking of the i ncoming beam, since blocks are high-Z materials (for lead, this is the main effect at energies >5 MeV)

32. Table 5.2 Relative Importance of Photoelectric (τ), Compton (σ), and Pair Production (Π) Processes in Water  Photon Energy ( MeV ) Relative Number of Interactions (%) τ σ Π  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.

33. Photodisintegration Occurs at above 10 MeV A high energy photon is absorbed by the nucleus The nucleus becomes excited and becomes radioactive To become stable, the nucleus emits negatrons, protons, alpha particles, clusters of fragments, or gamma rays

34. PHOTO-DISINTEGRATION An interaction of a high-energy photon with an atomic nucleus can lead to a nuclear reaction and to the emission of one or more nucleons. In most cases, this process of photodisintegration results in the emission of neutrons by the nuclei. An example of such a reaction is provided by the nucleus of bombarded with a photon beam: The above reaction has a definite threshold, 10.86 MeV. This can be calculated by the definition of threshold energy, namely the difference between the rest energy of the target nucleus and that of the residual nucleus plus the emitted nucleon(s). Because the rest energies of many nuclei are known for a very high accuracy, the photodisintegration process can be used as a basis for energy calibration of machines producing high-energy photons.  

35. Relative importance of Various types of Interactions The total mass attenuation coefficient( is the sum of four individual coefficients; Total Coherent Photoelectric Compton Pair Where , and are attenuation coefficients for Coherent scattering, Photoelectric effect , Compton effect and Pair production respectively.  

36. 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.  

37. Interactions Of Charged Particles With Matter Electrons, protons, alpha particles are examples of charged particles. Charged particle interaction or collisions mediated by coulomb force between the electric field of travelling particle and electric fields of orbital electrons and nuclei of atoms of the material. They interact primarily by ionization or excitation. The rate of kinetic energy loss per unit path length of the particle is known as the stopping power.

38. ELECTRONS Interaction can be 1. 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 or ‘braking radiation’).

39. Electrons All particles exhibit Bragg peak near end except electrons due to excessive scattering. Two fundamental interactions: Radiation (Bremsstrahlung) - bending of electrons around nucleus => shedding of energy as EM x-rays Ionization (Characteristic X-rays) - impact with orbital electron => electron release => vacancy fill => shedding of energy as Characteristic x-rays

40. Protons Incoming protons also lose energy mainly by interacting with orbital electrons; however, since they are much heavier (~1800x), they only lose very small fraction of their kinetic energy with each interaction, and thus scatter only minimally The interactions (and thus energy loss) become more frequent at slower energies. Thus the slower the proton moves, the more energy it loses to the tissue electrons, in a feed-forward loop, until it abruptly loses all energy. This region of rapid energy loss (and its deposition into the tissue) is called the Bragg peak. The distance at which Bragg peak occurs, and the energy is deposited, can be calculated very precisely (unlike electrons). The rapid drop-off in dose make it ideal for delivering dose precisely to the tumor, and not to the healty tissue beyond the tumor. Incoming protons also rarely interact with the nucleus, and may enhance cell kill by ~10%

41. Neutrons Interact by basically 2 processes: 1.Recoiling protons from hydrogen 2. Nuclear disintegrations The first process is similar to a billiard ball collision where energy transfer is very efficient if the colliding particles have the same mass The most efficient absorber of neutrons is a hydrogenous material such as water, paraffin wax, and polyethylene. Lead , on the other hand is an efficient absorber of x-rays but poor shielding material against neutrons.

42. Heavy ions A charged particle is called heavy if it’s rest mass is large compared to the rest mass of an electron Stopping power of ionization interactions is proportional to square of particle charge and inversly to square of its velocity They interact with tissue similarly to protons, but since they are heavier still, they scatter less initially, and have a faster dose fall-off (Bragg peak) at the end.

43. Conclusions The three major forms of interaction of radiation with matter, which are of clinical importance in radiotherapy are: 1. Compton effect. 2. Photoelectric effect. 3. Pair production. Out of these, the Compton effect is the most important in modern-day megavoltage radiation therapy. The reduced scattering suffered by high-energy radiation as well as the almost homogeneous tissue dosage is primarily due to the Compton effect. The photoelectric effect is of primary importance in diagnostic radiology and has only historical importance in present day radiotherapy. Despite several decades of research, photon-beam still constitute the main therapeutic modality in radiotherapy, because of several unresolved technical problems with the use of particulate radiation.

44. Thank you