INTERACTION OF RADIATION WITH MATTER DR ARNAB BOSE Dept. of Radiotherapy NRS Medical College, Kolkata
Part 1 : Introduction Importance of the knowledge of the fundamentals of interaction with matter3. Forms the basis of Radiobiology4. Forms the basis of Radiation protection5. Forms the basis of Radiation detection6. Ensures safe and effective methodologies in Radiology and Radiotherapy
Radiation The term radiation applies to the emission and propagation of energy through space or a material medium . Radiation may be Electromagnetic Radiation Particle Radiation When radiation passes through matter it may interact with the material , transferring some or all of its energy to the atoms of that material.
Electromagnetic Radiation Constitutes the mode of energy propagation for such phenomena as light waves, heat waves, radio waves, u v rays, x rays and γ rays . Spectrum of electromagnetic radiation ranges from 107 m (radio waves) to 10-13 m (ultra high energy X rays) . X rays and γ rays are the two major forms of electromagnetic radiation used in modern day radiotherapy. An X ray beam emitted from a target or a γ ray emitted from a radioactive source consists of a large number of photons , usually with a variety of energies .
Electromagnetic Wave with respect to Electric & Magnetic Field
Particulate Radiation Refers to the energy propagated by traveling corpuscles – which have definite rest mass , definite momentum and a defined position at any instant . Elementary atomic particles are electrons (charge – 1) , protons (charge + 1) and neutrons (zero charge) . Some common subatomic particles are positrons (charge + 1) , neutrinos (zero charge) and mesons .
Part 2 : Interaction of Photons with Matter When an X ray or γ ray beam passes through a medium , interaction occurs between the photon and the matter and energy is transferred to the medium . If the absorbing medium consists of body tissues sufficient energy may be deposited within the cells destroying their reproductive capacity .
Fate of the Photon Beam The photon beam may undergo the following four processes – Attenuation , Absorption , Scattering and Transmission . Attenuation refers to the removal of radiation from the beam by the matter . Attenuation may occur due to scattering and absorption . Absorption refers to the taking up of energy from the beam by the irradiated material .It is the absorbed energy which is important in producing the radiobiological effects . Scattering refers to the change in the direction of photons and it contributes to both attenuation and absorption . Any photon which does not suffer the above processes is transmitted .
Attenuation Coefficient (1) Fraction of photons removed from a mono energetic beam of x-ray or gamma ray per unit thickness of material is called linear attenuation coefficient (µ), typically expressed in cm-1 . Number of photons removed from the beam traversing a very small thickness ∆x: µ x n = N∆ where n = number removed from beam,N = number of photons incident on the material,and minus sign is placed before μ to indicate that no. of photons decreases as the absorber thickness increases.
Attenuation Coefficient (2) For a mono energetic beam of photons incident on either thick or thin slabs of material, an exponential relationship exists between number of incident photons (N0 ) and those transmitted (N) through thickness (x) without interaction: −µ N =N e 0 x The number of photons indicate the Intensity of the beam and can also be written as ( I ).
Attenuation Coefficient (3) Total Linear attenuation coefficient is the sum of individual linear attenuation coefficients for each type of interaction: µ = µRayleigh + µphoto + µCompton + µpair For a given thickness of material , probability of interaction depends on number of atoms the x ray or gamma ray encounter per unit distance. The density (ρ ) of material affects this number. Linear attenuation coefficient is proportional to the density of the material.
Mass Attenuation Coefficient For a given thickness probability of interaction is dependent on the number of atoms per volume. This dependency can be overcome by normalizing linear attenuation coefficient for density of material – Mass Attenuation Coefficient (μ / ρ ) = Linear attenuation coefficient Density of the material Mass attenuation coefficient is independent of density of the material.
Half Value Layer (1) Half value layer (HVL) defined as thickness of material required to reduce intensity of an x-ray or gamma-ray beam to one-half of its initial value. It is an indirect measure of the photon energies (also referred to as quality or penetrability) of a beam of radiation. For mono energetic photons, the probability of attenuation remains the same for each additional HVL thickness placed in the beam. Relationship between μ and HVL: HVL = 0.693/μ
List of Interactions Attenuation of a photon beam by an absorbing material is caused by 5 major types of interactions –2. Coherent Scattering3. Photoelectric Effect4. Compton Effect5. Pair Production6. Photonuclear Effect
Coherent Scattering X-rays passing close to the atom cause the bound electrons to vibrate momentarily at a frequency equal to that of the radiation. These in turn emit radiation of the same frequency in all directions . The energy is taken up from the beam and scattered in all direction, but none of the energy is absorbed. Thus this is a form of attenuation without absorption . This interaction is of little importance in practical radiotherapy, but is important in X-ray crystallography . Since it involves bound electrons, it occurs more in higher atomic number materials, and also more with low-energy radiations.
Photoelectric Effect (1) All of the incident photon energy is transferred to an electron, which is ejected from the atom. Kinetic energy of ejected electron called the photoelectron (EC ) is equal to incident photon energy (EO ) minus the binding energy of the orbital electron (EB ) EC =EO - EB
Photoelectric Effect (3) Incident photon energy must be greater than or equal to the binding energy of the ejected photon. The ionized atom regains electrical neutrality by rearrangement of the other orbital electrons. The electrons that undergo these rearrangements surrender some of the energy in form of a photon known as the characteristic radiation of the atom. Absorption of these characteristic radiation internally in the atom may result in emission of Auger electrons . These electrons are mono energetic in nature.
Photoelectric Effect (4) Probability of photoelectric absorption per unit mass is approximately proportional to 3 3 Z /E Energy dependence explains, in part, why image contrast decreases with higher x-ray energies. Process can be used to amplify differences in attenuation between tissues with slightly different atomic numbers, improving image contrast.
Photoelectric Effect (5) 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 sub shell . The phenomena of absorption edges is important for two different reasons:1) At these absorption edges, low-energy photons are less attenuated and therefore more penetrating than high energy photons. 2)A substance is relatively transparent to its own characteristic radiation. This effect is important when filters are considered as the filters will be “transparent” to their own characteristic radiation.
Compton Effect (1) Photon interacts with an atomic electron as though it were a free electron. Practically this means that energy of the incident photon must be large compared with the electron binding energy. The electron receives some energy from the photon and is emitted at an angle Φ while the photon with reduced energy is scattered at an angle θ .
Compton Effect (4) As incident photon energy increases, scattered photons and electrons are scattered more toward the forward direction. Probability of interaction increases as incident photon energy increases. Probability also depends on electron density ( no. of electrons per gram of matter ). Electron density fairly constant in tissue. Probability of Compton scatter/unit mass independent of Z.
Compton Effect (5) Maximum energy of photons with 90° scatter is 0.511 M e V while that for 180° scatter ( i.e.. Back scatter) is 0.255 M e V. Energy of the photons scattered at angles <90 ° will be more than 0.511 M e V and will gradually approach the incident photon energy. Energy of the scattered radiation is independent of the incident beam energy .This implies that as the photon energy increases there is a corresponding increase in the forward scatter of the beam. This results in better dose distribution. Direction of the scatter depends on the energy of the incident photon beam . This means that higher beam energies allow greater absorption of the dose in the body with less scattering of energy.
Compton Effect (6) If the angle by which the electron is scattered is θ and the angle by which the photon is scattered is Φ, then the following formula describes the change in the wavelength ( δ λ )of the photon: λ 1 – λ 2 = δ λ = 0.024 ( 1- c o s Φ) Å Thus the wavelength change depends neither on the material being irradiated nor on the radiation energy, but only upon the angle through which the radiation is scattered. The Compton effect results in both attenuation and absorption .
Compton Effect (7) 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.
Pair Production (1) When the photon with energy in excess of 1.02 M e V passes close to the nucleus of an atom, the photon disappears, and a positron and an electron appear. This effect is known as pair production. Pair production results in attenuation of the beam with absorption. The positron created as a result loses its energy by interaction with an electron to give rise to two annihilation photons, each having 0.511 M e V energy. Again because momentum is conserved in the process two photons are rejected in opposite directions. This reaction is known as an annihilation reaction.
Pair Production (3) Pair production results from an interaction with the electromagnetic field of the nucleus and as such the probability of this process increases rapidly with the atomic number (Z 2 ). In addition, the likelihood of this interaction increases as the photon energy increases, in contrast to the Compton effects and the photoelectric effect.
Photonuclear Reaction 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 possessed in excess of that needed to remove the particle becomes the kinetic energy of escape of that particle. The threshold energy for this effect is 10.8 M e V. The main use of this reaction is for energy calibration of machines producing high energy photons.
Relative Importance of the Various ProcessesThe relative importance of the 3 principal modes ofinteraction pertinent to radiation therapy- thePhotoelectric , Compton and Pair production processes- as a function of Incident beam energy and Atomicnumber of absorber matter shows - For an absorber with Z approximately equal to that ofsoft tissue - 7 , and for mono energetic photons ,Photoelectric effect is the dominant interaction belowabout 30 k e v.Above 30 k e v Compton effect remains dominant andremains so,Until about 24 M e v , after which Pair Production effectbecomes dominant .
Relative Importance of the Various Processes (3) In a graph plotted for total mass attenuation coefficient vs. photon energy it is seen that - The μ /ρ is large for low energies and high Z media (eg. Lead ) because of the predominance of Photoelectric interactions under these conditions. The μ /ρ decreases rapidly with energy until the photon energies far exceed the electron binding energies and Compton effect becomes the predominant mode of interaction. In the range of Compton effect the μ /ρ of lead and soft tissues do not vary greatly as Compton effect is independent of Z .The μ /ρ however decrease with energy until Pair production becomes important .
Plot of total mass att. Coef.As a function of photon energy
Part 3 : Interaction of Particle Radiation with Matter Interaction of Electrons with matter – The two different modes of interaction and energy transfer of electrons with matter include: Collision between the particle and the electron cloud resulting in ionization and excitation ( more important in low atomic number elements). This is called Collisional loss . Collision between the nucleus and the particle resulting in bremsstrahlung radiation (more in high atomic number elements). This is called Radiative loss .
Electron Interactions The ionization pattern produced by a beam of electrons is characterized by a constant value from the surface to a depth equal to about half the range, followed by a rapid falling off to almost zero at a depth equal to the range . The bremsstrahlung radiation produced when electrons slow down contributes to an insignificant dose beyond the range of any electron. This is specially seen in electrons in the energy range of 6 -15 M e V . These characteristics make electrons a useful treatment modality for superficial lesions.
Proton & Pi Meson Interactions Protons and pi mesons are charged particles that are being used in experimental set-ups only. These particles have a very high linear energy transfer (LET) that is they have a very high ionization density( Amount of energy deposited per unit path length is called the linear energy transfer (LET) ). Further, these charged particles also exhibit the phenomena of Bragg’s peak which refers to the increased ionization occurring near the end of the track with little effect beyond.
Neutron Interactions• Neutrons are indirectly ionizing uncharged radiations, which interact only with the nucleus in two ways: By recoiling protons from hydrogen and the nucleus in other elements. Nuclear disintegration , which contribute to ~30% of the total dose in tissues.
Part 4 : Practical ImplicationsThe three major forms of interaction of radiation with matter, which are of clinical importance in radiotherapy are: Compton effect. Photoelectric effect. 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.
Practical Implications Coherent scattering is of little importance in practical radiotherapy, but is important in X-ray crystallography . The photoelectric effect has several important implications in practical radiology: In diagnostic radiology , the primary mode of interaction is photoelectric. It is also responsible for the contrast effect. In therapeutic radiology , low-energy beams in orthovoltage irradiation causes excessive absorption of energy in bone.
Practical Implications The attenuation produced by the Compton effect is described by the mass scattering coefficient ( σ / ρ ), and is practically same for all substances except hydrogenous material, like water and soft tissue, where the Compton effect is greater (because of the higher electron density). Attenuation does not depend on the atomic number of absorber matter in Compton effect. Thus concrete is as good as lead in shielding of megavoltage equipment! The absorption in bones does not exceed that produced in the soft tissues – unlike in PE effect seen in orthovoltage radiation era. Port films produced in megavoltage equipment have very little detail.
Practical Implications The low mass of the electron leads to greater scattering. This is of practical importance as radioactive isotopes which are produce high energy beta radiation are better stored in low atomic number materials e.g. plastics as they will lead to lesser bremsstrahlung radiation. Also higher atomic number elements are better for x ray production. The amount of radiative loss is proportional to the square of the atomic number of the material This leads to the phenomenon of greater ionization in soft tissues relative to bones. Ionization and excitation are more for low atomic materials.
Practical Implications Protons and Heavy particle beams have the ability to concentrate dose inside the target volume and minimize dose to surrounding normal tissues because of the Bragg peak effect and minimal scattering. However there are several practical and theoretical difficulties with the use of these charged particles. Some of them include: The narrow Bragg peak makes a homogenous Tumor Dose difficult. Generation of these charged particles requires expensive and large machines. The method of the production ensures that the field size is very narrow. So, for treatment of cancers the beam has to be scanned back and forth across the treatment area.
Practical Implications Hydrogenous materials like fats absorb neutrons more than heavier materials and thus there is a 20% greater absorption in fat relative to muscle. Lower atomic materials (e.g. fats and paraffin) are better for neutron shielding as compared to lead as greater absorption occurs. Neutrons, being uncharged particles also penetrate deeply into matter. But neutrons are not commonly used in practical radiotherapy, because of technical difficulties in production of these beams as well as their complicated dosimetry.
Part 5 : Conclusion 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.