Interaction of Radiation with Matter

Interaction of radiation with matter Dr Santam Chakraborty Junior resident Department of radiotherapy. PGIMER Moderator: Dr. T.S. Kehwar

Nature of radiation The term radiation applies to the emission and propagation of energy through space or a material medium. Types of radiation : Electromagnetic radiation. Particulate radiation. Electromagnetic radiation: Mode of energy propagation for phenomena such as light waves, heat waves, x-rays, γ -rays etc. Was defined by Maxwell in terms of oscillating electrical and magnetic fields. Electromagnetic radiation has a dual nature The spectrum of the electromagnetic irradiation ranges from the wavelength of 10 7 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. The difference among the two lies in the mode of production: X rays are produced when high speed electrons collide with outer electrons. γ rays are produced by intranuclear disintegration. 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: electron, proton, neutron. Positron, neutrino and mesons are subatomic particles.

The smallest indivisible part of an element is known as Atom. The atom is made up of the nuclei and orbital electrons. The nucleus contains two types of particles, protons, which are positively charged and neutrons, which have no charge. The electrons are negatively charged and their number is equal to the number of protons, which makes the atom electrically neutral. Atoms are specified as Z X A where Z = atomic number, and A = mass number. 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. Here energy refers to the potential energy of the electron. This energy depends upon the magnitude of the coulomb forces of attraction between the nucleus and the orbital electrons. Higher the atomic number greater is this binding energy. Nature of matter Fig 1 : Bohr’s model of the atom Fig 2 : Energy level diagram (Hydrogen Nucleus)

When an x-ray or γ ray beam passes through a medium, interactions occur between the photon and the matter and energy is transferred to the medium. The initial step in energy transfer involves ejection of electrons from the atoms of the absorbing medium which in turn, transfer their energy by producing ionization and excitation of the atoms along their path. The photon-beam may undergo the following four processes: Attenuation. Absorption. Scattering. 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 the energy from the beam by the irradiated material. It is absorbed energy, which is important in producing the radiobiological effects. Scattering refers to a change in the direction of the photons and its contributes to both attenuation and absorption. Any photon, which does not suffer the above processes is transmitted . Radiation Interaction (overview): Fig. 3 Interaction of photons with matter Photon Matter Photoelectric effect Compton Scatter Pair production High Speed electrons Matter Ionization Excitation Heat Recombination X rays Chemical effects Biological effects

Attenuation: When monoenergetic radiation passes through any material, a reduction in the intensity of the beam occurs, which is known as attenuation. Attenuation occurs exponentially , that is a given fraction of the photos is removed for a given thickness of the attenuating material. Exponential attenuation of a photon-beam means that it is impossible to reduce this beam to nothing. The thickness of an absorber required to attenuate the intensity of a monoenergetic photon-beam to half its original value is known 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. However, X-rays produced by a generator consist of a spectrum of photon energies. Attenuation of this beam is not exponential. Fig 5: Semilog plot showing exponential attenuation of a monoenergetic photon beam. 2 nd HVL 1 st HVL

The fractional reduction produced in any monoenergetic photon-beam is constant for any given material per unit thickness This constant is known as the linear attenuation coefficient . The linear attenuation coefficient ( μ ) is an expression of the probability of the photon being removed by a given material. The linear attenuation coefficient is related to the half-value-layer by the following expression: μ = 0.693 / HVL The linear attenuation coefficient depends upon the density of the material, and this makes it’s a less fundamental coefficient. Thus 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 Other coefficients which are more fundamental include: Atomic attenuation coefficient. Electronic attenuation coefficient. Attenuation Coefficients: Various materials used for measurement of HVLs : Lead 500 kV- 2 MV Copper Aluminum Cellophane Material 120-600kV 30-150 kV ≤ 30kV Generation Energy

There are five major physical processes, which are responsible for photon-beam attenuation: Coherent scattering. Photoelectric effect. Compton effect. Pair production. Photo disintegration. Processes causing attenuation: Coherent scattering: This is also known as elastic scattering , Thomson scattering, unmodified scattering, classical scattering, Rayleigh scattering, etc. This is one of the processes, which can be more easily described by considering radiation as waves rather than as photons. In addition it is also one of the interactions, where bound electrons are involved. 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.

In this phenomena, the photon disappears altogether after interacting with the bound electron , some of the energy being used to remove the electrons from the shell, while the rest is imparted as kinetic energy to the photo-electron. h ν -  W + ½ m ν 2 The term W refers to the binding energy of the electron and the term ½ m ν 2 refers to the kinetic energy of the photo electron. 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. Absorption of these characteristic radiation internally in the atom may result in emission of Auger electrons . These electrons are monoenergetic in nature. The energy of the characteristic radiation ( fluorescent radiation ) varies from atom to atom, and for low atomic number elements, which make up most of the biological materials, it is of such low energy that it is probably absorbed by the same cell in which the initial event occurs. 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 Z 3 / E 3 The angular distribution of electrons emitted in the photoelectric process depend upon the photon energy. As the photon energy increases the photo electrons are emitted in a more forward direction. Photoelectric effect: Fig. 6 : The photo electric effect

Graphs of mass photoelectric attenuation coefficients plotted against photon energy, and for different materials reveal several important and interesting features. Photoelectric effect: As the graph on the right shows, they are discontinuities in the attenuation coefficient at specific photon energies. These are known as absorption edges . These absorption edges, correspond to the binding energies of the electrons in different shells. When the photon has an energy equal to the binding energy of the corresponding shell, resonance occurs and the probability of photoelectric absorption, involving the shell becomes very high. Beyond this the probability of absorption varies inversely, with the cube of energy (E 3 ). 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 caused excessive absorption of energy in bone. The phenomena of absorption edges is important for two different reasons: At these absorption edges, low-energy photons are less attenuated and therefore more penetrating than high energy photons. 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.

Also known as modified, incoherent and inelastic scattering. First elucidated by Arthur H. Compton in 1923,the Compton effect was accepted as the final proof for the dual nature of light and established the legitimacy of the quantum theory. Compton effect: Fig. 7 : Compton’s experiment In this type of interaction, photons interact with free electrons. The photon collides with electron and hands over part of its energy to it. The angle through which the photon is scattered, the energy handed on to the electron, and energy lost by the photon are interconnected. 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 ( δλ )of the photon: λ 1 – λ 2 = δ λ = 0.024 ( 1- cos 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 .

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). Compton effect (contd.): Practical Implications: This has several important implications in designing radiation protection. The maximum energy of photons with 90° scatter is 0.511 MeV while that for 180° scatter ( i.e.. Back scatter) is 0.255 MeV . The energy of the photons scattered at angles <90 ° will be more than .511 MeV 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. Thus with increasing photon energy greater absorption occurs relative to attenuation. The fraction of the energy imparted to the recoil electron increases as the beam energy increases Thus concrete is as good as lead in shielding of megavoltage equipment! The absorption in bones doesn't exceed that produced in the soft tissues – unlike in PE effect seen in orthovoltage radiation era. There is no Bone shielding phenomenon unlike that seen in orthovoltage radiation. Port films produced in megavoltage equipment have very little detail. Attenuation doesn't depend on the atomic number

When the photon with energy in excess of 1.02 MeV 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 particles tend to travel in a foreword direction related to the incident photon and while any energy distribution is possible the most probable distribution of energy is for each particle to acquire half of the available kinetic energy. The positron created as a result loses its energy by interaction with an electron to give rise to two annihilation photons, each having 0.51MeV energy. Again because momentum is conserved in the process to photons are rejected in opposite directions. This reaction is known as an annihilation reaction. Thus, the energy absorbed from the beam (with incident energy, E) is given by: E - 1.02 MeV 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. The pair production coefficient ( π ) is directly proportional to Z 2 and log of incident photon energy. Pair production: π = k Z 2 log (E)

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. The threshold energy for this effect is 10.8 MeV, and a maximum is reached about 5 MeV above this threshold. The main importance of this reaction lies in the unsubstantiated fear that ejection of a nuclear particle may result in the nucleus becoming radioactive. This had lead to the assumptions that patients may become radioactive following megavoltage radiotherapy in the earlier days of this technique. Nowadays, the main use of this reaction is for energy calibration of machines producing high energy photons. For this the following reaction is used: 29 Cu 63 + γ  29 Cu 62 + 0 n 1 Photo nuclear reaction:

The total mass attenuation coefficient is the sum of three individual coefficients; photoelectric coefficient, mass scattering coefficient and pair production coefficient: ( μ / ρ ) = ( τ / ρ )+( σ / ρ )+( π / ρ ) When we plot the total coefficient versus the photon energy, in different media, the following effects are seen: At low energies the mass attenuation coefficient is larger, especially in high atomic number media, because of the predominance of photoelectric interactions in these circumstances. That attenuation coefficient then decreases rapidly with the energy till the photon energy far exceeds the electron binding energy and Compton effect becomes the predominant mode of interaction. In between the ranges of 200 KeV- 4 MeV, Compton scattering is the predominant mode of interaction. At this energy range, the mass attenuation coefficients also become independent of the atomic number and actually become more for soft tissues, which have more hydrogen content. Beyond 4 MeV pair production results in increasing mass attenuation coefficients specially for high atomic number elements. Thus very high-energy radiations (> 20 MeV) are less-penetrating than some lower energy radiations and are not used in radiotherapy!! Up to 50KeV – PE effect is important. 60 KeV - 90 KeV – Both PE and Compton effects are important. 200 KeV – 4 MeV – Compton effect is increasingly important. Beyond 20 MeV – Pair production becomes important. Relative importance the reactions:

Most of electrons set in motion by the above interactions lose energy by inelastic collisions with the atomic electrons of the material. Some electrons will also loose energy by bremsstrahlung interactions with the nuclei. This energy is irradiated out of the local volume as x-rays and is therefore not included in the calculation of locally absorbed energy. Thus, the energy absorption coefficient( μ en ) is defined as the product of the energy transfer coefficient( μ tr ) and (1- g ) where g is the fraction of energy of secondarily charged particles lost to bremsstrahlung in the material. μ en = μ tr (1-g) In most interactions involving the soft tissues, the bremsstrahlung component is negligible , and the energy absorption coefficient is equal to the energy transfer coefficient under these conditions. The relationship between the mass attenuation coefficients and the mass absorption coefficient varies as per the radiation energy as follows: Absorption: Photon energy Mass coefficient 100 KeV 1 MeV 10 MeV 91% 15% 46% 71% 96% 10 KeV % of attenuated energy absorbed μ en μ / ρ

The mass absorption coefficients are practically identical for most biological materials over a wide range of energies in which the Compton process predominates as this effect does not depend upon the atomic number. In this energy range, the absorption per gram is maximum for hydrogen, because of its higher electron density. However in very high and very low energy ranges the high atomic number materials e.g. Bone absorb more radiation with several unfortunate consequences. The situation is remarkably different in case of low-energy radiation, where higher than atomic number greater is the energy absorbed. Also, in very high energy ranges where pair production predominates, the mass absorption coefficients again become higher for the higher atomic number elements. Absorption (contd.): Spatial distribution of secondary radiation: Recoil electrons : Usually travel forwards, never making an angle of more than 90° with the direction of the initial photon. Photoelectrons and electron pairs : Usually tend to travel forward, initially for higher-energy radiation. Characteristic and annihilation radiation : Isotropic, that is, given out equally in all directions. Coherently scattered photons : Isotropic. Compton scatter photons : Usually in a forward direction having suffered comparatively small angle scattering (lesser scattering for greater incident energy).

Particulate radiation can be classified into two categories: Ionizing or charged particles. Uncharged particles. The main charged particles used in radiotherapy are: Electron, Proton Pi – mesons (pions). 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 . This difference is due to the higher binding energy of the electrons and the fewer electrons per gram in higher atomic number elements . Ionization results in the stripping of electrons from atom and may produce ionization in it’s own turn – when it is called δ rays . Electrons are light particles with negligible mass and single negative charge. As a result they penetrate deeper than other charged particles but at the same time undergo greater scattering . 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 MeV – making these useful in clinical practice. These characteristics make electrons a useful treatment modality for superficial lesions. Interactions of particulate radiation:

Practical Implications (electrons): This leads to dosimetric inaccuracies when using air containing ion chambers. Polarization in heavier atomic weight elements. This leads to a “smudging” of the Bragg's peak which is not seen in electrons. 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

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. 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. The ionization produced by mesons at the end of the track is even more intense and is often referred to as star formation . 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, which complicates overall treatment. The large machines necessary for production of these beams often make it necessary to move the patient instead of the gantry! Interactions of particulate radiation:

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. The most efficient recoil is seen in the hydrogen nucleus and this leads to the maximum absorption . This is an advantage because most of the soft tissues in the body contains a large proportion of hydrogen. This phenomenon has some 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. The recoil protons, set in motion after interaction with neutrons. further cause ionization. The dense ionization produced by these particles in the vicinity, results in high LET values . Neutrons, being uncharged particles also penetrate deeply into matter Despite these attractive radiobiological and physical properties, neutrons are not commonly used in practical radiotherapy, because of technical difficulties in production of these beams as well as their complicated dosimetry. LET has certain important radiobiological implications: High LET radiation is more likely to induce lethal damage in the cells due to the dense ionization they produce. The oxygen enhancement ratio nears 1 as the LET increases – advantage in hypoxic tumors. The effect of fractionation reduces as LET increases. High LET radiation preferentially increase the repair independent damage in the cells. High LET radiation also leads to reduced variability in the cell cycle dependant radiosensitivity of cells. Interactions of neutrons:

Cellular damage may occur directly when the radiation interacts with the atom directly ( e.g. neutrons) or indirectly when interaction occurs by secondary electrons (e.g. photon beams). Electrons produced by the ionizing events lead to further ionizations as they move inside biological material --> these lead to the formation of highly reactive free radicals like OH - , H - radicals which in turn lead to chemical changes by breaking chemical bonds. Some of these reactions are potentially damaging to the cell, others effectively inactivate the radicals. The reactions that most commonly lead to cell damage usually occur at the level of the DNA although they may occur at the level of cell membranes, proteins etc. Biological correlates: DNA damage/ misrepair M Premitotic death (? Membrane damage – interphase) Senescence Clonogenic cell surviving after mitosis Post mitotic death after division(s): Apoptotic Necrotic Senescent Mitotic death – apoptotic / necrotic Death after failed mitosis : Failed division -> multimicronucleated cells ( mitotic catastrophe ) Multi polar division -> death Nondisjunction -> Change in chromosome content. Breakage and rejoining leads to -> dicentrics, rings, acentric fragments, translocations etc.

The 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. 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. Conclusions:

Interaction of Radiation with Matter

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