Radiation physics


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  • Fluorescent plates were made of barium platinocyanide.
  • Radiation physics

    1. 1. RADIATION PHYSICS Presented by- Dr. Urvashi Nikte (1st Year PG Student) Guided by- Prof.Dr.Mahendra Patait Dr. Kedar Saraf
    2. 2. COMPOSITION OF MATTER:  Matter is anything that has mass and occupies space.  It occurs in three states: Solid, Liquid and Gas  ATOM is the fundamental unit of matter that cannot be subdivided by chemical methods.
    3. 3. BOHR RUTHERFORD MODEL: In atomic physics, the Bohr model, introduced by Niels Bohr in 1913, depicts the atom as small, positively charged nucleus surrounded by electrons that travel in circular orbits around the nucleus—similar in structure to the solar system, but with attraction provided by electrostatic forces.
    5. 5. The quantum mechanical model is based on quantum theory, which says matter also has properties associated with waves. According to quantum theory, it’s impossible to know the exact position and momentum of an electron at the same time. This is known as the Uncertainty Principle. The quantum mechanical model of the atom uses complex shapes of orbitals (sometimes called electron clouds), volumes of space in which there is likely to be an electron. So, this model is based on probability rather than certainty. Four numbers, called quantum numbers, were introduced to describe the characteristics of electrons and their orbitals: Principal quantum number: n Angular momentum quantum number: l Magnetic quantum number: m1 Spin quantum number: ms
    7. 7. The number of protons only in a nucleus is called the atomic number ( the Z number). The atomic mass (A) is the total number of protons and neutrons in the nucleus of an atom. When the number of protons equals the number of electrons that atom is known to be in a stable or neutral state The electrons in the orbit are maintained by the electrostatic force between the positively charged nucleus and the negatively charged electrons on the one hand, balanced by the centrifugal force of the revolving electrons.
    8. 8. ELECTROSTATIC FORCE: It is the force of attraction between protons and electrons.
    9. 9. CENTRIFUGAL FORCE: It is the force that pulls electrons away from the nucleus.
    10. 10. CF EF Balance between ELECTROSTATIC and CENTRIFUGAL forces keeps the electrons in orbit around the nucleus.
    11. 11. ELECTRON BINDING ENERGY: The amount of energy required to remove an electron from a given shell must exceed the electrostatic force of attraction between it and the nucleus. This is called electron binding energy of the electron/ionization energy It is specific for each shell of each element. The electron shell closest to the nucleus will have the greatest binding energy and it will decrease successively in each shell
    12. 12. For an electron to move from a specific orbit to another orbit farther from the nucleus, energy must be supplied in an amount equal to the difference in the binding energies between the two orbits. In contrast, in moving an electron from an outer orbit to the closer to the nucleus, energy must be lost and this energy is the difference between the binding energies of the two orbits. This lost energy is given up in the form of electromagnetic radiation.
    13. 13. IONIZATION:  When the number of orbiting electrons in an atom is equal to the number of protons in its nucleus, the atom is electrically neutral. If an electrically neutral atom loses an electron, it becomes a positive ion and the free electron is a negative ion. This process of forming an ion pair is termed ionization.
    14. 14. NATURE OF RADIATION: DEFINITION: Radiation is the transmission of energy through space and matter. PARTICULATE RADIATION ELECTROMAGNETIC
    15. 15. PARTICULATE/CORPUSCULAR RADIATION Particulate radiation consists of atomic nuclei or subatomic particles moving at high velocity. e.g. alpha particles, beta particles, and cathode rays are examples of particulate radiation.
    16. 16. The capacity of particulate radiation to ionize atoms depends on its mass, velocity, and charge. The rate of loss of energy from a particle as it moves along its track through matter (tissue) is its linear energy transfer (LET) . A particle loses kinetic energy each time it ionizes adjacent matter; the greater its physical size and charge and the lower its velocity, the greater is its LET. For example, alpha particles, with their high charge and low velocity, lose kinetic energy rapidly and have short path lengths (are densely ionizing); thus they have a high LET. Beta particles are much less densely ionizing because of their lighter mass and lower charge and thus have a lower LET. They penetrate through tissue more readily than do alpha particles.
    17. 17. ELECTROMAGNETIC RADIATION: Electromagnetic radiation is the movement of energy through space as a combination of electric and magnetic fields. It is generated when the velocity of an electrically charged particle is altered. E.g. Gamma rays, x rays, ultraviolet rays, visible light, infrared radiation(heat), microwaves, and radio waves.
    18. 18. Some of the properties of electromagnetic radiation are best expressed by wave theory, whereas others are most successfully described by quantum theory. Wave theory: Electromagnetic wave Wavelength Electric field Electric Field Magnetic Magnetic Field Field Direction Direction •All electromagnetic waves travel at the velocity of light (3.0 x 108 m/s ) • Waves of all kinds exhibit the properties of wavelength (λ) and frequency (v). • λ x v = c = 3 X 108 meters/second
    19. 19. Wavelength (λ) Time One oscillation (frequency is number of oscillations per second)
    20. 20. According to wavelengths radiation can differ in properties: Short wavelength OR Long wavelength
    21. 21. The short wavelength Increased frequency increased energy accompanied with it increase the power of penetration. They are termed as Hard Radiation which is characterised by low absorption and low ionisation potential. The long wavelength decreased frequency decreased energy accompanied with it less power of penetration. They are termed as Soft Radiation which is characterised by high absorption and high ionisation potential.
    22. 22. HISTORY: X-RAYS:  X-Rays were first discovered by Wilhelm Conrad Roentgen on 8th November, 1895.  He was a professor of physics at the University of Wuzberg in Germany.  He was working with Hittorf-Crookes tube, through which an electric current from a battery was flowing in a darkened room with black cardboard covering the tube. There were many fluorescent plates in the room.  One evening, while he was working on the tubes, he noticed something coming from the tube causing the fluorescent plates to glow.  He did not know what it was, he called it X-rays.  He won the Nobel prize for his discovery in 1901.
    23. 23.  He first conducted the experiment on his wife’s hand with a 15 minutes exposure.  He published 3 papers: • A new kind of Rays: a preliminary communication. • A new kind of Rays: continued. • Further observations on a new kind of Rays.  In June 1896, within 6 months after Roentgen announced his discovery, X-Rays were being used by battlefield physicians to locate bullets in wounded soldiers.
    24. 24.  Within 2weeks of its discovery, the first dental radiograph was taken by German dentist Dr. Otto Walkoff and Prof.Wilhelm Koening in 1896 with the help of a radiochemist Giesel.  He used a small glass photographic plate wrapped in black paper and covered with rubber dam that he placed in his own mouth, because of the plate’s positioning his mouth the image showed parts of the upper and lower teeth and he was actually taking a bitewing radiograph.
    25. 25.  Dr. C. Edmond Kells, a New Orleans dentist, is credited with taking the first intra oral radiographs in the US in April 1896.  Dr. William Rollins developed the first dental X-ray unit in 1896. He developed burns on his skin of his hands and recommended lead shielding of both the tube and the patient.  Dr. Howard Riley was the first to introduce radiology into the dental school curriculum at the University of Indiana. In 1913, film was used instead of glass photographic plates.
    26. 26. PROPERTIES OF X-RAY: 1. They have a very short wavelength. It has wavelength of 0.10.001nm. 2. They have a selective penetration and absorption power. 3. It causes certain substances to fluorescence. 4. They cause ionization of atoms. 5. They have biological damaging effects. 6. They travel at the speed of light i.e. 3x108 m/s 7. Invisible, odorless, cannot be felt or heard. 8. Weightless, massless, charge less. 9. They cannot be focused or collected by lens. 11.They cannot be reflected or refracted. 12.They cannot be deviated by a magnetic field.
    30. 30. THE X-RAY TUBE: All dental and medical x-ray tubes are called Coolidge tubes because they follow the original design of W. C. Coolidge introduced in 1913. The basic apparatus for generating x rays, the x-ray tube, is composed of a cathode and an anode.
    31. 31. THE TUBE: The tube is an Evacuated glass tube with two extending arms or electrodes extending in two opposite directions, which are the cathode and the anode. The tube is evacuated for the following reasons: 1) This will prevent the collision of the moving electrons with air molecules. 2) This evacuation will prevent the oxidation and burnout of the filaments.
    32. 32. THE X-RAY TUBE
    33. 33. CATHODE: It is the negative electrode of the tube, which serves as the source of electrons. It consists of: The filament Focusing cup
    34. 34. CATHODE
    35. 35. THE FILAMENT: •The filament is the source of electrons within the x-ray tube. • It is a coil of tungsten wire about 2mm in diameter and 1cm or less in length. •It is mounted on two stiff wires that support it and carry the electric current. These two mounting wires lead through the glass envelope and connect to both the high- and low-voltage electrical sources. •The filament is heated to incandescence by the flow of current from the low-voltage source and emits electrons at a rate proportional to the temperature of the filament.
    36. 36. FOCUSSING CUP: •The filament lies in a focusing cup, a negatively charged concave reflector made of Molybdenum. •The focusing cup electro statically focuses the electrons emitted by the incandescent filament into a narrow beam directed at a small rectangular area on the anode called the focal spot. •The electrons move in this direction because they are repelled by the negatively charged cathode and attracted to the positively charged anode.
    38. 38. ANODE: It is the positive electrode of the tube. It consists of: TARGET COPPER BLOCK
    40. 40. TARGET: •The purpose of the target in an x-ray tube is to convert the kinetic energy of the electrons generated from the filament into x-ray photons. •This is an inefficient process with more than 99% of the electron kinetic energy converted to heat. •The target is made of tungsten, a material that has several characteristics of an ideal target material. It has a high atomic number (74)- Best for production of X-Rays High melting point (3410o C)- Withstand heat generated at anode  High thermal conductivity- dissipate heat to copper stem Low vapour pressure at the working temperatures of an x-ray tube.maintains vacuum.
    41. 41. COPPER BLOCK: •The tungsten target is typically embedded in a large block of copper to dissipate heat. •Copper, a good Thermal conductor, dissipates heat from the tungsten, thus reducing the risk of the target melting. •In addition, insulating oil between the glass envelope and the housing of the tube head carries heat away from the copper stem. This type of anode is a stationary anode.
    42. 42. LINE FOCUS PRINCIPLE:  The focal spot is the area of the anode from which the x-rays are emitted. The focal spot impacts the geometric resolution of the x-ray image.  By angling the anode target, one makes the effective focal spot much smaller than the actual area of interaction. The angling of the target is know as the line focus principle.  The Effective Focal Spot is the beam projected onto the patient. As the anode angle decreases, the effective focal spot decreases. Diagnostic tube target angles range from 5 to 15°.  Smaller target angles will produce smaller effective focal spots and sharper images. To cover a 17” the angle must be 12°. To cover 36” the angle must be 14°.
    43. 43. HEEL EFFECT:  Because of the use of line-focus principle the consequence is that the radiation intensity on the cathode side of the x-ray field is higher than that on the anode side.  Because the e- on the anode side must travel further than the ethat are close to the cathode side of the target, the anode side xrays have slightly lower energy than the cathode side x-rays.  The smaller the anode angle, the larger the heel affect.
    44. 44. A SIMPLIFIED DIAGRAM OF X-RAY TUBE Step Down Transformer Cathode Filament Anode Target Copper Block Insulating Oil Focussing Cup X-Ray Metal Housing Evacuated Glass Tube Useful Beam Step Up Transformer
    45. 45. POWER SUPPLY: The primary functions of the power supply of an x-ray machine are to (1) provide a low-voltage current to heat the x-ray tube filament by use of a step-down transformer. (2) generate a high potential difference between the anode and cathode by use of a high voltage transformer. These transformers and the x-ray tube lie within an electrically grounded metal housing called the head of the x-ray machine. An electrical insulating material, usually oil, surrounds the transformers.
    46. 46. TRANSFORMER: It is an electric device, which increases or reduces the voltage of an alternating current by mutual induction between primary and secondary coils. Step down Transformer: A transformer in which the secondary voltage is less than primary voltage. Step up Transformer: A transformer in which the secondary voltage is greater than the primary voltage.
    47. 47. The Principles of X-Ray Production When an electric current, which is composed of a stream of negatively charged electrons having kinetic energy passes through a filament or wire, it will be heated so the orbiting electrons within its atom will acquire sufficient energy to escape from their shells. Finally, this electron cloud will be given from the heated wire of filament. If these electrons are suddenly stopped, they will lose the accompanying kinetic energy and convert it into heat and XRay radiation.
    48. 48. Application of this Principle on dental X-Ray machine: The step down transformer will decrease the electric voltage to 812 volts This voltage is sufficient enough to heat the tungsten filament of the cathode and produce electrons according to the degree of heating by thermo ionic emission. These electrons will form a cloud around the cathode, which will be collected by the concave focussing cup but they have no velocity to move.
    49. 49. The step-up transformer will raise the potential difference between the cathode and the anode by raising the voltage to 60-70 kVP. This increase in potential difference will accelerate the electron cloud to move towards the anode, as there is a force of attraction between the cathode and anode. By the action of the focussing cup, the electrons will hit only the tungsten target of the anode, losing their kinetic energy in the form of 99.8% heat and only 0.2% X-Rays. The produced X-rays are conducted out from the tube housing through the filters and collimators to be used as a useful beam.
    50. 50. FACTORS CONTROLLING X-RAY BEAM: EXPOSURE TIME: When it increases, there is increase in the number of photons generated keeping current and voltage constant. When the exposure time is doubled, the number of photons generated at all energies in the x-ray emission spectrum is doubled, but the range of photon energies is unchanged. Therefore changing the time simply controls the quantity of the exposure, the number of photons generated. TUBE CURRENT: The number of photons is directly proportional to the tube current (mA)
    51. 51. TUBE VOLTAGE: Increasing the kVp increases the potential difference between the cathode and anode, thus increasing the energy of each electron when it strikes the target. This results in an increased efficiency of conversion of electron energy into x-ray photons, and thus an increase in (1) the number of photons generated, (2) their mean energy, and (3) their maximal energy. The increased number of photons produced per unit time by use of higher kVp results from the greater efficiency in the production of bremsstrahlung photons that occurs when increased numbers of higher-energy electrons interact with the target.
    52. 52. X-Ray Machine Components
    53. 53. PRODUCTION OF X-RAYS: Electrons traveling from the filament to the target convert some of their kinetic energy into x-ray photons by the formation of Bremsstrahlung and Characteristic radiation.
    54. 54. BREMSSTRAHLUNG:  Bremsstrahlung interactions, the primary source of x-ray photons from an x-ray tube, are produced by the sudden stopping or slowing of high-speed electrons at the target. (Bremsstrahlung means "braking radiation“ in German.)  Most high-speed electrons, however, have near or wide misses with atomic nuclei.  In these interactions, a negatively charged high-speed electron attracted toward the positively charged nuclei.  The closer the high speed electron approaches the nuclei, the greater is the electrostatic attraction on the electron, the braking effect, and the energy of the resulting bremsstrahlung photons.
    55. 55. Bremsstrahlung Radiation
    57. 57. BREMS
    58. 58. CHARACTERISTIC RADIATION: •Characteristic radiation occurs when an electron from the filament displaces an electron from a shell of a tungsten target atom, thereby ionizing the atom. •When this happens, a higher energy electron in an outer shell of the tungsten atom is quickly attracted to the void in the deficient inner shell. •When the outer-shell electron replaces the displaced electron, a photon is emitted with an energy equivalent to the difference in the two orbital binding energies.
    60. 60. FILTRATION: A thin sheet of pure aluminum (1.5-2.5mm thickness) is placed in the way of the X-ray beam at the end of the X-ray tube in order to improve the quality of the beam. Inherent filtration consists of the materials that x-ray photons encounter as they travel from the focal spot on the target to form the usable beam outside the tube enclosure. These materials include the glass wall, insulating oil, barrier material Total filtration is the sum of the inherent filtration plus any added external filtration supplied in the form of aluminum disks placed over the port in the head of the x-ray machine.
    62. 62. COLLIMATION: A collimator is a metallic barrier with an aperture in the middle used to reduce the size of the x-ray beam and therefore the volume of irradiated tissue within the patient.
    63. 63. COLLIMATION
    64. 64. INVERSE SQUARE LAW: The intensity of an x-ray beam at a given point (number of photons per crosssectional area per unit exposure time) depends on the distance of the measuring device from the local spot. For a given beam the intensity is inversely proportional to the square of the distance from the source. The reason for this decrease in intensity is that the x-ray beam spreads out as it moves from the source. The relationship is as follows: I1 / I2 = D22 / D2 1 where l is intensity and D is distance. Therefore changing the distance between the x-ray tube and patient has a marked effect on beam intensity. Such a change requires a corresponding modification of the kVp or mA if the exposure of the film is to be kept constant.
    65. 65. INTERACTIONS OF XRAYS WITH MATTER: • The intensity of an x-ray beam is reduced by interaction with the matter it encounters. This attenuation results from interactions of individual photons in the beam with atoms in the absorber. • The x-ray photons are either absorbed or scattered out of the beam. • In a dental x-ray beam there are three means of beam attenuation: (1) COHERENT SCATTERING (2) PHOTOELECTRIC ABSORPTION (3) COMPTON SCATTERING
    66. 66. COHERENT SCATTERING: • Coherent scattering (also known as classical, elastic, or Thompson scattering) may occur when a low-energy incident photon passes near an outer electron of an atom. • The incident photon interacts with the electron causing it to become momentarily excited at the same frequency as the in coming photon.
    69. 69. • This interaction accounts for only about 8% of the total number of interactions (per exposure) in a dental examination. • Coherent scattering contributes very little to film fog because the total quantity of scattered photons is small and its energy level is too low for much of it to reach the film.
    70. 70. PHOTOELECTRIC ABSORPTION: • It is critical in Diagnostic Imaging. • This process occurs when an incident photon collides with a bound electron in an atom of the absorbing medium. • The electron is ejected from its shell and becomes a recoil electron (photoelectron). • The kinetic energy imparted to the recoil electron is equal to the energy of the incident photon minus that used to overcome the binding energy of the electron.
    71. 71. • The absorbing atom is now ionized because it has lost an electron. • The recoil electron acquires most of the energy of the incident photon. • Most photoelectric interactions occur in the K shell because the density of the electron cloud is greater in this region and a higher probability of interaction exists. • About 30% of photons absorbed from a dental x-ray beam are absorbed by-the photoelectric process.
    72. 72. • Although this is beneficial in producing high-quality radiographs, because no scattered radiation fogs the film, it is potentially deleterious for patients because of increased radiation absorption. • The difference in absorption in various tissues appears as a difference in optical density in the radiographic image.
    73. 73. PHOTOELECTRIC ABSORPTION Incident Photon
    74. 74. COMPTON SCATTERING • In this interaction the incident photon collides with an outer electron, which receives kinetic energy and recoils from the point of impact. • The path of the incident photon is deflected by its interaction and is scattered from the site of the collision.  The energy of the scattered photon equals the energy of the incident photon minus the sum of the kinetic energy gained by the recoil electron and its binding energy.
    75. 75. • Compton scattering results in the loss of an electron and ionization of the absorbing atom. • The probability of a Compton interaction is directly proportional to the electron density of the absorber. Therefore the probability of Compton scattering is correspondingly greater in bone than in tissue. • In a dental x-ray beam, approximately 62% of the photons undergo Compton scattering.
    76. 76. COMPTON SCATTERING Incident Photon Recoil Electron Deflected Photon
    77. 77. REFERENCES: 1. Stuart C. White, Michael J. Pharoah; Oral Radiology: Principles and Interpretation; Mosby Elsevier Publication, 5th edition, 2004. 2. Malin, Shimon (2012). Nature Loves to Hide: Quantum Physics and the Nature of Reality, a Western Perspective (Revised ed.). World Scientific. ISBN 978-981-4324-57-1. 3, Bushberg J.T. : The essential physics of medical imaging, ed.2 Baltimore, 2001Lippincott Williams an Wilkin. 4. Bushbong SC: Radiologic science for technologists: physics, biology and protection, ed 7. St Louis, 2001, Mosby. 5. Curry TS, Dowdey JE, Murry RC: Christensen’s introduction to the physics of diagnostic radiology, ed 4. Philadelphia, 1990, Lea & Febiger 6. International Commission on Radiological Protection & Safety In Medicine; ICRP Publication 73, 1996, Elsevier Science.
    78. 78. 7. WJ Merdith, JB Massey, Fundamental Physics of Radiology, Second Edition, Bristol: John Wright and Sons 1972. 8. Akhlesh Lakhtakia (Ed.); Salpeter, Edwin E. (1996). "Models and Modelers of Hydrogen". American Journal of Physics (World Scientific) 65 (9): 933. Bibcode:1997AmJPh..65..933L 9. Frommer Stabulas- Savage: Radiology For The Dental Professional, Elsevier Mosby, ed 8, 2005.
    79. 79. THANK YOU