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MBP 1028: Radiation Physics

MBP 1028: Radiation Physics. 1) Intro, Matter, and Radiation a) Radiation uses: diagnosis, treatment, power b) Nuclear Reactor for power i) Cherenkov radiation c) Forces ( Patel ) d) Matter i) Electrons, neutrons, protons, atomic structure, binding energy e) Radiation i) EM radiation

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MBP 1028: Radiation Physics

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  1. MBP 1028: Radiation Physics 1) Intro, Matter, and Radiation a) Radiation uses: diagnosis, treatment, power b) Nuclear Reactor for power i) Cherenkov radiation c) Forces (Patel) d) Matter i) Electrons, neutrons, protons, atomic structure, binding energy e) Radiation i) EM radiation ii) Photons iii) Inverse square law iv) Ionization f) Radionuclides i) Radon (Rotenber) ii) Decay iii) Half-life Activity Brachytherapy applications Co-60 Unit Electron Interactions Kristy K. Brock Radiation Physics

  2. Acknowledgements • Review of Radiologic Physics, Walter Huda and Richard Slone • Slide sets based on the book authored byE.B. Podgorsak of the IAEA publication (ISBN 92-0-107304-6):Review of Radiation Oncology Physics: A Handbook for Teachers and Students • Slides from Geordi Pang • Wikipedia

  3. Radiation Uses • Power • Nuclear Reactors • Diagnosis • CT • X-ray (fluoroscopy, planar x-ray, dental x-ray) • Nuclear medicine (PET, SPECT • Treatment • External beam radiation therapy • Brachytherapy • Orthovoltage radiation therapy

  4. Fusion History • 1917, Ernest Rutherford first split the atom by bombarding nitrogen with alpha particles • 1932, English physicist and Nobel laureate James Chadwick discovered the neutron • few years later, Enrico Fermi and his collaborators discovered neutron bombardment = new elements • 1939, Otto Hahn and Fritz Strassman, bombarded solutions of uranium salts with neutrons • Few weeks later, Lise Meitner and Otto Frisch showed that a uranium nucleus, having absorbed a neutron, could split, with the release of energy, into two roughly equal parts

  5. Power: Nuclear Reactors • 1939: Nuclear Fission discovered • Split atomic nucleus in half = ++++ Energy • Catalyst (i.e. moderator) + uranium = fission chain reaction • Canada developed a heavy water based reactor • Heavy water = ++++ deuterium (2H2O) • In 1997: Ontario power = 48% nuclear • 17 Power reactors, CANDU design

  6. Cherenkov Radiation • EM radiation emitted when a charged particle passes through an insulator at a constant speed greater than the speed of light in that medium • Characterized by Pavel Alekseyevich Cherenkov in 1958 Advanced Test Reactor (wikipedia)

  7. Diagnosis • CT • Planar X-ray • Fluoroscopy • PET • SPECT

  8. Diagnosis • CT • Planar X-ray • Fluoroscopy • PET • SPECT

  9. Diagnosis • CT • Planar X-ray • Fluoroscopy • PET • SPECT

  10. Diagnosis • CT • Planar X-ray • Fluoroscopy • PET • SPECT

  11. Diagnosis • CT • Planar X-ray • Fluoroscopy • PET • SPECT

  12. Treatment: Radiation Therapy (RT) • ~ 50% of cancer patients received RT • External Beam Radiation Therapy • Brachytherapy • Systemic Therapy • Thyroid & non-Hodgkin Lymphoma

  13. Matter • Atoms: protons, neutrons, electrons • Electron: - change, relative mass = 1, rest mass E = 511 keV • Isolate e- is stable • Protons: + charge, relative mass = 1,836, rest mass E = 938 MeV • Isolate p+ is stable • Neutrons: no charge, relative mass = 1,839, rest mass E = 940 MeV • Isolated n is not stable, half-life = 11 min

  14. Electronic Structure • Nucleus: protons and neutrons (nucleons) • Majority of the atomic mass • Bohr Model (1913) : electrons surround the nucleus in shells (K, L, M…) • Each shell has a principal quantum number (n) • K = 1, L =2… • Max electrons per shell = 2n2 • Number of e- in the outer shell determins the chemical properties

  15. Electronic Structure • Atomic mass number A Number of nucleons (Z+N) in an atom, where • Z is the number of protons (atomic number) in an atom • N is the number of neutrons in an atom.

  16. Electronic Structure In nuclear physics the convention is to designate a nucleus X as , where A is the atomic mass number Z is the atomic number For example: • Cobalt-60 nucleus with Z = 27 protons and A = 33 neutrons is identified as . • Radium-226 nucleus with 88 protons and 138 neutrons is identified as .

  17. Electronic Structure Nickel www.lapp-plats.com

  18. Electron Binding Energy • Electrostatic pull keeps e- orbiting the nucleus • Work required to remove an e- is electron binding energy • Binding E of outer shell is ~ eV • Binding E of inner shell is ~ keV • K-shell Binding E increases with increasing atomic number (Z) (H = 0.01 keV, Pb = 88 keV)

  19. Nuclear Binding Energy • Strong forces hold nucleons together • Total Binding Energy of the entire nucleus is the energy required to separate all the nucleons • Average Binding Energy per nucleon • Total Binding Energy/# nucleons • Ranges from ~ 1 MeV (deuterium) to 7-9 MeV for nuclei with mass number > 20 • Increases after radioactive decay (daughter is more stable than the parent) • High Binding Energy  nuclear stability

  20. Forces

  21. RADIATION = energy in transit from one location to another p a r t i c l e w a v e • sound (mechanical wave) • elementary particles, ions • heat (conduction) • heat (convection)  EM radiation   Maxwell’s eqn photons  Particle - Wave Duality NATURE OF RADIATION

  22. Radiation of sufficient energy to cause the formation of an ion as it traverses matter • Most commonly by removal of an electron from a neutron atom or molecule (typically requires ~ 10eV to overcome the electron binding energy) • Directly ionizing radiation • charged particles of sufficient energy • electrons • protons •  particles • etc • Indirectly ionizing radiation • uncharged particles that liberate • directly ionizing radiation from • the matter • EM radiation • neutrons Ionizing Radiation

  23. Electromagnetic Radiation • Radiation: transport of energy through space • Wave Definitions: • Wavelength (): distance between crests of waves [m] • Amplitude: intensity defined by height of wave • Frequency (f): number of waves per unit time [Hz] • Period (1/f): time for one wavelength to pass • Velocity (v) = f x  [m/s] • EM radiation travels in a straight line at the speed of light in a vacuum • c = f x 

  24. Photons • EM radiation is quantized • Discrete quantities of E, photons • Photons: waves or particles • No mass • E  f, 1/  •  ~ angstrom, 10-10 m [Å] • E [keV] = h x f = h x (c/) = 12.4/ [Å] • H: Planck’s constant: h = 6.63 x 10 -34 J-s • 10 keV photon,  = 1.24 Å (~ 1 atom) • 100 keV photon,  = 0.124 Å • What is the difference between x-rays and gamma rays?

  25. 4x10-1 8x10-1 1x106 •  (µm) = 1x10-6 1.5x10-4 1x10-2 1x103 Electro-Magnetic (EM) Radiation • Anything in common among the following? • TV and radio transmissions • microwaves • light • x rays • gamma rays They are all EM but with different frequencies !

  26. Inverse Square Law • X-ray beam intensity  with distance • Divergence •  intensity  distance2 http://en.wikipedia.org/wiki/Inverse-square_law

  27. Ionization • Neutral atom – ejected electron = positive ion • EM radiation with high enough E = ‘ionizing radiation’ • X-rays and Gamma rays • Directly ionizing radiation • ? • Indirectly ionizing radiation • ? • Air + 33 eV  e- + ion pair

  28. Directly Ionizing Radiation • Charged particles (e-, e+, p+, ) • Lose E when passing through matter by interacting with e- in nearby atoms • E loss  with  charge and mass • E loss  with  velocity • E loss  ejected e-, raise atomic e- to higher energies

  29. Ionization • Linear Energy Transfer (LET): E loss of a charged particle [keV] per [m] distance traveled • e- and e+: 0.5 keV/m soft tissue (Low LET) • : 100 keV/m soft tissue (High LET) • p+ and n (ionization through recoil p+) intermediate LET • Ionizing radiation  E to e-  DNA damage

  30. How do we get Radiation? • Naturally occurring • Radon, cosmic… • Generate radioactive material • Radionuclides • Generators/bombardment • X-ray tube • Linear accelerator

  31. Radioactivity • Radioactivity is a process by which an unstable nucleus (parent) decays into a new nuclear configuration (daughter) that may be stable or unstable. • If the daughter is unstable it will decay further through a chain of decays until a stable configuration is attained.

  32. Radioactivity • Henri Becquerel discovered radioactivity in 1896. • Other names used for radioactive decay are: • Nuclear decay • Nuclear disintegration • Nuclear transformation • Nuclear transmutation • Radioactive decay

  33. Radioactivity • Radioactive decay involves a transition from the quantum state of the parent (P) to a quantum state of the daughter (D). • The energy difference between the two quantum states is called the decay energy (Q) • The decay energy Q is emitted in the form of either: • electromagnetic radiation (gamma rays) • kinetic energy of the reaction products.

  34. Radioactivity

  35. Radioactivity • Activity represents the total number of disintegrations (decays) of parent nuclei per unit time. • The SI unit of activity is the becquerel (1 Bq = 1 s-1). • Both becquerel and hertz correspond to s-1 • hertz expresses frequency of periodic motion • becquerel expresses activity. • The older unit of activity is the curie • originally defined as the activity of 1 g of radium-226. • Currently, the activity of 1 g of radium-226 is 0.988 Ci.

  36. Radioactivity • Decay of radioactive parent P into stable daughter D: • The rate of depletion of the number of radioactive parent nuclei is equal to the activity at time t: • Where is the initial number of parent nuclei at time t = 0.

  37. Radioactivity • The number of radioactive parent nuclei as a function of time t is: • The activity of the radioactive parent as a function of time t is: • where is the initial activity at time t = 0.

  38. Radioactivity • Parent activity plotted against time t illustrating: • Exponential decay of the activity • Concept of half life • Concept of mean life (avg life of all parent atoms at t = 0)

  39. Radioactivity • Half life (t1/2)P of radioactive parent P is the time during which the number of radioactive parent nuclei decays from the initial value Np(0) at time t = 0 to half the initial value: • The decay constant P and the half life (t1/2)P are related as follows:

  40. Radioactivity • Decay of radioactive parent P into unstable daughter D which in turn decays into granddaughter G: • The rate of change dND/dt in the number of daughter nuclei D equals to supply of new daughter nuclei through decay of P given as PNP(t) and the loss of daughter nuclei D from the decay of D to G given as - DND(t)

  41. Radioactivity • The number of daughter nuclei is: • Activity of the daughter nuclei is:

  42. Radioactivity At t = tmax the parent and daughter activities are equal and the daughter activity reaches its maximum. and • Parent and daughter activities against time for

  43. Radioactivity Special considerations for the relationship: • For General relationship (no equilibrium) • For Transient equilibrium for • For Secular equilibrium

  44. Activation of nuclides • Radioactivation of nuclides occurs when a parent nuclide P is bombarded with thermal neutrons in a nuclear reactor and transforms into a radioactive daughter nuclide D that decays into a granddaughter nuclide G. • The probability for radioactivation to occur is governed by the cross section  for the nuclear reaction and the neutron fluence rate . • The unit of  is barn per atom where 1 barn = 1b = 10-24 cm2 • The unit of is cm-2s-1

  45. Activation of nuclides • An important example of nuclear activation is the production of the cobalt-60 radionuclide through bombarding stable cobalt-59 with thermal neutrons • For cobalt-59 the cross section  is 37 b/atom • Typical reactor fluence rates are of the order of 1014 cm-2s-1. or

  46. Modes of radioactive decay • Radioactive decay is a process by which unstable nuclei reach a more stable configuration. • There are four main modes of radioactive decay: • Alpha decay • Beta decay • Beta plus decay • Beta minus decay • Electron capture • Gamma decay • Pure gamma decay • Internal conversion • Spontaneous fission

  47. Modes of radioactive decay • Nuclear transformations are usually accompanied by emission of energetic particles (charged particles, neutral particles, photons, neutrinos) • Radioactive decay Emitted particles • Alpha decay  particle • Beta plus decay + particle (positron), neutrino • Beta minus decay - particle (electron), antineutrino • Electron capture neutrino • Pure gamma decay photon • Internal conversion orbital electron • Spontaneous fission fission products

  48. Neutrino () • Postulated in 1930 by Wolfgang Pauli • Preserve conservation of E, momentum, and angular momentum in beta decay • Name coined by Fermi • Elementary particle • Travels close to the speed of light • No charge • Small cross section with most matter • Nonzero mass (but very small) • Generated from radioactive decay and nuclear reaction

  49. Modes of radioactive decay • In each nuclear transformation a number of physical quantities must be conserved. • The most important conserved physical quantities are: • Total energy • Momentum • Charge • Atomic number • Atomic mass number (number of nucleons)

  50. Modes of radioactive decay • Alpha decay is a nuclear transformation in which: • An energetic alpha particle (helium-4 ion) is emitted. • The atomic number Z of the parent decreases by 2. • The atomic mass number A of the parent decreases by 4.

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