Lesson1a

Lesson1a - Lesson 01-contd Ionizing Radiation MP200...

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Unformatted text preview: Lesson 01-contd. Ionizing Radiation MP200 Radiation Physics - 2010 Duke Medical Physics Graduate Program 1 Ionizing Radiation Subatomic particles or electromagnetic waves that are energetic enough to detach the electrons from atoms or molecules are called ionizing radiation. Radiological physics is the science of ionizing radiation and its in- teractions with matter, with special interest in the energy absorbed by the matter. Radiation dosimetry has to do with the quantitative determination of that energy. Ionizing radiation are characterized by their ability to excite and ionize atoms of matter which they interact. The energy needed to cause a valence electron to escape from an atom is 4 - 25 eV. Ionizing radiation must have energy greater than that. For EM waves (UV) this range is up to 320 nm of wave length. Type and Sources of Ionizing Radiation The important types of ionizing radiation are: • Gamma-ray ( γ ray ) • X-ray • Fast electrons • Heavy Charged Particles • Neutrons 2 Gamma ray Electromagnetic radiation emitted from a nucleus or an annihilation reaction between matter and antimatter ( electron and positron) E γ = hν = hc λ = 1 . 2398 keV.nm λ h = 6 . 626 × 10- 34 J.s = 4 . 136 × 10- 18 keV.s = Planck’s Constant c = 2 . 99 × 10 8 m.s- 1 = 2 . 998 × 10 17 nm.s- 1 = Speed of light in vacuum X-ray Electromagnetic radiation emitted by electrons changing atomic levels ( characteristic X ray) or slowing down in a Coulomb field ( bremsstrahlung). Energy range: • 0.1 - 20 kV - low energy or soft X-ray or Grenz rays • 20 - 120 kV - Diagnostic X-ray • 120 - 300 kV - Orthovoltage • 200 kV - 1 MV - Intermediate • 1 MV up - Megavoltage Fast Electrons or positrons If emitted from a nucleus they are referred to as β rays . If they resulted from a charged particle collision they are called ” δ rays” . Heavy Charged Particles Heavy charged particles are usually obtained from acceleration by a Coulomb force field in a Cyclotron, or a linear accelerator. 3 • Proton - the hydrogen nucleus • Deuteron - Deuterium nucleus ( p + n ) • Triton - ( p + 2n ) • Alpha - ( 2p + 2n ) • Other heavy charged particles consisting of nucleus of heavier atoms with charge • Pions - Produced by interaction of electrons or protons with nuclei Neutrons Obtained from Nuclear reactions[ ( p,n) or fissions] Two types of Ionizing Radiation 1. Directly Ionizing Radiation 2. Indirectly Ionizing Radiation Directly Ionizing Radiation Fast charged particles which deliver their energy to matter directly through Coulomb force interactions along the particle’s track.through Coulomb force interactions along the particle’s track....
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Lesson1a - Lesson 01-contd Ionizing Radiation MP200...

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