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Basic Physics of Ionizing Radiation What is “Radiation”? • Radiation is energy travelling through space. • Sunshine is one of the most familiar forms of radiation, without it we would not exist. • But too much of it can be dangerous, therefore we limit our exposure (Radiation Protection) • There are many forms of radiation from particles to waves (commonly termed ElectroMagnetic Radiation). What is “Radiation”? • Radiation can be thought of as the transmission of energy through space. • Two major forms of radiation: – Electromagnetic (EM) radiation – Particulate radiation • Both forms can interact with matter, and transfer their energy to the matter. Electromagnetic Radiation • Electromagnetic radiation has no mass, and moves through space at the speed of light (3.0 x108 meters per second). • Electromagnetic radiation can be described by two models: – Wave Model – Photon Model EM Radiation: Wave Model • EM radiation is a pair of perpendicular, timevarying electric and magnetic fields traveling through space with the velocity of light (c). • The distance between maxima of the EM fields is the wavelength (λ). • The frequency (ν) of the wave is given by: ν=c/λ Wavelength • Distance between two peaks or troughs in a wave. l l • Wavelength (l) = Colour • Wavelength (l) = Energy (E) – (As l increases, E decreases) Ultraviolet 390-425nm 425-445nm 445-500nm 500-575nm 575-585nm 585-620nm 620-740nm Infrared Scientific Notation Value Symbol Value (Scientific Notation) 1,000,000,000 Giga (G) 1 x 109 1,000,000 Mega (M) 1 x 106 1,000 Kilo (k) 1 x 103 1 x 100 1 0.001 Milli (m) 1 x 10-3 0.000001 Micro (m) 1 x 10-6 0.000000001 Nano (n) 1 x 10-9 The de Broglie Wave Hypothesis h = Plank’s Constant = 6.6 x 10-34 J/s p = momentum = mass x velocity l = wavelength Value Bullet Electron Mass 0.03kg 9.1 x 10-31 kg Velocity 330 m/s 1 x 108 m/s 6.6 x 10-35 m 7 x 10-12 m Wavelength (l) EM Radiation: Photon Model E=hc/l Electromagnetic radiation can also be described as discrete packets of energy called photons. The energy (E) is related to the wavelength (l) in the wave model through Planck’s Constant (h) and the speed of light (c). Ionizing EM Radiation • EM radiation with wavelengths shorter than 100 nanometers can remove electrons from the outer atomic shells. • This process produces ions. • Ions can interact with living tissue to produce biological damage. • A major source of ionizing radiation is nuclear transformation. Nuclear Transformation - Δm Radioactive Stable Ionizing Radiation: α, β, or γ Gamma Rays Z, M Z, M g Gamma rays are electromagnetic radiation resulting from nuclear transformation. Production of X-Rays Electron or beta X-Ray Target Nucleus (Heavy metal) X-rays are produced when a charged particles (electrons or betas) are decelerated by a strong electrostatic field, such as that found near the nuclei of heavy metals (tungsten, lead). Types of Ionizing Radiation Alpha Particles Stopped by a sheet of paper Radiation Source Beta Particles Stopped by a layer of clothing or less than an inch of a substance (e.g. plastic) Gamma Rays Stopped by inches to feet of concrete or less than an inch of lead Particulate Radiation • Charged particles are emitted from the atomic nucleus at high energy in some nuclear transformations. These include alpha and beta particles. • Uncharged particles (neutrons) are produced by fission or other nuclear reactions. • Both types of particles produce ionization. Alpha Particles Z - 2, M - 4 4 ++ a 2 Z, M Alpha Particle (Helium Nucleus) Beta Particles 0 0n Antineutrino Z+1, M Z, M 0 b -1 Beta Particle Four Primary Types of Ionizing Radiation: Alpha Particles Alpha Particles: 2 neutrons and 2 protons They travel short distances, have large mass Only a hazard when inhaled Four Primary Types of Ionizing Radiation: Beta Particles Beta Particles: Electrons or positrons having small mass and variable energy produced inside the nucleus. Electrons form when a neutron transforms into a proton and an electron or: Gamma Rays Gamma Rays (or photons): Result when the nucleus releases Energy, usually after an alpha, beta or positron transition X-Rays X-Rays: Occur whenever an inner shell orbital electron is removed and rearrangement of the atomic electrons results with the release of the elements characteristic X-Ray energy Neutrons Neutrons: Have the same mass as protons but are uncharged They behave like bowling balls Four Primary Types of Ionizing Radiation • • • • • Alpha particles Beta particles Gamma rays (or photons) X-Rays (or photons) Neutrons Radioactive Atom Ionizing Radiation alpha particle X-ray beta particle gamma ray Radioactive Atom Ionizing Radiation alpha particle X-ray beta particle gamma ray Direct Ionization Caused By: • Protons • Alpha Particles • Beta Particles • Positron Particles Indirect Ionization Caused By: • Neutrons • Gamma Rays • X-Rays Specific forms of ionizing radiation Directly ionizing Particulate radiation Indirectly ionizing consisting of atomic or subatomic particles (electrons, protons, etc.) which carry energy in the form of kinetic energy of mass in motion. Electromagnetic radiation in which energy is carried by oscillating electrical and magnetic fields traveling through space at the speed of light. Interaction of Charged Particles with Matter: Ionization Interaction of x or g rays (photons) with matter Concept of Physical Half-life • Radioactive nuclei undergo disintegration at a rate that is proportional to the number of untransformed nuclei present. • The physical half-life is the time required for one-half of the remaining nuclei to transform. • The half-life is characteristic of the radionuclide. Each second a fraction of the parent atoms decay each atom which decays throws out a radioactive particle each atom which decays produces a new daughter atom. After a time that we call the element’s half life half the parent atoms have decayed the radioactivity count rate has dropped by one half count rat e/ decays per s 600 500 400 Half original decay rate 300 Half life = about 45s 200 100 0 0 50 100 150 200 t ime / s 250 300 350 400 After another half life a further half of the parent atoms have decayed the radioactivity count rate has dropped by a further half count rat e/ decays per s 600 500 400 Half original decay rate 300 A further half of original decay rate 200 – so now only one quarter of original 100 0 0 50 100 150 200 t ime / s 250 300 350 400 Half Lives can have many different values e.g. Uranium 238 5000 million years Cobalt 60 5 years Iodine 131 8 days Barium 143 12 seconds Polonium 213 4 millionths of a second Half-Life • The Half-Life of a radioactive isotope is the time taken for its activity to drop to half its initial value. • 100 atoms of Tc-99m (used in Nuclear Medicine Departments). • The half-life of Tc-99m is 6 hours. • How many atoms have decayed in 12 hours? • Example 1 : • A radioiodine compound has 5 microcuries of radioactivity on a given date. How much radioactivity remains after 40 days may be found • Example 2 : • A 5 microcurie dose of radioiodine is administrated to a patient for diagnostic purpose. If the physical halflife of the isotope is 8 days and the biological half-life is 2 days, what activity will remain after 8 days