Radiation and the Atom PHYS Lecture Departamento de Física Instituto Superior de Engenharia do Porto cav@isep.ipp.pt
Overview SI Units and Prefixes Radiation Electromagnetic Radiation Electromagnetic Spectrum Wave-Particle Duality Radiological Units Particles in Medicine Periodic Table of The Elements Atomic Nucleus Electronic Structure Electron Binding Energy Radiation from Electron Transitions Exercises 2
SI Units and Prefixes 3
Derived Units 4
Radiation Radiation propagation of energy through space and matter, as either electromagnetic waves or particles (e.g., electron). Radioactivity the characteristic of various materials to emit ionizing radiation. Ionization the removal of electrons from an atom. The essential characteristic of high energy radiation when interacting wit matter. 5
Electromagnetic Radiation Electromagnetic radiation consists of two components: Electric (E) field Magnetic (M) field Both fields vary sinusoidally as a function of space and time, at 90º angle to one another and perpendicularly to the direction of wave propagation 6
Electromagnetic Radiation Amplitude Related to the intensity of the wave. Wavelength: λ distance between identical points on adjacent cycles (1 nm = 10-9 m) Period: T Time required to complete one cycle (λ) of a wave Frequency: ν = 1/T Number of periods per second Hertz (Hz) (1 Hz = 1 s -1 ) (m s -1 ) 7
Electromagnetic Radiation Characterized by wavelength λ, frequency ν, or energy E Unaffected by external E or M fields All EM radiation has zero mass Wave-particle duality: the manifestation depending on E and relative dimensions of the detector to λ. Interaction with matter via either absorption or scattering *X-rays are ionizing radiation, removing bound electrons - can cause either immediate or latent biological damage 8
Electromagnetic Spectrum Physical manifestations are classified in the EM spectrum based on energy (E) or wavelength (λ) and comprise general categories. 9
Wave-Particle Duality Electromagnetic Wave Planck Hypothesis Quantum energy of a photon: Electromagnetic waves (radiation) explain phenomena such as Interference Diffraction... Electromagnetic radiation act as energy particle or quanta (photons) and explains the Photoelectric Effect 10
Relativistic Energy Non-relativistic Kinetic Energy The kinetic energy of a high speed particle can be calculated from Einstein mass-energy relationship The relativistic energy of a particle can also be expressed in terms of its relativistic momentum: For a particle, it includes both the kinetic energy and rest mass energy 11
DeBroglie Hypothesis Einstein formula Momentum-wavelength relationship for a photon (applies to other particles as well) For a particle of zero rest mass For a photon 12
Electromagnetic Spectrum Speed of light 13
Electromagnetic Spectrum Speed of light Above near-ultraviolet E is sufficient to remove bound electrons (ionizing EM) 12.6 ev required to ionize H 2 O 14
Radiological Units 15
Radiological Units 16
Radiological Units Unit Conversion roengten-to-rad conversion factor f f factor depends on: Energy of the radiation Density of material Atomic number of material 17
Particles in Medicine 18
Particles and Radiation Energy equivalence of rest mass For electron at speed of light: m = 9.109 x 10-31 kg c = 2.998 x 10 8 m/s E = 8.187 x 10-14 J Corpuscular radiations are comprised of moving particles of matter the energy of which is based on the mass and velocity of the particles Interactions with matter are collisional in nature and are governed by the conservation of Energy (E) momentum (p) E = 0.511 MeV = 511 kev 19
http://physics.nist.gov/physrefdata/pertable/index.html 20
Atomic Nucleus Composition of the Nucleus Isotopes same Z, but different A A X uniquely defines an isotope (also written as X-A): Example (Iodine-131) 21
Isotopes Stable isotopes found along line N/Z = 1 at low Z N/Z = 1.5 at high Z N Odd N and odd Z tend to be unstable Odd Z elements offer potential for NMR (unpaired p+) Z 22
Nuclear Binding Energy (Mass Defect) Difference between the mass of an atom and the rest mass of all its constituent components due to nuclear binding energy Example: N-14 Mass of 7 p+, 7 n, 7 e- = 14.11536 amu Mass of N-14 = 14.00307 amu Difference = 0.11229 amu x 931 MeV/amu = 104.5 MeV 23
Electronic Structure Bohr Model Atom central nucleous electrons at discrete energy levels Orbital electron shells K, L,, Q: Pauli Exclusion Principle No two electrons in an atom may have identical quantum numbers max. electrons per shell 24
Electronic Structure Quantum Numbers n: principal q.n. which e- shell n = 0,1,2, l: angular momentum q.n. l = 0, 1,..., n-1 ml: magnetic q.n. orientation of the e- magnetic moment in a magnetic field ml = -l, -l+1,..., 0,... +l-1, +l ms: spin q.n. direction of the e- spin (ms = +½ or -½) 25
Electron Binding Energy Decreases with distance from nucleus Increases with Z 26
Radiation from Electron Transitions Characteristic X-rays Spectral lines Auger Electrons Preference at low Z ~0 Auger e- from soft tissue 27
Exercises The wavelength of a photon is 1.24 x 10-1 Å. What is its energy in kev? A. 124 kev B. 1 kev C. 10 kev D. 100 kev E. 12.4 kev The energy of characteristic radiation produced by an L to K transition will be that produced by an M to K transition. A. Greater than B. The same as C. Less than The energy of a photon is: A. Proportional to its wavelength B. Proportional to its frequency C. Inversely proportional to the exposure time D. Inversely proportional to its wavelength E. Can be expressed in terms of potential difference (volts) 28
End of Lecture!