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Title: EN250: Lecture 2 Radiation Physics


1
EN250 Lecture 2Radiation Physics
  • X-ray Production
  • Disclaimer These slides are a compilation of
    pictures obtained from the WWW and various books,
    etc, and none is original. For example the flash
    demos are obtained from http//learntech.uwe.ac.uk
    /radscience/xray_prod/production_of_xrays03_files/
    frame.htm

2
Overview
  • Review of the structure of the atom
  • Electromagnetic Radiation
  • Production of X-rays
  • Interaction of X-ray with matter
  • Properties of X-rays
  • Formation of an X-ray image
  • Potential for 3D?

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History of the Development of Atomic Models
  • 1. Democritus (400 B.C.) matter is made of
    atomos.
  • 2. John Dalton (1800s) proof for atoms.
  • 3. J.J. Thomson (1904) discovered electrons in
    atoms his model was of a positive sphere with e-
    embedded in it.
  • 4. Ernest Rutherford (1911) discovered nucleus
    proposed that e- orbited a nucleus that had
    almost all the mass.
  • 5. Niels Bohr (1913) said e- orbited in fixed
    paths.
  • 6. James Chadwick (1932) nucleus contained
    protons neutrons.
  • 7. Erwin Schrodinger (1926) electron cloud
    model.

4
Review of the Structure of the Atom
  • Atom is mostly empty space
  • Mass is concentrated in its Nucleus
  • Nucleus consists of Protons and Neutrons, the
    Nucleons
  • Electrons orbiting around the nucleus

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ELECTRONS PROTONS NEUTRONS
ELECTRONS PROTONS NEUTRONS
Negative charge Positive charge Zero or neutral charge
of electrons determines the charge of the atom of protons determines type of element the atom is of neutrons determines type of isotope the atom is
Much much smaller than protons Much much bigger than electron Much much bigger than electron
Contributes little to mass or weight of atom-hardly anything Contributes significantly to mass or weight of atom Contributes significantly to mass or weight of atom

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Atomic Number
  • Atomic Number A Number of Nucleons
  • Mass number Z number of protons

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Characterizing Atoms
  • A Nuclide is characterized by A and Z

9
ORBITAL BASICS
  • (1) A shell is sometimes called an orbital or
    energy level.(2) Shells are areas that surround
    the center of an atom.(3) The center of the atom
    is called the nucleus.(4) Electrons live in
    something called shells.

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SHELLS ONLY HOLD SOME ELECTRONS
  • Not all shells hold the same number of electrons.
    For the first 18 elements, there are some rules.
  • The k-shell only holds two electrons.
  • The l-shell only holds eight electrons.
  • The m-shell only holds eight electrons (for the
    first 18 elements). The m-shell can actually hold
    up to 18 electrons as you move further along the
    periodic table.
  • YOU CAN'T KNOW WHERE AN ELECTRON IS
  • Niels Bohr came up with all these ideas in 1913.

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Valence Shell
  • The outermost shell is the valence shell
  • Valence Shell determines chemical, thermal,
    optical, and electrical properties of the element
  • X-rays involves inner shells
  • Radioactivity and Gamma rays involve the nucleus
  • Valence shells have at most 8 electrons
  • Metals have 1, 2, or 3, one of which can be
    easily detached, the free electron, responsible
    for the excellent conduction of heat and
    electricity

14
Binding Energy
  • An atom is ionized when one of is electrons is
    completely removed
  • The Binding Energy E is the energy required to
    remove this electron
  • Usually measured in electronvolts (eV)
  • Example Tungsten (W Z74)
  • EK 70, EL 11, EM 2, All in Kev
  • Binding Energy EK for various atoms (usually lt
    100 kev)
  • W Z 74 70 Kev
  • I Z 53 33 Kev
  • Mo Z 42 20 Kev
  • Cu Z 29 9 Kev
  • eV is the amount of energy gained by a single
    unbound electron when it accelerates through an
    electrostatic potential difference of one volt

15
Exciting an Atom
  • Atom is called excited when an electron is raised
    from one shell to another further out, as a
    result of expenditure of energy
  • When the electron falls back it emits this energy
    in a packet of energy, a photon of light (visible
    or ultraviolet)
  • A Photon is an elementary particle, the quantum
    of the electromagnetic field and the basic unit
    of light and all other forms of electromagnetic
    radiation
  • Developed by Einstein (1905-1917) to explain the
    non-wave properties of EM waves

16
Electromagnetic Radiation
  • X-rays and Gamma rays are EM waves
  • Quantum aspects Travel in a straight line,
    photons packets or quanta of energy
  • Wave aspects Electric and Magnetic Fields at
    right angle to each other and to the direction of
    wave travel of the wave field strength
    sinusoidal frequency f, period T, wavelength

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Electromagnetic Specturm
  • Radio Waves and X-Ray/Gamma rays are the only EM
    Waves that can penetrate the body

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Combined View
  • E h f, h Plancks constant
  • E (in Kev) 1.24 / Lambda (in nm)
  • Blue Light lambda 400nm E 3ev
  • X-ray lambda 0.1 nm E 140Kev
  • Rays travel from source in all directions
  • A beam is a collimated set of rays
  • Energy flux Total energy (total number of
    photons ) per unit area
  • Intensity is inversely proportional to distance
    from source

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Discovery of X-ray Wilhelm Conrad Roentgen
(1845-1923)Physical Institute of the University
of Wurzburg
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Roentgens early tube design
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Roentgens Laboratory
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Nov 1895 Penetrating solids
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First X-Ray Picture 22 Dec 1895
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Modern X-ray Tubes
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Production of X-rays
  • X-Rays are produced when fast moving electrons
    are suddenly stopped by impact on a metal target
  • The kinetic energy of the electrons is converted
    into heat (99) and X-rays (1)
  • Structure of an X-ray Tube
  • A Negative Electrode, Cathode, fine Tungsten coil
    or filament, heated to incandescence (2200 C)
    which gives off electrons (thermionic emission)
  • A Positive Electrode, Anode, smooth flat metal
    target, usually Tungsten, collects these
    electrons which hit it with about half the speed
    of light

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  • Filament heating voltage about 10V and using
    about 10A
  • The accelerating voltage in the range 30-150 kV
    and current of 0.5-1000 mA

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Processes underlying X-ray formation
  • Elastic collision with Atoms
  • Inelastic collision with electrons in the outer
    shell of an atom
  • Excitation
  • Ionization
  • Inelastic collision with electrons in the inner
    shell of an atom (Characteristic Radiation)
  • Inelastic collision with the nuclei of the atom
    (Bremsstrahlung)

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Elastic collision with target
  • The electron is deflected but looses very
    little kinetic energy because its mass is
    negligible, and continues in tortuous path
    because of successive interactions

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Interaction between filament electron and outer
shell electron (1)
  • Excitation
  • i) This results in an outer shell electron
    gaining energy being raised to a higher level.
  • ii) Heat is produced as the electron falls back
    into its original path.
  • No contribution to x-ray production

38
Interaction between filament electron outer
shell electron (2)
  • Outer shell electron ejected from target atom
    results in an outer shell electron being
    completely removed from the target atom.
  • Both the filament the ejected electrons may
    interact in either the first or second of these
    interaction processes with other target atoms
  • Ultimately, this type of interaction may cause
    the target material to heat up

39
Illustration (2)
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Inelastic collisions with electrons in the inner
shell of an atom (characteristic radiation)
  • The incoming electron transfers sufficient energy
    to remove an inner shell electron from its atom
    in the target.
  • In order for this to occur the electron must
    possess energy at least as great as the binding
    energy of the inner shell.
  • Any surplus energy appears as additional kinetic
    energy in the ejected electron
  • The inner shell vacancy is quickly filled by an
    electron falling inwards from a shell further out
    from the nucleus
  • This transition is accompanied by a burst of
    electromagnetic radiation with energy equal to
    the difference in binding energies of the two
    shells.
  • This type of x-ray production is termed
    characteristic because the exact photon energy is
    characteristic of the element of which the target
    is made.

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Illustration (3)
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Energy levels
  • The difference in the binding energies of the K
    and L shells in tungsten is 70 keV - 11 keV 59
    keV.So a characteristic photon of 59 keV is
    emitted.
  • The difference in the binding energies of the M
    and K shells in tungsten is 70keV - 2 keV 68
    keV.
  • These two electron transitions are the most
    likely to occur, producing x-ray photons of 68
    and 59 keV.
  • Characteristic x-rays contribute less than 10 of
    an x-ray beam. The majority of x-ray production
    results from inelastic collisions of incoming
    electrons with the nuclei of the target atoms

43
Inelastic collisions with the nuclei of the atom
(Bremsstrahlung Radiation)
  • The incoming electron passes very close to the
    nucleus of a target atom (1).
  • The attraction causes the electron to deviate in
    its course (2)
  • The sudden change of direction stimulates the
    electron to release energy in the form of a
    photon of electromagnetic radiation (3)
  • The emission of radiation results in a reduction
    in the electrons kinetic energy causing it to
    slow down.
  • The energy of the radiation depends on the degree
    of deviation the electron suffers.
  • In an extreme case the electron may actually be
    brought to rest. Thus the photon energy can be of
    any value from zero up to a maximum equal to the
    initial kinetic energy of the incoming electron.
  • This gives rise to a continuous spectrum of
    x-radiation and is known as braking
    (Bremsstrahlung) radiation.

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Illustration (4)
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Interaction of X-Ray with matter
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? Example X-ray images ?
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End of Lecture 2
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Additional Slides
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  • Collimated vs non-collimated x-ray

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  • Dose Limits
  • Occupational Whole Body5,000 mrem per
    yearExtremity and Skin of Whole Body 50,000 mrem
    per year Lens of the Eye 15,000 mrem per year Any
    single organ (e.g., Thyroid) 50,000 mrem per year
    Fetus of Declared Pregnant Worker500 mrem per
    term of pregnancyGeneral Public100 mrem per
    yearFor additional information see Section 6 of
    the University of Washington Radiation Safety
    Manual

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http//www.amptek.com/xrf.html
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