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Production of Xrays 1

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Production of X-rays (1) ... Electrons from a hot element are accelerated ... Using crystal as a wavelength selector, the intensity of different wavelengths ... – PowerPoint PPT presentation

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Title: Production of Xrays 1


1
Production of X-rays (1)
  • X-rays are produced when rapidly moving electrons
    that have been accelerated through a potential
    difference of order 1 kV to 1 MV strikes a metal
    target.

2
Production of X-rays (2)
  • Electrons from a hot element are accelerated onto
    a target anode.
  • When the electrons are suddenly decelerated on
    impact, some of the kinetic energy is converted
    into EM energy, as X-rays.
  • Less than 1 of the energy supplied is converted
    into X-radiation during this process. The rest is
    converted into the internal energy of the target.

3
Properties of X-rays
  • X-rays travel in straight lines.
  • X-rays cannot be deflected by electric field or
    magnetic field.
  • X-rays have a high penetrating power.
  • Photographic film is blackened by X-rays.
  • Fluorescent materials glow when X-rays are
    directed at them.
  • Photoelectric emission can be produced by X-rays.
  • Ionization of a gas results when an X-ray beam is
    passed through it.

4
X-ray Spectra (1)
  • Using crystal as a wavelength selector, the
    intensity of different wavelengths of X-rays can
    be measured.

5
X-ray Spectra (2)
  • The graph shows the following features.
  • A continuous background of X-radiation in which
    the intensity varies smoothly with wavelength.
    The background intensity reaches a maximum value
    as the wavelength increases, then the intensity
    falls at greater wavelengths.
  • Minimum wavelength which depends on the tube
    voltage. The higher the voltage the smaller the
    value of the minimum wavelength.
  • Sharp peaks of intensity occur at wavelengths
    unaffected by change of tube voltage.

6
Minimum wavelength in the X-ray Spectra
  • When an electron hits the target its entire
    kinetic energy is converted into a photon.
  • The work done on each electron when it is
    accelerated onto the anode is eV.
  • Hence hf eV and the maximum frequency

Therefore,
7
Characteristic X-ray Spectra
  • Different target materials give different
    wavelengths for the peaks in the X-ray spectra.
  • The peaks are due to electrons knock out
    inner-shell electrons from target atoms.
  • When these inner-shell vacancies are refilled by
    free electrons, X-ray photons are emitted.
  • The peaks for any target element define its
    characteristic X-ray spectrum.

8
Uses of X-rays
  • In medicine
  • To diagnose illness and for treatment.
  • In industry
  • To locate cracks in metals.
  • X-ray crystallography
  • To explore the structure of materials.
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