X - RAYS - Form 4 Physics Notes

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Introduction

  • X-rays were discovered by a German scientist named Roentgen in 1985.
  • They can pass through most substances including soft tissues of the body but not through bones and most metals.
  • They were named X-rays meaning 'unknown rays'.


X-ray Production

  • They are produced by modified discharge tubes called X-ray tubes. The cathode is in the form of a filament which emits electrons on heating.
  • The anode is made of solid copper molybdenum and is called the target.
  • A high potential difference between the anode and the cathode is maintained (10,000 v to 1,000,000 or more) by an external source.
  • The filament is made up of tungsten and coiled to provide high resistance to the current.
  • The electrons produced are changed into x-rays on hitting the anode and getting stopped. Only 0.2% of the energy is converted into x-rays.
  • Cooling oil is led in and out of the hollow of the anode to maintain low temperature.
  • The lead shield absorbs stray x-rays.
    x ray production1


Energy Changes in an X-ray Tube

  • When the cathode is heated electrons are emitted by thermionic emission.
  • They acquire electrical energy which can be expressed as E = eV. Once in motion the electrical energy is converted to kinetic energy, that is eV = ½ mev2 .
  • The energy of an electromagnetic wave can be calculated using the following equation
    Energy = hf, where h - Planck’s f –frequency constant, of the wave .
  • The highest frequency of the X-rays released after an electron hits the target is when the greatest kinetic energy is lost, that is
    hfmax = eV
  • Lower frequencies are released when the electrons make multiple collisions losing energy in stages, the minimum wavelength, λmin, of the emitted X-rays is given by;
    (hc)min = eV
  • These expressions can be used to calculate the energy, frequencies and wavelengths of X-rays.

    Examples
  1. Determine the energy possessed by X-rays whose frequency is 4 × 1017 Hz.

    Solution
    E = hf
    = 6.63 × 10-34 ×4 × 1017
    = 2.652 × 10-16 J.
  2. An X-ray tube operates at 60 kV and the current through it is 4.0 mA. Calculate the,
    1. Number of electrons striking the target per second.
    2. Speed of the electrons when they hit the target.

      Solution
      a) Current through the tube is given by I = ne , where n - number of electrons striking target per second and e- electronic charge (e = 1.6 × 10-19 coulombs)
      So, n = 1/e = (4.0 × 10-3)/1.6 × 10-19 = 2.5 × 1016 electrons.
      b) Kinetic energy = electrical energy
      ½ mev2 = eV, then v = 2.13 × 108 m/s
  3. An 18 kV accelerating voltage is applied across an X-ray tube. Calculate;
    1. The velocity of the fastest electron striking the target
    2. The minimum wavelength in the continuous spectrum of X-rays produced. (mass of electron - 9 × 10-31 kg, charge on an electron - 1.6 × 10-19 C, h - 6.6 × 10-34 J/s, c - 3 × 108 m/s)

      Solution
      a) V = 18 × 103 V
      me = 9 × 10-31 kg
      e = 1.6 × 10-19 C
      h = 6.6 × 10-34 J/s
      c = 3 × 108 m/s 
      ½ mev2 = ev; therefore v = 8 × 107 m/s
      b) (hc)/λmin = eV; λmin = hc/eV
      λ min = (6.6 × 10-34 × 3 × 108 )/(1.6 × 10-19 × 18 × 103) = 6.9 × 10-11 m.


Properties of X-rays

  1. They travel in straight lines
  2. They undergo reflection and diffraction
  3. They are not affected by electric or magnetic fields since they are not charged particles.
  4. They ionize gases causing them to conduct electricity
  5. They affect photographic films
  6. They are highly penetrating, able to pass easily through thin sheets of paper, metal foils and body tissues
  7. They cause fluorescence in certain substances for example barium platinocynide.


Hard X-rays

  • These are x-rays on the lower end of their range (10 - 11 – 10 - 8 m) and have more penetrating power than normal X - rays.
  • They are capable of penetrating flesh but are absorbed by bones.


Soft X-rays

  • They are on the upper end of the range and are less penetrative.
  • They can only penetrate soft flesh and can be used toshow malignant growth in tissues.


Dangers of X-rays and the Precautions

  1. They can destroy or damage living cells when over exposed.
  2. Excessive exposure of living cells can lead to genetic mutation.
  3. As a precautionary measure X-ray tubes are shielded by lead shields.


Uses of X-rays

  1. Medicine – X-ray photos called radiographs are used as diagnostic tools for various diseases. They are also used to treat cancer in radiotherapy.
  2. Industry – they are used to photograph and reveal hidden flaws i.e. cracks in metal casting and welded joints.
  3. Science –since the spacing of atomic arrangement causes diffraction of x-rays then their structure can be studied through a process called X-ray crystallography.
  4. Security –used in military and airport installations to detect dangerous metallic objects i.e. guns, explosives, grenades etc.

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