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IAEA 1
Interaction of radiation with matter - 2
Charged Particle Radiation (Beta Particles)
Day 2 – Lecture 2
IAEA 2
• To discuss the following as they relate to beta particle interactions
Mechanisms of Energy Transfer Bremsstrahlung Cerenkov Radiation Shielding
Objective
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Ionizing radiation removes orbital electrons from atoms
This creates an ion pair – an electron and the atom that has lost an electron
Ionization
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Ionizing radiation includes photons, but the result is the same – an ion pair is produced
This section focuses on the electron interactions
Ionization
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Electrons are much lighter than the nucleons – the neutron and proton in the nucleus
Electrons
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All of the photon interactions
photoelectric effect Compton scattering pair production
Electrons
result in the production
of electrons. These are ionizing radiation just like beta particle sources
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• Electron interactions are comparable to those of other charged particles
• More energetic electrons travel faster and so create a lower ionization density
• Energetic electrons deposit less energy so the dose is lower until they slow down
• Dose is the amount of energy deposited per mass of material (joules/kg)
Electrons
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• In addition to the energy of the electron, the stopping power depends on the material in which the electron is interacting
Electrons
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Bremsstrahlung
• When an electron interacts close to a nucleus, it accelerates and changes direction
• The result is that a photon is produced. This process is called “Bremsstrahlung” which means ‘braking radiation’
• Bremsstrahlung photons have a continuous energy distribution
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The fraction of electrons producing Bremsstrahlung follows the relationship:
F = 3.5 x 10-4 (Z)(E)
Empirical Relationship
Note that the value of “E” is the maximum energy for beta particles
Beta particles that have higher energy will have a greater fraction of Bremsstrahlung photons created
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F = 3.5 x 10-4 (Z)(E)
Bremsstrahlung
Carbon-14, phosphorous- 32 are tritium (hydrogen-3) are all beta emitters but only one of these presents a radiation hazard due to bremsstrahlung radiation. Which radionuclide and Why do you think this is the case?
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Bremsstrahlung
Carbon-14 and tritium (hydrogen-3), are not likely to produce bremsstrahlung due to their low energy beta particles of 0.156 MeV and 0.018 MeV, respectively.
Conversely, a higher energy beta emitting nuclide, like phosphorous-32, is very likely to create bremsstrahlung photons due to it’s 1.7 MeV beta particle.
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Cerenkov Radiation
• Cerenkov radiation is the visible light that is created when charged particles pass through a material at a velocity greater than the velocity of light for that material
• Cerenkov radiation is observable in spent fuel pools of reactors and in irradiator source storage pools
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• While no particle can exceed the speed of light in a vacuum (3.0x108 m/s), it is possible for a particle to travel faster than the speed of light in certain mediums such as water
• When the charged beta particle moves through the water it tends to "polarize" (or orient) the water molecules
Cerenkov Radiation
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• After the beta particle has passed, the molecules realign themselves in their original, random charge distribution
• A pulse of electromagnetic radiation in the form of blue light is emitted as a result of this reorientation
• The intensity of the blue glow is directly proportional to the number of fissions occurring and the reactor power level
Cerenkov Radiation
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• Although most of the Cerenkov radiation is in the ultraviolet region, it is visible to us with a distinctive soft blue glow
• The blue glow persists for a short time after the reactor has been shut down
• This property may be used to inspect spent fuel to see if it is actually spent fuel or dummies used to mask a diversion of material
Cerenkov Radiation
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Where to Get More Information
Cember, H., Johnson, T. E, Introduction to Health Physics, 4th Edition, McGraw-Hill, New York (2009)
International Atomic Energy Agency, Postgraduate Educational Course in Radiation Protection and the Safety of Radiation Sources (PGEC), Training Course Series 18, IAEA, Vienna (2002)