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Interaction of radiation with matter - 2

Charged Particle Radiation (Beta Particles)

Day 2 – Lecture 2

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• 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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Recall that unlike photons, electrons have a charge (-) and mass

Electrons

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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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Bremsstrahlung

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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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Shielding for Beta Sources

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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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Cerenkov RadiationReactor Spent Fuel Pool

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Cerenkov RadiationIrradiator Source Rack

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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)