Radiation basics
The dose to the workers assessed with VISIPLAN covers the external exposure with gamma and x-ray radiation. In the following, we describe the origin of this radiation and the way in which shielding and doses to the workers are calculated.
Gamma radiation, X-ray radiation Gamma radiation and X-ray radiation are both forms of electromagnetic radiation with energies above 10 keV. This kind of radiation is characterised by its high penetrability through matter.
| Gamma Radiation |
Gamma radiation originates from unstable isotopes (natural or artificial) which after disintegration through α-decay or β-decay remain in an excited state.
The isotope can reduce its excess energy by emitting a photon of gamma (γ)-radiation (see fig.1. below).
Fig.1. : Sources of gamma-radiation |
| X-ray radiation |
X-ray radiation is produced when the electrons around the nucleus of an atom are rearranged. This rearrangement can be caused by collision of a particle with the atom or through interaction with radiation.
X-ray radiation is also produced through the energy loss of an electron moving in the electrical field of a heavy nucleus (see fig.2.)
Fig.2. : Sources of X-ray radiation |
Interaction of radiation with matter Different interactions between radiation and matter are possible, s.a. the photo-electric effect, the Compton-effect and pair production.
Photoelectric
effect |
In the photoelectric effect the incoming photon collides with an electron of the atom. The photon is absorbed and the electron is ejected from the atom. The energy of the photon is completely transferred to the electron. Part of the energy is spent to compensate the binding energy of the electron, the remaining part is taken up as kinetic energy of the electron. This type of interaction is predominant for low energy photons. |
| Compton Scattering |
The Compton effect occurs when a photon colliding with an electron of the atom loses only a fraction of its energy. The photon survives the collision but is deviated from its trajectory. Compton scattering is predominant in the energy range from 1 MeV to 10 MeV for elements of low and intermediate atomic number. |
| Pair production |
Pair production is the most likely process for photons of high energy. During this interaction the photon creates an electron positron pair. This process only occurs when the energy of the photon is greater then 1.02 MeV. |
All these effects can be seen on the following figure (3.). |
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Fig.3. : Photoelectric effect and pair production
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Gamma-ray attenuation In order to describe the attenuation of radiation through shielding matter we introduce the quantity called fluence rate.
| Fluence rate |
The fluence rate Φ gives the intensity of the radiation at a certain point i.e. the number of photons passing a unit surface per unit time (cm-2.s-1). |
Narrow beam attenuation Consider a narrow collimated beam of photons passing through matter. Part of the incoming photons will be removed from the beam, in a distance dx, due to processes s.a. the photoelectric effect, Compton scattering and pair production.
The fluence rate reduction is found to be proportional to dx, the distance travelled through the matter and can be written as :
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(formula 1.) |
| Linear attenuation |
The linear attenuation coefficient µ is determined by the above mentioned interaction processes, and is a function of the material composition and photon energy.
The photon fluence rate after passing a distance t through a homogeneous medium can thus be written as :
 |
(formula 2.) | With Φ0 the photon fluence rate of the incoming beam.
This equation determines the number of uncollided photons that arrive at the dose point. |
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Fig.4. : Narrow beam attenuation
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Broad beam attenuation