Gamma Absorption
Heavy materials for the absorption of gamma rays
In terms of radioprotection mitigate gamma rays is not enough: they have to be absorbed. When the gamma disappears by interacting, it transmit its energy to other particles that will deposit this energy in the medium if they are charged. If they are neutral, they will continue their way until they interact themselves. The way the energy is absorbed is therefore complex.
The simplest case occurs when the photon interacts by photoelectric effect, that’s to say when it is absorbed in one stroke by an atom of which it ejects an electron. In a dense screen matter, the path traveled by this electron is very short so that we can consider that its energy is locally absorbed. For its part, the absorbing atom that has lost an electron will restore the energy gained by emitting one or more photons. These secondary photons, which can be X rays if the electron belonged to the innermost shell (layer) of the atom, have lost the memory of the direction of incident gamma and are emitted in all directions.
The most complicated case occurs when the photon interacts by Compton effect. As for the photoelectric effect, the photon is absorbed, but a new lower energy photon is also emitted with the ejected electron. The Compton electron as the photoelectric electron deposits its energy locally, but the new gamma escapes, taking its energy fraction. It is emitted preferably in the direction of the incident gamma. If the screen is thick, it has a better chance to be absorbed later on by interacting itself. But it can also exit the screen, in which case only a part of the energy – the electron one – has been absorbed.
These secondary gamma, which are emitted at different angles and with various energies complicate the absorption law. They not only extend the life of the beam, but they degrade it. However, it is customary to consider an absorption coefficient of the deposited energy (the electron one). The absorptive capacity ignores the deposited energy of the secondary gamma absorbed further into the screen. Concerning radiation protection, it does not matter: the screen thicknesses thus calculated overestimate the risk.
Lead is a very efficient absorber. First it is a very dense material. Then the lead nucleus is a heavy nucleus whose property is to favour the photoelectric effect. This effect plays a predominant role for gamma which, up to an energy of 200-300 keV, are mostly stopped. Lead performances are less good for more energetic gamma. The relevant figure is that only 1.5 cm screen is enough to absorb 50% of the energy of 1 MeV gamma.
Light absorbers (water, concrete, air) require greater thicknesses because they are less dense and that the photoelectric effect – beneficial to stop the gamma – plays a lesser role.
NEXT : Gamma Attenuation
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