The following provides some background information on the physics associated with our shielding and cryogenic work:
Shielding systems are designed to control the negative impact of two fundamental phenomena – that of electromagnetic radiation and magnetism, to a point whereby any residual levels will remain within safe, non-hazardous and acceptable limits.
Satisfying these conditions necessitates placing a suitable physical barrier of appropriate material, form and thickness in between the offending source and the designated protected 'clean' area so as to create a benign, safe environment.
For EMI / RFI applications, a 'Faraday Cage' – that is a continuously conductive electrical barrier, is formed around the source of emission or the 'zone' to be protected from such emissions, such that:
- Any unwanted signals penetrating or escaping from an enclosure do not compromise the electromagnetic compatibility (EMC) of equipment and that equipment will function as intended without unacceptable degradation
- A safe environment can exist for reasons of product testing, research, safety or security
Where there are service penetrations, then techniques exist to allow such services to enter or exit the protected zone without compromising the integrity of the shielding.
Materials used for radiation risks
The selection of shielding material is dependent upon the intensity and classification of the radiation present and other such practical factors as:
- Whether the work is an upgrade of existing facilities or a new build
- What space is available
- Structural / physical constraints
- Neutron scatter effect
All these influence the choice of shielding material. Generally lead, high density concrete, steel or a combination of these are used, but other more exotic shielding materials exist, such as boronated polyethylene are also used when appropriate.
Gamma rays and X-rays differ only in their source of origin but both are part of the higher end of the electromagnetic spectrum and require high density shielding materials such as lead or concrete.
Neutron shielding is complex and generally requires shielding materials with a low atomic number i.e. they possess a high hydrogen content. Examples are concrete and boronated polyethylene; the latter being particularly suited for a linac bunker's maze.
There is also Alpha and Beta radiation. Alpha particles have a short range and are the least penetrating and can be stopped by a sheet of paper or aluminium foil. However, if they enter the body, then this can be fatal. Beta particles have a longer range than alpha particles but can be stopped by a few mm of aluminium or low density plastic. Under certain conditions, Beta particles will trigger the production of secondary x-ray radiation, called Bremsstrahlung.