The physics are complicated. EMR interacting with matter will produce different effects depending where within the electromagnetic spectrum the event occurs. The critical condition is whether the radiation is 'ionising' (so called because as it creates charged particles – 'ions') or not. It is these 'ions' that interact with other material to produce hazardous 'radiation'.
The two fundamental categories of radiation are:
- 'Non-ionising' which includes visible light, radio waves, micro-waves etc
- 'Ionising' radiation – Alpha and Beta particles, X-rays and Gamma rays – all of which are hazardous, as are Neutrons
All ionising radiation – which is commonly referred to as simply 'radiation', can be harmful and the degree to which it impacts on health is a factor of:
- The type of radiation
- Total exposure time
Spectrum 2 – ionising and non-ionising radiation
As often in physics, nothing is quite what is seems at first. Neutrons have no 'charge' so they cannot react with electrons and, technically, are non-ionising. Nevertheless neutrons can interfere with the atomic nuclei of matter, which includes living cells, to produce harmful radiation.
The unwanted neutrons (a bi-product of the linear accelerator process) will, if the energy level is sufficient, induce additional gamma radiation from the material within a bunker. This neutron activated secondary radiation is likely to be triggered where the linac energy is 10MV or above.
Whilst a linac bunker's primary shielding protects those outside the bunker, within the room there will be neutron scatter off the walls requiring special attention be paid to the shielding within the maze and the entrance door that generally leads to a high occupancy area. This is even more so if space restrictions prevent the construction of a maze.
There is also the need to counter the effect of 'Bremsstrahlung'* radiation, produced when the high energy electrons created by the accelerator undergo rapid deceleration when they hit the target plate, with the knock-on effect of generating even more radiation; x-ray, gamma and neutron.
* German: 'breaking radiation'
Areas where there is a risk of ionising radiation being present are designated by the internationally accepted yellow background trefoil symbol for radiation. The International Atomic Energy Agency (IAEA) have now introduced a new supplementary sign (right) as an intuitive warning to those not educated in the significance of the three cornered trefoil.
Note: A magnetic field hazard is designated bysign or similar.