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Frequently Asked Questions

  • Electromagnetic Compatibility (EMC)

    Ensuring that when an electrical or electronic device or system is functioning, there will be no unintentional interference with other electrical or electronic devices or systems.

  • Electromagnetic Interference (EMI)

    This is an external disturbance interrupting or interfering with the effective performance of a piece of electrical equipment or associated circuit. Sources of interference may take the form of radio waves, power circuits, rotating electrical equipment and similar. These events may be natural or man-made and span the electromagnetic spectrum.

  • Radio Frequency Interference (RFI)

    A different name for EMI.

  • Electromagnetic Radiation (EMR)

    This is energy that travels outward from a given source in straight lines and in all directions, and thus it 'radiates'.

    In terms of the modern quantum theory, electromagnetic radiation consists of particles called photons, which are packets ("quanta") of energy which move at the speed of light. However, electromagnetic radiation has a dual nature and can behave both as a particle and a wave.

    Visualisations of a photon
    Entrance to lead working area
    Visualisations of a photon
    Site dress PPE (lead)
    Visualisations of a photon

    All matter produces radiation which can be considered as a wave vibrating at different frequencies. This oscillating radiation also has an electric and magnetic component and carries energy and momentum that may interact with any body through which it passes or by which it is absorbed.

    Electromagnetic radiation can be described by any of the following three physical properties:

    • Frequency* f
    • Wavelength* λ
    • Photon energy E which is directly proportional to the frequency of a wave

    * The relationship between frequency and wavelength is that they are inversely proportional to each other and valid over the whole of the electromagnetic spectrum.

    For convenience, EMR is divided into basic frequency bands which have their corresponding wavelength – the higher the frequency, the higher the energy. As wavelength is inversely proportional to frequency, longer wavelengths have lower energy and vice versa.

    Spectrum 1 – Frequency groups
    Spectrum 1 – Frequency groups

    EEP shielding work is primarily concerned with the first two and the last two bands.

  • The impact of Electromagnetic Radiation (EMR)

    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
    • Intensity
    • Total exposure time
    Spectrum 2 – ionising and non-ionising radiation
    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.

    Radiation Shield
    Fig 1.
    Visualisations of a photon

    Note: A magnetic field hazard is designated bysign or similar.

  • The Electromagnetic Spectrum

    This is a general statement to describe the entire frequency range A, B of electromagnetic energy C that can be radiated. This commences at just above zero D Hz and extends to infinity; all travelling at the same speed in space. For convenience, the spectrum is divided into different groupings, these are purely arbitrary and there is no specific defined point between one band and another.

    A Or wavelength, which is inversely proportional to frequency

    B This is generally considered to range from 2.4×1023 Hz* down to ~1 kHz, the local plasma frequency of the ionized interstellar medium

    C All electromagnetic radiation is governed by the same laws of physics which apply across the whole of the spectrum

    D In electrical terms, zero equates to 'direct current' such as that used in MRI scanner magnet coils

    * Producing gamma rays at 1 GeV (109 ) at one end down to ultra low energy radio waves in the 10-15 eV (femto, quadrillionth), region.