Various cutting-edge experiments can be adversely affected by magnetic fields and may require shielding from electromagnetic fields with a magnetically shielded room (MSR). Tiny magnetic impulses of the active nerves of the human body may be the focus of a biological experiment. These impulses can be detected but their signals may be swamped by background fields.
Electron microscopes can also suffer from external magnetic fields. Just as optical microscopes exploit photon wavelengths to illuminate a sample of interest, electron microscopes make use of the shorter wavelength of electrons in an electron beam to more highly resolve the sample. These highly sensitive imaging devices benefit from magnetic shielding.
Shielding Electron Microscopes
An electron moving in a magnetic field experiences a force tending to change its direction of motion unless that motion is parallel to the field. A beam of electrons can be focused using electromagnetic lenses. Optical lenses diffract photons travelling through them and converge the outgoing beam. The principle of an electromagnetic lens is similar; the beam of electrons can be focused by altering the path of the incoming electrons.
The basic design of an electromagnetic lens consists of a solenoid through which the beam can pass on its way towards the sample. Applying a current to the solenoid induces a magnetic field according to Ampere’s law which, since electrons are extremely sensitive to magnetic fields, deflects the electrons to a focused point. Both scanning electron microscopes (SEM) and transmission electron microscopes (TEM) rely on this process.
The resolving power of a microscope is inherently dependent on the wavelength of the electromagnetic radiation used to create the image. As the wavelength of the radiation is decreased, the resolving power is increased. Since electron wavelengths are approximately 100,000 times shorter than photon wavelengths, electron microscopes offer superior resolving power to optical microscopes.
While the precision of electron microscopes is of huge benefit, there are disadvantages of electron microscopy that require addressing. External magnetic fields disturb the electron beam and diminish the overall resolving power of the microscope. Housing the microscope in a magnetically shielded room ensures the microscope can achieve its upper limit of resolution.
Shielding Medical Imaging Devices
Magnetoencephalography (MEG) employs the use of incredibly sensitive magnetometers, such as SQUIDs (superconducting quantum interference devices) or OPMs (optically pumped magnetometer) to pick up on low level fields induced by the synchronized ionic neural currents within the brain.
MEG records data temporally so that a vast array of neurological brain processes can be effectively watched in real-time and researched; from mapping the brain’s responses to stimuli to investigating structural abnormalities within the brain. While MEG is valuable for a wide range of non-invasive neurological research, it is also extensively used in conjunction with other forms of imaging techniques such as positron emission tomography (PET) and electroencephalography (EEG).
The ambient Earth’s field is approximately 50μT; the field produced by the brain is of the order 10-9μT (or 1fT). It is clear to see that to achieve any sort of useful reading of these small fields, it is necessary to shield from the much greater ambient field.
We wish to remove the field signal which is of no interest; this can be done using an appropriately designed magnetic shield. Small shields are used commonly in numerable applications based upon the principle that a highly permeable material can divert field around the volume of interest, so that this volume is nearly or totally field free.
If we were to simply scale the shield to the dimensions of a room, with large planar surface areas we would see a reduction in the efficiency of the shield. However, the increasing demand for magnetically shielded rooms with high field attenuation has led to us to design optimal shielded rooms involving multiple µMetal layers and the introduction of copper or aluminium sheet metal to shield RF fields, allowing impressive shielding factors to be achieved.