Permanent magnets, but also magnets generated by coils, generate strong static magnetic fields, which can amount to several teslas in the core. NMR magnets belong to this category, among others.
NMR spectroscopy is a method for investigating the electronic environment of individual atoms and their interactions with neighbouring atoms. The sample is placed in a strong, static magnetic field caused by a current-carrying coil.
To reduce the strong magnetic fields of such DC magnets, stray field shields with high saturation must be used. This limits areas around the magnets to <5 Gauss (500µT), which is the prescribed limit for electrical implants such as pacemakers. However, the application is not only limited to adhering to the 5 Gauss line, but also depends on the customer’s specifications. In the field of biological research, for example, values in the order of magnitude of the earth’s magnetic field, i.e. 50µT, are often required so that biological experiments can be carried out in a “natural” environment.
With stray field shielding, the fields of strong magnets, caused by unshielded and shielded NMRs, are effectively reduced. Pure iron, which has a high saturation flux density, is usually used as shielding material for strong magnetic DC fields.
The thickness of the material and the required shielding area is determined by the Finite element method (FEM) simulations. Compared to alternating field shielding, DC shielding is typically much thicker and correspondingly heavier.
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