Abstract
Neutrons have zero net electrical charge and can thus penetrate deeply into matter, but their intrinsic magnetic moment makes them highly sensitive to magnetic fields. These properties have been combined with radiographic (2D) and tomographic (3D) imaging methods to provide a unique technique to probe macroscopic magnetic phenomena both within and around bulk matter. Based on the spin-rotation of a polarized neutron beam as it passes through a magnetic field, this method allows the direct, real-space visualization of magnetic field distributions. It has been used to investigate the Meissner effect in a type I (Pb) and a type II (YBCO) superconductor, flux trapping in a type I (Pb) superconductor, and the electromagnetic field associated with a direct current flowing in a solenoid. The latter results have been compared to predictions calculated using the Biot–Savart law and have been found to agree well.
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GENERAL SCIENTIFIC SUMMARY Introduction and background. The use of neutron imaging to investigate the spatial distribution of matter based on chemical content and material density is widespread in both science and industry. Combining those methods with measurements of the nuclear spin-rotation of a polarized neutron beam as it traverses a magnetic field, a unique technique has been developed for the direct, real-space visualization of magnetic field distributions both in free space and within the bulk of massive objects.
Main results. The feasibility and potential of this method have been verified through investigations of a variety of macroscopic magnetic phenomena, including flux expulsion (the so-called Meissner effect) and flux trapping in superconductors. Images of the electromagnetic field associated with a direct current flowing in a solenoid have been compared with theoretical predictions and have been found to agree well.
Wider implications. Spin-polarized neutron imaging is easily adaptable to different experimental environments and could find a wide range of applications. To understand high temperature superconductivity, for example, it is vital to understand how magnetic fields are established and distributed within materials. Further possible fields of investigation include magnetoelastic and magnetostrictive stress and strain; current densities in conductors; and magnetic domain distributions in crystals.
Figure. A spin-polarized neutron radiograph showing a dipole magnet. The magnetic field decreases in strength with distance from the magnet, resulting in a series of maxima (minima) in intensity, caused by spin rotations of 2nπ (nπ).