Three-dimensional imaging of magnetic fields with polarized neutrons

Neutrons are highly sensitive to magnetic fields owing to their magnetic moment, whereas their charge neutrality enables them to penetrate even massive samples. The combination of these properties with radiographic and tomographic imaging enables a technique that is unique for investigations of macroscopic magnetic phenomena inside solid materials. Here, we introduce a new experimental method yielding two- and three-dimensional images that represent changes of the quantum-mechanical spin state of neutrons caused by magnetic fields in and around bulk objects. It opens up a way to the detection and imaging of previously inaccessible magnetic field distributions, hence closing the gap between high-resolution two-dimensional techniques for surface magnetism and scattering techniques for the investigation of bulk magnetism. The technique was used to investigate quantum effects inside a massive sample of lead (a type-I superconductor).