Abstract
Precession of neutron spin in a magnetic field can be used for mapping of a magnetic field distribution, as demonstrated previously for static magnetic fields at neutron beamline facilities. The fringing in the observed neutron images depends on both the magnetic field strength and the neutron energy. In this paper we demonstrate the feasibility of imaging periodic dynamic magnetic fields using a spin-polarized cold neutron beam. Our position-sensitive neutron counting detector, providing with high precision both the arrival time and position for each detected neutron, enables simultaneous imaging of multiple phases of a periodic dynamic process with microsecond timing resolution. The magnetic fields produced by 5- and 15-loop solenoid coils of 1 cm diameter, are imaged in our experiments with ∼100 μm resolution for both dc and 3 kHz ac currents. Our measurements agree well with theoretical predictions of fringe patterns formed by neutron spin precession. We also discuss the wavelength dependence and magnetic field quantification options using a pulsed neutron beamline. The ability to remotely map dynamic magnetic fields combined with the unique capability of neutrons to penetrate various materials (e.g., metals), enables studies of fast periodically changing magnetic processes, such as formation of magnetic domains within metals due to the presence of ac magnetic fields.
Highlights
Neutrons have zero net electrical charge and can penetrate deeply into matter, but their intrinsic magnetic moment makes them highly sensitive to magnetic fields
It can be shown that this polarization vector behaves exactly like a classical magnetic moment [24]
Experiments were performed on the cold neutron radiography and tomography station (CONRAD) at the Helmholtz Centre Berlin for Materials and Energy
Summary
Experiments were performed on the cold neutron radiography and tomography station (CONRAD) at the Helmholtz Centre Berlin for Materials and Energy (figure 2). The maximum (minimum) intensity will be measured when the beam polarization and the analyzer are aligned perfectly parallel (anti-parallel). From (7), the total precession angle (though not the rate of precession) is dependent on the neutron wavelength and it is necessary to use a monochromatic beam; a polychromatic beam would dephase upon precession, resulting in a loss of polarization (figure 3). The adjustable positioning and angling of the PGCs ensured that the beam propagation direction was unperturbed and allowed the selection of wavelengths in the range 2.0–6.0 Å ( λ/λ =0.12). The wavelength selection decreases the beam intensity to ∼1% of that of the polychromatic beam, which still allows single radiographs to be recorded within several minutes
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