Martin Dawson

Advances in neutron radiography and tomography

Neutron imaging can provide two- or three-dimensional, spatially resolved images of the internal structure of bulk samples that are not accessible by other techniques, making it a unique tool with many potential applications. The method is now well established and is available at neutron sources worldwide. This review will give a survey of the technique of neutron imaging with a special focus on neutron tomography; the basics of the method as well as the technology of instrumentation will be outlined, and the techniques will be illustrated by representative applications. While the first part of the paper focuses on conventional attenuation contrast imaging, the second part reviews and critically assesses recent methodical developments.

Imaging with polarized neutrons

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.

Observation of Magnetic Domains in Insulation-Coated Electrical Steels by Neutron Dark-Field Imaging

High-resolution neutron dark-field imaging has been applied to study magnetic domains in sheets of commercial grain-oriented electrical steel enveloped in an insulating layer. The resulting images have been compared to those obtained using conventional magnetic powder patterns and a clear correlation has been found, with the neutron technique providing complementary information about the internal structures of the material. An understanding of the magnetic structure of these materials is important for improving the performance characteristics – particularly minimizing core loss – in the electrical devices in which they are used.

Visualization of embolism formation in the xylem of liana stems using neutron radiography

BACKGROUND AND AIMSnCold neutron radiography was applied to directly observe embolism in conduits of liana stems with the aim to evaluate the suitability of this method for studying embolism formation and repair. Potential advantages of this method are a principally non-invasive imaging approach with low energy dose compared with synchrotron X-ray radiation, a good spatial and temporal resolution, and the possibility to observe the entire volume of stem portions with a length of several centimetres at one time.nnnMETHODSnComplete and cut stems of Adenia lobata, Aristolochia macrophylla and Parthenocissus tricuspidata were radiographed at the neutron imaging facility CONRAD at the Helmholtz-Zentrum Berlin für Materialien und Energie, with each measurement cycle lasting several hours. Low attenuation gas spaces were separated from the high attenuation (water-containing) plant tissue using image processing.nnnKEY RESULTSnSevere cuts into the stem were necessary to induce embolism. The formation and temporal course of an embolism event could then be successfully observed in individual conduits. It was found that complete emptying of a vessel with a diameter of 100 µm required a time interval of 4 min. Furthermore, dehydration of the whole stem section could be monitored via decreasing attenuation of the neutrons.nnnCONCLUSIONSnThe results suggest that cold neutron radiography represents a useful tool for studying water relations in plant stems that has the potential to complement other non-invasive methods.

NEW VIEWS OF PLANT FOSSILS FROM ANTARCTICA: A COMPARISON OF X-RAY AND NEUTRON IMAGING TECHNIQUES

Abstract A fossil plant of Eocene age from Antarctica was studied using X-ray and neutron tomography to reveal the three-dimensional plant structures encased within carbonate nodules. The fossil was identified as a branch and leaves of an araucarian conifer, which grew on the volcanic highlands of the Antarctic Peninsula region approximately 50 million yr ago. Both X-ray and neutron imaging techniques successfully exposed the full three-dimensional structure of the fossil without destroying the original specimen, revealing that most of the fossil was present as voids in the concretion and little organic matter was present. However, neutron tomography was found to produce images with superior quality and detail.

Imaging of dynamic magnetic fields with spin-polarized neutron beams

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.

Visualisierung dreidimensionaler magnetischer Feldverteilungen mit spin-polarisierten Neutronen

Kurzfassung Röntgenstrahlen und Neutronen sind zueinander komplementäre Sonden, die sich in vielen Bereichen der Werkstoffforschung gegenseitig ergänzen. Neutronen zeichnen sich im Besonderen durch eine sehr hohe Sensitivität gegenüber Magnetfeldern aus. Vor kurzem konnte gezeigt werden, dass diese Eigenschaft auch für die Bildgebung genutzt werden kann. In Verbindung mit der teilweise hohen Eindringtiefe von Neutronen ist es dadurch möglich, magnetische Feldverteilungen im Inneren massiver Materialien (wie zum Beispiel in Metallen) zwei- und dreidimensional zu visualisieren und zu quantifizieren. Auch Stromdichteverteilungen in elektrischen Leitern können auf diese Weise indirekt über das Magnetfeld untersucht werden. Dies ermöglicht die Erschließung neuer Anwendungsbereiche in der Werkstoffforschung. Einige Beispiele hierfür werden in diesem Artikel vorgestellt.