[show abstract][hide abstract] ABSTRACT: Here we report on a mathematical description for the neutron dark-field image (DFI) contrast based on the influence of the thickness-dependent beam broadening caused by scattering interactions and multiple refraction in the sample. We conduct radiography experiments to verify that the DFI signal exponentially decays as a function of thickness for both magnetic and nonmagnetic materials. Here we introduce a material-dependent parameter, the so-called linear diffusion coefficient Ω. This allows us to perform a quantitative DFI-computed tomography. Additionally, we conduct correlative small-angle neutron-scattering experiments and validate the mathematical assumption that the angular broadening of the direct beam is proportional to the square root of the number of discrete layers.
[show abstract][hide abstract] ABSTRACT: We report on the observation of dendritic flux avalanches in a large Niobium
single crystal. In contrast to avalanches observed in thin films, they appear
only in a very narrow temperature interval of about a tenth of a Kelvin near
the critical temperature of Nb. At a fixed temperature, we find two sets of
dendritic structures, which differ by the magnetic field required for their
formation and by the maximum distance the dendrites penetrate into the sample.
The effect is caused by dendritic flux penetration into thin superconducting
surface layers formed in the single crystal close to the critical temperature.
[show abstract][hide abstract] ABSTRACT: Neutron imaging is a non-invasive method for material research on the macroscopic level. It is carried out at laboratories equipped with powerful neutron sources, suitable neutron beam lines and neutron detection systems. Decades ago neutron radiography began capturing images with film techniques. These techniques yielded excellent spatial resolution even over large fields of view. In the recent years, improvements in the detection techniques and their digitization have been the main forces driving successes in neutron imaging. Several detector options have been developed, implemented and used in practical applications in order to achieve digital information from the neutron transmission process which is needed for a quantitative evaluation of image data by sophisticated methods like neutron tomography, phase contrast imaging, neutron interferometry and time dependent studies. The most common approach in digital neutron imaging is a conversion of the neutron field information into visible light by a scintillation process, where a neutron converter is needed because neutrons do not excite directly due to their neutral charge. Low level light signals can be observed either with sensitive camera systems or by using amorphous silicon based semiconductor plate devices. However, these now established detection techniques are still limited in respect to spatial and time resolution. The best possible spatial resolution which can be achieved today is available by a system built at PSI with about 10 μm pixel size. Recently, it was upgraded with a tilted option for an increased resolution by a factor of 4 in one direction. Scintillator based techniques are limited by the dissipation of the secondary particles. This limitation has motivated the search for new detector options. One approach is a pixilated system where the readout per incoming neutron can be used to calculate precisely the position of its impact. Such devices are realized as the TIMEPIX system already. The system was tested successfully at PSI and other neutron imaging facilities. Other options might be to use MEDIPIX devices with neutron absorbing/converting materials. A similar pixel detector, EIGER, has been developed recently at PSI and its performance in the field of neutron imaging is under investigation. For future applications at the upcoming pulsed spallation sources the time = energy resolving aspect becomes even more important. This will push the future use of pixilated systems. This article intends to describe the present state-of-the-art even if most of the presented results are obtained at PSI's leading facilities, sometimes in collaboration with our partners.
Journal of Instrumentation 01/2011; 6(01):C01050. · 1.66 Impact Factor
[show abstract][hide abstract] ABSTRACT: The beamline for Imaging with COld Neutrons (ICON) at Swiss spallation neutron source (SINQ) at Paul Scherrer Institut has a flexible design to meet the requests from a wide user community. The current status of the beamline and its characteristics are described. The instrumentation includes three experimental positions from which two are equipped with digital camera based imaging detectors. Tomographic imaging is among the standard methods available at the beamline. Advanced methods such as energy-selective imaging and grating interferometry are available as instrument add-ons which are easily installed.
Nuclear Instruments and Methods in Physics Research Section A Accelerators Spectrometers Detectors and Associated Equipment 01/2011; 659(1):387-393. · 1.14 Impact Factor
[show abstract][hide abstract] ABSTRACT: Magnetic domains have been the subject of much scientific investigation since their theoretical existence was first postulated by P.-E. Weiss over a century ago. Up to now, the three-dimensional (3D) domain structure of bulk magnets has never been observed owing to the lack of appropriate experimental methods. Domain analysis in bulk matter thus remains one of the most challenging tasks in research on magnetic materials. All current domain observation methods are limited to studying surface domains or thin magnetic films. As the properties of magnetic materials are strongly affected by their domain structure, the development of a technique capable of investigating the shape, size and distribution of individual domains in three dimensions is of great importance. Here, we show that the novel technique of Talbot-Lau neutron tomography with inverted geometry enables direct imaging of the 3D network of magnetic domains within the bulk of FeSi crystals.
[show abstract][hide abstract] ABSTRACT: We report how a grating interferometer yields neutron dark-field scatter images for tomographic investigations. The image contrast is based on ultrasmall-angle scattering. It provides otherwise inaccessible spatially resolved information about the distribution of micrometer and submicrometer sized structural formations. Three complementary sets of tomographic data corresponding to attenuation, differential phase, and small-angle scattering can be obtained from one measurement. The method is compatible with conventional imaging and provides significantly higher efficiency than existing techniques.
[show abstract][hide abstract] ABSTRACT: Here we introduce a novel neutron imaging method, which is based on the effect that the spatial coherence of the neutron wave front can be changed through small-angle scattering of neutrons at magnetic domain walls in the specimen. We show that the technique can be used to visualize internal bulk magnetic domain structures that are difficult to access by other techniques. The method is transferable to a wide variety of specimens, extendable to three dimensions, and well suited for investigating materials under the influence of external parameters, as, e.g., external magnetic field, temperature, or pressure.
[show abstract][hide abstract] ABSTRACT: Imaging with visible light today uses numerous contrast mechanisms, including bright- and dark-field contrast, phase-contrast schemes and confocal and fluorescence-based methods. X-ray imaging, on the other hand, has only recently seen the development of an analogous variety of contrast modalities. Although X-ray phase-contrast imaging could successfully be implemented at a relatively early stage with several techniques, dark-field imaging, or more generally scattering-based imaging, with hard X-rays and good signal-to-noise ratio, in practice still remains a challenging task even at highly brilliant synchrotron sources. In this letter, we report a new approach on the basis of a grating interferometer that can efficiently yield dark-field scatter images of high quality, even with conventional X-ray tube sources. Because the image contrast is formed through the mechanism of small-angle scattering, it provides complementary and otherwise inaccessible structural information about the specimen at the micrometre and submicrometre length scale. Our approach is fully compatible with conventional transmission radiography and a recently developed hard-X-ray phase-contrast imaging scheme. Applications to X-ray medical imaging, industrial non-destructive testing and security screening are discussed.
Nature Material 03/2008; 7(2):134-7. · 35.75 Impact Factor
[show abstract][hide abstract] ABSTRACT: We report on how a neutron grating interferometer can yield projection images of the internal domain structure in bulk ferromagnetic samples. The image contrast relies on the ultrasmall angle scattering of unpolarized neutrons at domain wall structures in the specimen. The results show the basic domains of (110)-oriented sheets in an FeSi test sample. The obtained domain structures could be correlated with surface sensitive magneto-optical Kerr effect micrographs.
[show abstract][hide abstract] ABSTRACT: We report on the demonstration experiment of the multiple small angle neutron scattering (MSANS) technique at a 5.6 m long neutron beam line, leading to a q resolution of 3×10−4 Å−1. The MSANS technique is based on two two-dimensional multihole apertures placed at the front end of the collimator and close to the sample, respectively. By choosing the proper MSANS geometry, individual diffraction patterns are superimposed leading to a large gain in intensity. Using MSANS as an option for standard small angle neutron scattering beam lines, the q resolution could be increased to 10−5 Å−1 without dramatically sacrificing intensity.
[show abstract][hide abstract] ABSTRACT: We report on the fabrication and application of a novel neutron imaging test device made of gadolinium. It is designed for a real time evaluation of the spatial resolution, resolution direction, and distortions of a neutron imaging detector system. Measurements of the spatial resolution of (6)LiF doped ZnS scintillator screens with different thicknesses and of imaging plates were performed. The obtained results are in good agreement with comparison measurements using the standard knife edge detection method.
Review of Scientific Instruments 06/2007; 78(5):053708. · 1.60 Impact Factor
[show abstract][hide abstract] ABSTRACT: In x-ray radiography, particularly for technical and industrial applications, a scanning setup is very often favorable when compared to a direct two-dimensional image acquisition. Here, we report on an efficient scanning method for grating based x-ray phase contrast imaging with tube based sources. It uses multiple line detectors for staggered acquisition of the individual phase-stepping images. We find that the total exposure time does not exceed the time needed in an equivalent scanning setup for absorption radiography. Therefore, we conclude that it should be possible to implement the method into a scanning system without affecting the scanning speed or significant increase in cost but with the advantage of providing both the phase contrast and the absorption information at once.
Review of Scientific Instruments 05/2007; 78(4):043710. · 1.60 Impact Factor
[show abstract][hide abstract] ABSTRACT: We have developed a method for X-ray phase contrast imaging, which is based on a grating interferometer. The technique is capable of recording the phase shift of hard X-rays travelling through a sample, which greatly enhances the contrast of low absorbing specimen compared to conventional amplitude contrast images. Unlike other existing X-ray phase contrast imaging methods, the grating interferometer also works with incoherent radiation from a standard X-ray tube. The key components are three gratings with silicon and gold structures, which have dimensions in the micrometer range and high aspect ratios. The fabrication processes, which involve photolithography, anisotropic wet etching, and electroplating, are described in this article for each of the three gratings. An example of an X-ray phase contrast image acquired with the grating interferometer is given.
[show abstract][hide abstract] ABSTRACT: An interferometric method to record quantitative X-ray phase contrast images has been developed that can be used at polychromatic and incoherent X-ray sources such as laboratory tubes. With respect to previously presented results, in this work we report on recent developments and results that have been achieved in view of potential future applications such as in medicine or biology. In particular, due to improvements in the fabrication process large area diffraction gratings with high aspect ratio were achieved. Thereby, the field of view of the interterometer has been drastically increased to 64 x 64 mm(2) and the design value of the photon energy for the gratings could be increased up to 28 keV. Moreover, the use of a Medipix2 single photon-counting pixel detector shows a considerable improvement in image quality and sensitivity over the integrating detector used so far.
physica status solidi (a) 01/2007; 204(8):2728. · 1.21 Impact Factor
[show abstract][hide abstract] ABSTRACT: We report how a setup consisting of three gratings yields quantitative two- and three-dimensional images depicting the quantum-mechanical phase shifts of neutron de Broglie wave packets induced by the influence of macroscopic objects. Since our approach requires only a little spatial and chromatic coherence it provides a more than 2 orders of magnitude higher efficiency than existing techniques. This dramatically reduces the required measurement time for computed phase tomography and opens up the way for three-dimensional investigations of previously inaccessible quantum-mechanical phase interactions of neutrons with matter.