Wolfgang Treimer

Quasi–in situ neutron tomography on polymer electrolyte membrane fuel cell stacks

Quasi–in situ neutron tomography is applied to polymer electrolyte membrane fuel cell stacks for a cell-by-cell detection of liquid water agglomerates. Water distributions in the corresponding anodic and cathodic flow fields are analyzed separately. The influence of the membrane thickness as well as effects of the electro-osmotic drag and of back-diffusion from the cathode to the anode on the water distribution are investigated. Furthermore, the well-known engineering problem of the anomalous behavior of the outermost cells in long multistacks is addressed. The suitability of neutron tomography to support the development of fuel cells is shown.

Wavelength tunable device for neutron radiography and tomography

A special double monochromator system was tested for a conventional operating tomography setup in order to use a broad wavelength band of monochromatic neutrons for radiography and tomography. Scanning through the wavelength region of Bragg edges, it is possible to make series of radiographs and tomographs at different wavelengths from 2.0 until 6.5A. So no beam hardening influences the measurements and is not to be corrected. With this instrument for cold neutron radiography and tomography, energy selecting quantitative radiography, stress and strain mapping, and phase radiography were performed.

X-ray and neutron imaging – Complementary techniques for materials science and engineering

Abstract Using X-ray and neutron radiography and tomography, images of material and component inhomogeneities and their development with time can be obtained. Due to their non-destructiveness and non-invasive nature both methods give insight into the function of devices and their decay processes. Fundamentals of X-ray and neutron radiography and tomography are briefly outlined, examples for both techniques are given, their complementarities are highlighted and emerging techniques and frontiers are discussed.

Refraction as imaging signal for computerized (neutron) tomography

Refraction of thermal neutrons was used as imaging signal for computerized neutron tomography. Using a special instrument, attenuation and refraction could be observed simultaneously, extracted and separated from each other, and two-dimensional (2D) images reconstructed from pure refraction data. Even in the case of no attenuation contrast, perfect 2D images of objects could be reconstructed using the refraction data only.

Magnetic field induced differential neutron phase contrast imaging

Besides the attenuation of a neutron beam penetrating an object, induced phase changes have been utilized to provide contrast in neutron and x-ray imaging. In analogy to differential phase contrast imaging of bulk samples, the refraction of neutrons by magnetic fields yields image contrast. Here, it will be reported how double crystal setups can provide quantitative tomographic images of magnetic fields. The use of magnetic air prisms adequate to split the neutron spin states enables a distinction of field induced phase shifts and these introduced by interaction with matter.

Small angle scattering signals for (neutron) computerized tomography

Small angle neutron scattering is a well-established tool for the determination of microscopic structures in various materials. With the ultrasmall angle neutron scattering technique (USANS), structures with sizes of approximately 50nm to 50μm can be resolved by a double crystal diffractometer (DCD). USANS signals recorded with a special DCD were used for tomographic purposes investigating the macroscopic structure of a sample with a maximum resolution of 200μm. Thereby, macroscopic regions within the sample with different ultrasmall angle scattering properties, i.e., with different microscopic structures, could be imaged by the means of tomographic reconstruction from projections (on a macroscopic scale).

Observation of partial Meissner effect and flux pinning in superconducting lead containing non-superconducting parts

Partially suppressed Meissner effect and magnetic flux trapping in the bulk of poly-crystalline Pb samples with virtually zero demagnetization and large (cm3) volume containing non-superconducting parts could be visualized by polarized neutrons and quantified with respect to the shape and amount of the trapped flux in the intermediate state of lead. These measurements provided a unique look at the macroscopic Quantum mechanics effects and the coexistence of superconducting state, intermediate state, and normal conducting parts in the same sample, respectively.

A polarised SUSANS facility to study magnetic systems

Using a right-angled magnetic air prism, we have achieved a separation of ∼10 arcsec between ∼2 arcsec wide up and down-spin peaks of 5.4 Å neutrons. The polarised neutron option has thus been introduced into the SUSANS instrument. Strongly spindependent SUSANS spectra have been observed over ±1.3 × 10−4 Å−1 range for several magnetic alloy samples. Spatial pair-distribution functions for the up and down-spins as well as the nuclear and magnetic scattering length density distributions in the micrometer domain, have been deduced from these spectra.

Setup for polarized neutron imaging using in situ 3He cells at the Oak Ridge National Laboratory High Flux Isotope Reactor CG-1D beamline

In the present study, we report a new setup for polarized neutron imaging at the ORNL High Flux Isotope Reactor CG-1D beamline using an in situ 3He polarizer and analyzer. This development is very important for extending the capabilities of the imaging instrument at ORNL providing a polarized beam with a large field-of-view, which can be further used in combination with optical devices like Wolter optics, focusing guides, or other lenses for the development of microscope arrangement. Such a setup can be of advantage for the existing and future imaging beamlines at the pulsed neutron sources. The first proof-of-concept experiment is performed to study the ferromagnetic phase transition in the Fe3Pt sample. We also demonstrate that the polychromatic neutron beam in combination with in situ 3He cells can be used as the initial step for the rapid measurement and qualitative analysis of radiographs.

First Sub-arcsecond Collimation of Monochromatic Neutrons

We have achieved the tightest collimation to date of a monochromatic neutron beam by diffracting neutrons from a Bragg prism, viz. a single crystal prism operating in the vicinity of Bragg incidence. An optimised silicon {111} Bragg prism has collimated 5.26A neutrons down to 0.58 arcsecond. In conjunction with a similarly optimised Bragg prism analyser of opposite asymmetry, this ultra-parallel beam yielded a 0.62 arcsecond wide rocking curve. This beam has produced the first SUSANS spectrum in Q ~ 10−6 A−1 range with a hydroxyapatite casein protein sample and demonstrated the instrument capability of characterising agglomerates upto 150 μm in size. The super-collimation has also enabled recording of the first neutron diffraction pattern from a macroscopic grating of 200 μm period. An analysis of this pattern yielded the beam transverse coherence length of 175 μm (FWHM), the greatest achieved to date for A wavelength neutrons.