Neutron Phase Imaging Facility (NPIF)
Problem: Neutron-imaging techniques in general have seen limited application because of poor attenuation contrast and the requirement of intense neutron sources. Poor attenuation contrast limits choice of samples that can be investigated and consequently neutron imaging has been limited primarily to highly absorbing or scattering samples like hydrogen. In important applications like imaging of fuel cell water transport, the limitations become pronounced and critically important studies of membrane assembly interface dynamics becomes very difficult and unreliable. In addition, there is an immense potential of direct, unambiguous and model independent (unlike SANS and Reflectometry) study of grain boundaries and interfaces, residual stress in metals, micro cracks due to fatigue, domains in magnetic materials, forensic investigations, non-destructive elemental characterization of biological tissues and mass density of thin polymers films, just to name a few, that can performed if a facility incorporating high resolution and quantitatively precise methods can be developed. Such technological and facility development is now required to augment and go beyond the current capabilities; in particular to assist in the rapid development of fuel cells and characterization of hydrogen storage materials and devices.
Neutron Phase Imaging Facility Solution: Using a monochromatic cold neutron beam (both above and below the Bragg cutoff), we plan to build world's first state of the art experimental facility solely dedicated to Neutron Phase Imaging. This facility will help overcome current performance limitations and operational constraints in conventional imaging, and allow investigation of material structures and properties that are invisible to attenuation based neutron or photon imaging techniques. Neutron Phase contrast imaging is a technique that records neutron intensity variation due to the phase modulation of neutrons. It was first demonstrated at NIST nearly a decade ago, is typically three orders of magnitude more sensitive than normal radiographs and is ideally suited for very precise quantitative neutron imaging measurements. Once the phase information is obtained the structural variation as well as grain boundaries, interfaces and edges are well characterized. This allows not only determining the structural variation within the sample but also allows the properties (magnetic and nuclear for example) of the sample to be determined. This technique provides complementary as well as overlapping structural information normally obtained through the SANS or Reflectivity measurements, but with much greater sensitivity and with quantification of the result that is model independent. We also plan to extend these measurements to 3D and obtain phase tomography of the specimens to allow selective analysis of different parts of the sample as is normally done in normal medical CT scans. Our goal is to fully develop, perfect and utilize a suite of phase imaging methods in this facility. Brief descriptions of these methods follow.
A. Imaging with Pinhole Geometry: As shown in figure below, a neutron beam is defined by a pinhole to provide transverse coherence. After passing through a sample the beam is recorded in two different positions. The recorded images in these two positions provide the intensity distribution and also the intensity derivative at the plane of interest. The phase modulation of the beam as caused by the sample is then mathematically extracted based on the retrieval process known as Transport Intensity Equation (TIE).
B. Imaging with a Coded Source: We also plan to apply the coded aperture technique to reduce the time taken to obtain an image using a single pinhole. Coded aperture is a superposition of images from multiple pinholes in a known matrix. Using a Fourier reconstruction process, the original image is then obtained with enhanced intensity and resolution.
C. Imaging with Gratings: The schematic below shows the experimental imaging setup. The gratings denoted by Gs, Gp, and Ga represent the source grating, phase grating, and analyzer grating, respectively. They are typically constructed from silicon and coated with neutron absorbing material. Each vertical slit in the source grating provides spatially coherent but mutually incoherent neutron beams to make the best use of neutrons while preserving the spatial coherence. The phase grating (Gp) shifts the neutron beam phase by 2π and an intensity pattern of the same period with the grating is formed at the plane of the analyzer grating. When a sample is introduced, due to refraction in the sample this intensity pattern is perturbed. The neutron absorbing lines on the analyzer grating have half the period of the phase grating and using a technique called 'phase stepping' that involves translating the analyzer grating with respect to the phase grating the phase modulation due to the sample is scanned and a resulting intensity pattern is recorded by a conventional detector.
For all these methods discussed above, current detector technology restricts the spatial resolution to about 20 mm. However, with emerging alternative detector technologies, micron to sub- micron-level spatial resolution can be expected in the future, particularly for small samples.
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