[en] Commonly in neutron image experiments, the interpretation of the point spread function (PSF) is limited to describing the achievable spatial resolution in an image. In this article it is shown that for various PSF models, the resulting blurring due to the PSF affects the quantification of the neutron transmission of an object and that the effect is separate from the scattered neutron field from the sample. The effect is observed in several neutron imaging detector configurations using different neutron scintillators and light sensors. In the context of estimation of optical densities with an algorithm that assumes a parallel beam, the effect of blurring fractionates the neutron signal spatially and introduces an effective background that scales with the area of the detector illuminated by neutrons. Examples are provided that demonstrate that the illuminated field of view can alter the observed neutron transmission for nearly purely absorbing objects. It is found that by accurately modeling the PSF, image restoration methods can yield more accurate estimates of the neutron attenuation by an object

[en] During reactor operation, the neutron flux distribution is modified by the reactor control mechanisms and in the case of the reactor at the National Institute of Standards and Technology, this is determined by the angular position of the Cd shim arms and the vertical position of an Al regulating rod. The changing flux distribution results in a change in the optical axis of neutron beams, whose view is a fixed position within the reactor core. The changing optical axis results in two noticeable image artifacts: poor registration between images of a static object taken at different times and a change in the shape of the flat field intensity. These two effects were measured during the first four days of reactor operation. Both measurements show correlation with the reactor control mechanisms, with combined correlation coefficients during the first two days after reactor startup approaching 1. The change in the edge position is well below the image spatial resolution, and has more uncertainty associated with it. However, the change in the flat-field shape demonstrates a clear correlation with both shim arm angle and regulating rod position.

[en] An experimental investigation on a water filled 6-turn oscillating heat pipe (OHP) was conducted using neutron imaging. Using the neutron images, a method was developed to determine the volume fraction of liquid phase in different portions of the OHP while in operation. The results show that the volume fraction of liquid phase is highly nonuniform once the thermally excited fluid motion starts. Over the heat input range studied, during the steady state oscillating motion in an OHP, the volume fraction distribution of liquid phase was always less than 2.5% in the evaporator and greater than 80% in the condenser. Using this method, the startup behavior, pulsing and pause motions, circulation, and the relationship between the pulsing motion and circulation and temperature variations in the OHP were all determined as well. There is a notable synchronization between the entrance of liquid into the evaporator portion of the heat pipe and temperature oscillations. (authors)

[en] We investigate Co/Nb multilayers to explore the spontaneous π-phase shift between the superconducting (SC) layers, which is attributed for causing the non-monotonic change of the SC transition temperature (T _c) with the ferromagnetic (FM) layer thickness (t _FM) in several FM/SC multilayered systems. The issue of interfacial roughness is also explored by growing Co/Nb multilayers at various sputtering pressures. Transport measurements show a non-monotonic dependence of T _c on t _FM, and this dependence is insensitive to the structural variation present in the samples, as measured by X-ray scattering

[en] Dense samples (10-100 bar cm) of nuclear spin polarized ³He are utilized in high energy physics, neutron scattering, atomic physics, and magnetic resonance imaging. Metastability exchange optical pumping can rapidly produce high ³He polarizations (≅80%) at low pressures (few mbar). We describe a polarized ³He gas compressor system which accepts 0.26 bar l h^-1 of ³He gas polarized to 70% by a 4 W neodymium doped lanthanum magnesium hexaluminate (Nd:LMA) laser and compresses it into a 5 bar cm target with final polarization of 55%. The spin relaxation rates of the system's components have been measured using nuclear magnetic resonance and a model of the ³He polarization loss based on the measured relaxation rates and the gas flow is in agreement with a ³He polarization measurement using neutron transmission

[en] The predominant geometry for a neutron imaging experiment is that of a pinhole camera. This is primarily due to the difficulty in focusing neutrons due to the weak refractive index, which is also strongly chromatic. Proof of concept experiments demonstrated that neutron image forming lenses based on reflective Wolter mirrors can produce quantitative, high spatial resolution neutron images while also increasing the time resolution compared to the conventional pinhole camera geometry. Motivated by these results, we report the design of a neutron microscope where two Wolter mirrors replace condensing and objective lenses, in direct analogy with typical visible light microscopes. Ray tracing results indicate that this system will yield $3\mathrm{\mu }\mathrm{m}$ spatial resolution images with an acquisition time of order 4 faster than currently possible at this spatial resolution) with a field of view of about 5 mm in diameter.

[en] The design objective of the thermal neutron radiography facility at the National Institute of Standards and Technology (NIST) Center for Neutron Research was to provide a large beam diameter and a high fluence rate in order to produce images of dynamic systems. A thermal neutron beam with a 14 cm diameter thimble was chosen. The beam was initially filtered by a 10 cm thick single crystal bismuth filter cooled with liquid nitrogen. The beam exiting the port is shaped using either a 1 cm or 2 cm diameter pinhole to form a uniform high fluence rate beam at the sample. The resulting neutron beam at the sample has an L/D ratio of 280 with a fluence rate of 1.84x10⁷ cm^-2 s^-1 and 560 with a fluence rate of 4.75x10⁶ cm^-2 s^-1 uniformly spread over a 26 cm diameter beam. To capture the neutron beam image a scintillator and CCD camera is used. The current neutron camera system is limited to a 2.5 s frame rate; however, a high frame rate detector system based on amorphous silicon will allow frame rates to meet the design goal. Samples can be rotated and translated in situ for radiography and tomography applications. This facility became operational in early 2003. Since then the facility has been translated backwards by ∼2.13 m and 5 cm of bismuth was added to the filter. The design of this facility and the impact of the later changes are discussed