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Results of experimental investigations of a neutron resonances width in planar waveguides using the time-of-flight reflectometer REMUR of the IBR-2 pulsed reactor are reported and comparison with theoretical calculations is presented. The intensity of the neutron microbeam emitted from the waveguide edge was registered as a function of the neutron wavelength and the incident beam angular divergence. The possible applications of this method for the investigations of layered nanostructures are discussed.
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Peculiarities of the formation of a neutron enhanced standing wave in the structure with a thin highly absorbing layer of gadolinium are considered in the article. An analogue of the poisoning effect well known in reactor physics was found. The effect is stronger for the Nb/Gd/Nb system. Despite of this effect, for a Nb/Gd bilayer and a Nb/Gd/Nb trilayer placed between Al2O3 substrate and Cu layer, it is shown theoretically and experimentally that one order of magnitude enhancement of neutron density is possible in the vicinity of the Gd layer. This enhancement makes it possible to study domain formation in the Gd layer under transition of the Nb layer(s) into the superconducting state (cryptoferromagnetic phase).
Neutron planar waveguides are focusing devices generating a narrow neutron beam of submicron width. Such a neutron microbeam can be used for the investigation of local microstructures with high spatial resolution. The essential parameter of the microbeam is its angular width. The main contribution to the microbeam angular divergence is Fraunhofer diffraction on a narrow slit. We review and discuss various ways to characterize the angular divergence of the neutron microbeam using time-of-flight and fixed wavelength reflectometers.
We investigate neutron propagation in a middle layer of a planar waveguide which is a tri-layer thin film. A narrow divergent microbeam emitted from the end face of the film is registered. The neutron channeling length is experimentally measured as a function of the guiding channel width. Experimental results are compared with calculations.
Far-ultraviolet (FUV) scintillation signals have been measured in heavy noble gases (argon, krypton, xenon) following boron-neutron capture ($^{10}$B($n,alpha$)$^7$Li) in $^{10}$B thin films. The observed scintillation yields are comparable to the yields from some liquid and solid neutron scintillators. At noble gas pressures of 107 kPa, the number of photons produced per neutron absorbed following irradiation of a 1200 nm thick $^{10}$B film was 14,000 for xenon, 11,000 for krypton, and 6000 for argon. The absolute scintillation yields from the experimental configuration were calculated using data from (1) experimental irradiations, (2) thin-film characterizations, (3) photomultiplier tube calibrations, and (4) photon collection modeling. Both the boron films and the photomultiplier tube were characterized at the National Institute of Standards and Technology. Monte Carlo modeling of the reaction cell provided estimates of the photon collection efficiency and the transport behavior of $^{10}$B($n,alpha$)$^7$Li reaction products escaping the thin films. Scintillation yields increased with gas pressure due to increased ionization and excitation densities of the gases from the $^{10}$B($n,alpha$)$^7$Li reaction products, increased frequency of three-body, excimer-forming collisions, and reduced photon emission volumes (i.e., larger solid angle) at higher pressures. Yields decreased for thicker $^{10}$B thin films due to higher average energy loss of the $^{10}$B($n,alpha$)$^7$Li reaction products escaping the films. The relative standard uncertainties in the measurements were determined to lie between 14 % and 16 %. The observed scintillation signal demonstrates that noble gas excimer scintillation is promising for use in practical neutron detectors.
Several recent methods for tomographic reconstruction of stress and strain fields from Bragg-edge neutron strain images have been proposed in the literature. This paper presents an extension of a previously demonstrated approach based on Gaussian Process regression which enforces equilibrium in the method. This extension incorporates knowledge of boundary conditions, primarily boundary tractions, into the reconstruction process. This is shown to increase the rate of convergence and is more tolerant of systematic errors that may be present in experimental measurements. An exact expression for a central calculation in this method is also provided which avoids the need for the approximation scheme that was previously used. Convergence of this method for simulated data is compared to existing approaches and a reconstruction from experimental data is provided. Validation of the results to conventional constant wavelength strain measurements and comparison to prior methods shows a significant improvement.
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