Description of the line shape of Bragg peak in neutron powder diffractometers taking into account spatial effects

Based on a universal mathematically correct approach previously developed, the compact analytical expression for the line shape of Bragg reflexes taking into account spatial effects has been obtained and a geometrical interpretation is given.     

Determination of the characteristics of a double monochromator in neutron powder diffractometers with allowance for spatial effects

Analytical expressions for the characteristics of double monochromators have been derived on the basis of a universal mathematically correct approach developed previously. These expressions are combined with simple geometric diagrams added to explain the features of instrument operation.     

Improved resolution and powder neutron diffraction research on the KSN-2 diffractometer

The diffractometer KSN-2 placed at the 10 MW reactor VVRS-M in e near Prague provides the possibility for powder structure analysis of silicates. The characteristic parameters of the device are given.     

A stopping layer concept to improve the spatial resolution of gas-electron-multiplier neutron detector

In recent years, gas electron multiplier (GEM) neutron detectors have been developing towards high spatial resolution and high dynamic counting range. We propose a novel concept of an Al stopping layer to enable the detector to achieve sub-millimeter (sub-mm) spatial resolution. The neutron conversion layer is coated with the Al stopping layer to limit the emission angle of ions into the drift region. The short track projection of ions is obtained on the signal readout board, and the detector would get good spatial resolution. The spatial resolutions of the GEM neutron detector with the Al stopping layer are simulated and optimized based on Geant4GarfieldInterface. The spatial resolution of the detector is 0.76 mm and the thermal neutron detection efficiency is about 0.01% when the Al stopping layer is 3.0 μ m thick, the drift region is 2 mm thick, the strip pitch is 600 μ m, and the digital readout is employed. Thus, the GEM neutron detector with a simple detector structure and a fast readout mode is developed to obtain a high spatial resolution and high dynamic counting range. It could be used for the direct measurement of a high-flux neutron beam, such as Bragg transmission imaging, very small-angle scattering neutron detection and neutron beam diagnostic.     

Analytic description of optical bistability including spatial effects

Steady-state features for both absorptive and dispersive bistability are obtained analytically with the inclusion of spatial effects. Both Fabry-Perot and ring-cavity geometries are treated using an analytic integration of coupled field equations for counter-propagating waves in a homogeneously broadened two-level medium. The mean-field limit is rigorously introduced starting from exact solutions. Mean-field state equations for both Fabry-Perot and ring cavities are obtained. In the former case explicit comparisons are made with results arising in the approximate treatment of standing waves.     

Resolution function of neutron diffractometers with focusing monochromators: I. General equations

A general rigorous method is developed for determining resolution functions of powder neutron diffractometers. In this method, unlike the conventional approach, spatial effects are taken into account. The derived equations are the basis for the determination of resolution functions of instruments equipped with vertical and horizontal focusing monochromators. The integration of accessories into the instrument design is considered.     

Resolution function of neutron diffractometers with focusing monochromators: II. Vertical focusing

Based on the general approach proposed previously, the resolution function and the Bragg peak parameters are determined in an instrument with a vertical focusing monochromator. The consideration takes into account a wide range of instrumental parameters (various collimations, mosaicity, and scanning trajectories). Some issues of instrument tuning optimization are considered.     

Resolution function of neutron diffractometers with focusing monochromators: III. Horizontal focusing

The resolution functions of a neutron powder diffractometer with horizontal focusing monochromator and Bragg peak parameters are calculated on the basis of a previously proposed universal approach. The calculations take into account a wide set of instrumental parameters (various collimators, mosaicity, scanning trajectories).     

The angular and spatial distributions of the thermal neutron source description of the THOR BNCT beam

This paper presents a way to determine the angular and spatial distributions of the thermal neutron source strength of a boron neutron capture therapy (BNCT) beam. The experiments applied 1) the indirect neutron radiography, 2) the cadmium difference method, and 3) the instrumental neutron activation analysis. The measured data were processed by the spectrum deconvolution technique to resolve into a proper set of angular and spatial distributions. This paper took the epithermal neutron beam of the BNCT facility at the Tsing Hua Open-pool Reactor as an example.     

中子半影成像的闪烁体阵列空间分辨力模拟

为了比较3种闪烁体探测器的性能,利用蒙特卡罗方法计算了不同光纤直径的闪烁体阵列的空间分辨和能量沉积。模拟结果表明:相同光纤直径的普通液体闪烁体阵列空间分辨力比塑料闪烁体阵列提高了约0.1 mm,而氘代液体闪烁体阵列空间分辨力约为普通液体闪烁体阵列的1/2;富氢闪烁体的能量沉积比氘代闪烁体高;闪烁体光纤直径越小,分辨力越好;闪烁体越厚,光产额越高。     

中子半影成像的闪烁体阵列空间分辨力模拟

为了比较3种闪烁体探测器的性能,利用蒙特卡罗方法计算了不同光纤直径的闪烁体阵列的空间分辨和能量沉积。模拟结果表明:相同光纤直径的普通液体闪烁体阵列空间分辨力比塑料闪烁体阵列提高了约0.1 mm,而氘代液体闪烁体阵列空间分辨力约为普通液体闪烁体阵列的1/2;富氢闪烁体的能量沉积比氘代闪烁体高;闪烁体光纤直径越小,分辨力越好;闪烁体越厚,光产额越高。     

Self-consistent description of the inner crust of a neutron star with allowance for superfluidity effects

Within the Wigner-Seitz approximation, a self-consistent fully quantum-mechanical calculation of the structure of the inner crust of a neutron star is performed over a wide range of densities with allowance for superfluidity effects. Within the approach used, the Wigner-Seitz cell consists of a nuclear-like cluster surrounded by a nearly uniform neutron gas. An effective energy functional is constructed by matching, at the cluster surface, the realistic phenomenological nuclear functional for the cluster due to S. A. Fayans and his coauthors and the energy functional calculated microscopically for neutron matter. The microscopic component of the functional is calculated within the Brueckner method by using the v18 Argonne interaction.     

Heat capacity anomaly in UO2 in the vicinity of 1300 K: an improved description based on high resolution X-ray and neutron powder diffraction studies

X-ray and neutron powder diffraction studies of UO₂ were performed under controlled oxygen partial pressure between room temperature and 1673 K. More than 40 neutron diffraction patterns were recorded. The thermal expansion coefficient of UO₂ and the temperature dependence of Debye-Waller factors for oxygen and uranium atoms were determined. The dependence of Debye-Waller factors as a function of temperature is linear and the thermal expansion coefficient follows the classical Debye regime within the temperature range 300-1000 K. Above 1200 K, a departure from this quasi-harmonic behavior is clearly observed. Both an abnormal increase of the thermal expansion and of the oxygen sublattice disorder are evidenced. The departure of the lattice parameter from a linear thermal variation is found to be thermally activated with an effective activation energy close to 1 eV, very similar to the activation energy already found for the electrical conductivity. This new result suggests that polarons may affect the mean lattice parameter. A new thermodynamic model is then proposed to explain the heat capacity thermal variation by only three contributions: harmonic phonons, thermal expansion and polarons.     

Inclusion of the causality principle in the dynamical description of nonequilibrium systems

In the nonequilibrium-statistical-operator (NSO) method a correction to the Hamiltonian of a system is found, corresponding to a source in the Liouville equation, which destroys the invariance of this equation with respect to time reversal. Analysis of this correction enables one to conclude that the forces leading to the irreversible behavior of a system are of an essentially nonpotential nature. Inclusion of the causality principle in the Hamiltonian of a system also makes it possible to give a new representation of the NSO.