Thermal Neutron Scattering Cross Sections Research Articles

Modern Techniques for Inelastic Thermal Neutron Scattering Analysis

A predictive approach based on ab initio quantum mechanics and/or classical molecular dynamics simulations has been formulated to calculate the scattering law, S(κ⇀,ω), and the thermal neutron scattering cross sections of materials. In principle, these atomistic methods make it possible to generate the inelastic thermal neutron scattering cross sections of any material and to accurately reflect the physical conditions of the medium (i.e, temperature, pressure, etc.). In addition, the generated cross sections are free from assumptions such as the incoherent approximation of scattering theory and, in the case of solids, crystalline perfection. As a result, new and improved thermal neutron scattering data libraries have been generated for a variety of materials. Among these are materials used for reactor moderators and reflectors such as reactor-grade graphite and beryllium (including the coherent inelastic scattering component), silicon carbide, cold neutron media such as solid methane, and neutron beam filters such as sapphire and bismuth. Consequently, it is anticipated that the above approach will play a major role in providing the nuclear science and engineering community with its needs of thermal neutron scattering data especially when considering new materials where experimental information may be scarce or nonexistent.

Investigation of thermal neutron scattering cross sections for LiH

LiH is designated as a promising moderator and shielding material because of its low density, high melting point and large fraction of H atoms. However, lack of the thermal neutron cross sections of LiH makes numerical calculation deviate from experimental data to some extent. As a result, it is necessary to study LiH thermal kernel effect. The phonon property of LiH has been investigated by first-principles calculations using the plane-wave pseudo potential method with CASTEP code. The scattering law and the thermal neutron scattering cross sections for Li and H have been generated using this distribution. The results have been compared with zirconium hydride data. The GASKET and NJOY/LEAPR codes have been used in the calculation of scattering law, whose results have been compared with the reference; the discrepancy mainly comes from phonon spectrums and its expansion. By studying LiH phonon spectrum and constructing the model of LiH thermal kernel and scattering matrix, the ACE format LiH thermal neutron cross sections for MCNP code could be made and used for reactor Neutronics calculations.

Investigation of the Impact of Simple Carbon Interstitial Formations on Thermal Neutron Scattering in Graphite

In both the prismatic and pebble bed designs of very high temperature reactors, the graphite moderator is expected to reach exposure levels of 1021 to 1022 n/cm2 over the lifetime of the reactor. This exposure results in damage to the graphite structure. Studies of the thermal properties of irradiated graphite show changes in the thermal conductivity and (to a lesser extent) the heat capacity at fluences <1021 n/cm2. In graphite, these properties depend on the behavior of atomic vibrations (phonons) in the solid. Therefore, it can be expected that alterations in the phonon behavior that would produce changes in these properties would have an impact on the thermal neutron scattering behavior of that material. In this work, an atomistic ab initio investigation is performed to explore the potential impact of simple carbon interstitial formations on the inelastic thermal neutron scattering behavior of graphite. Using the VASP/PHONON code system, graphite supercells were modeled with and without either a single carbon interstitial or a di-interstitial (C2) molecule between the graphite planes. This resulted in the production of the phonon frequency spectra for these structures. From the phonon data, the inelastic thermal neutron scattering cross sections were generated, using the NJOY code system, at temperatures of 300 and 1200 K. A comparison of the generated cross sections shows that accounting for the interstitials in the calculations affects the cross sections mainly in the energy range from 0.01 to 0.1 eV.

Cold and thermal neutron scattering cross sections of liquid H 2 and D 2. II: improvement on coherent scattering

In the previous paper, a cross section model has been developed for cold and thermal neutron scattering in liquid H 2 and D 2 at temperatures between melting and boiling points. In the present paper, the model is improved to describe adequately coherent scattering in liquid para-H 2 and D 2 in terms of a mean field approximation. A screened response function and an effective potential are successfully determined by use of an already-ascertained self scattering function and an experimentally-estimated static structure factor. It is shown that the improved model is in good agreement with experimental cross sections, both total and double-differential. This makes it possible to use the model for generation of a reliable cross-section library in developing an advanced cold-neutron source.

00/01329 Cold and thermal neutron scattering cross sections of liquid H 2 and D 2 at temperatures between melting and boiling points

00/01329 Cold and thermal neutron scattering cross sections of liquid H 2 and D 2 at temperatures between melting and boiling points

Cold and thermal neutron scattering cross sections of liquid H 2 and D 2 at temperatures between melting and boiling points

Cold and thermal neutron scattering cross sections of liquid H 2 and D 2 at temperatures between melting and boiling points

Scaling and final-state interactions in deep-inelastic neutron scattering

The momentum distributions of atoms in condensed matter can be determined by neutron inelastic scattering experiments if the momentum transfer $\ensuremath{\hbar}q$ is large enough for the scattering to be described by the impulse approximation. This is strictly true only in the limit $q\ensuremath{\rightarrow}\ensuremath{\infty}$ and, in practice, the experimentally determined momentum distributions are distored by final-state interactions by an amount that is typically 2% to 8%. In this paper we develop a self-consistent method for correcting for the effect of these final-state-interaction effects. We also discuss the Bjorken-scaling and $y$-scaling properties of the thermal-neutron scattering cross section and demonstrate, in particular, the usefulness of $y$ scaling as an experimental test for the presence of residual final-state interactions.

Self correlations in gases of linear molecules with application to thermal-neutron scattering

A phenomenological linear Boltzmann equation is derived to study the classical motion of a rigid linear test molecule in a gaseous medium. The state of the molecule is defined by its center- of-mass coordinate and velocity, rotation frequency, orientation of its angular momentum and a rotor phase angle. The thermal-neutron scattering cross section is derived from a Fourier transform of a density distribution function which satisfies the Boltzmann equation with a suitable initial condition. Cross sections are calculated for some models to indicate the effect of coupling of center of mass and internal behavior.

The Thermal-Neutron Scattering Cross Sections of Uranium-233 and Thorium-232

The Thermal-Neutron Scattering Cross Sections of Uranium-233 and Thorium-232

わが国で利用できる熱中性子散乱コード

A brief review is given of the present status of computer codes for the thermal neutron scattering cross section now available in Japan. It can be stated that at least for room temperature moderators the thermal neutron scattering cross section can be easily calculated with sufficient accuracy by the use of these available computer codes.

Thermal-Neutron Coherent Scattering Amplitudes and Cross Sections of Nickel-61 and Nickel-64

The values of coherent nuclear scattering amplitudes obtained by neutron diffraction studies of isotopically enriched samples of nickel-61 and nickel-64 are: ${{b}_{\mathrm{Ni}}}^{61}=+0.760\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}12}$ cm and ${{b}_{\mathrm{Ni}}}^{64}=\ensuremath{-}0.037\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}12}$ cm. Since nickel-62 is already known to possess an anomalous negative scattering amplitude, the value obtained for ${{b}_{\mathrm{Ni}}}^{64}$ shows that the nickel element has two isotopes, ${\mathrm{Ni}}^{62}$ and ${\mathrm{Ni}}^{64}$, which possess anomalous scattering amplitudes for slow neutrons. These results suggest a further search for nuclear scattering resonance effects on the observed neutron scattering for the nickel isotopes.

Measurement of the Electron-Neutron Interaction by the Asymmetrical Scattering of Thermal Neutrons by Noble Gases

The electron-neutron interaction has been obtained from measurements of thermal neutron scattering by argon, krypton, and xenon. In terms of the slope of the electric structure factor of the neutron at zero momentum transfer, the values obtained were 0.0196\ifmmode\pm\else\textpm\fi{}0.0013 ${\mathrm{F}}^{2}$ from Ar, 0.0197\ifmmode\pm\else\textpm\fi{}0.0007 ${\mathrm{F}}^{2}$ from Kr, and 0.0190\ifmmode\pm\else\textpm\fi{}0.0005 ${\mathrm{F}}^{2}$ from Xe. The final result is 0.0193\ifmmode\pm\else\textpm\fi{}0.0004 ${\mathrm{F}}^{2}$, which corresponds to an electron-neutron scattering length of -1.34\ifmmode\pm\else\textpm\fi{}0.03\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}16}$ cm and an effective potential of -3720\ifmmode\pm\else\textpm\fi{}90 eV. During the course of the experiment, the neutron-absorption cross sections of krypton and xenon (at 2200 m/sec) were measured by transmission and found to be 25.0\ifmmode\pm\else\textpm\fi{}0.8 b and 25.1\ifmmode\pm\else\textpm\fi{}1.0 b, respectively. Also, some free-atom thermal-neutron scattering cross sections were measured. The results were 2.415\ifmmode\pm\else\textpm\fi{}0.010 b for neon, 0.647\ifmmode\pm\else\textpm\fi{}0.003 b for argon, 7.61\ifmmode\pm\else\textpm\fi{}0.04 b for krypton, 4.30\ifmmode\pm\else\textpm\fi{}0.02 b for xenon, 73.7\ifmmode\pm\else\textpm\fi{}0.4 b for $^{36}\mathrm{Ar}$, 1.705\ifmmode\pm\else\textpm\fi{}0.009 b for $^{22}\mathrm{Ne}$, and 5.1\ifmmode\pm\else\textpm\fi{}0.3 b for $^{21}\mathrm{Ne}$.