Cold neutron interaction with a classical electric field: Some basic theoretical and experimental considerations

We explore the feasibility of performing an experiment to measure the interaction of cold neutrons with a given classical electric field. Bound and scattering states could be detected by means of an approximate Aharonov-Casher configuration. The theoretical background is presented and then some primary elements for building a neutron detector of this nature are proposed.     

聚变脉冲中子源时间谱测量的非弹散射技术

介绍DT聚变反应产生的非单能的高能脉冲中子时间谱的一种测量技术——非弹散射法。利用安装在脉冲中子源附近的氧化铍(BeO)材料作为辐射转换体,测量DT聚变反应产生的高能脉冲中子与^16O的非弹散射(n,n’γ)过程中产生的γ辐射时间谱,实现对DT反应脉冲中子时间谱的测量。     

Investigation of thermal neutron scattering data for BeF2 and LiF crystals

The influences of thermal neutron scattering data for BeF₂ and LiF crystals on molten salt reactor physics are investigated in this work. Based on the structure parameters of BeF₂ and LiF, the coherent scattering for both crystals is added to NJOY source code. The ENDF6 format thermal neutron scattering sub-libraries for both crystals are evaluated with their phonon spectra using LEAPR; the ACE format data are produced by NJOY subsequently. Finally, the effect of thermal neutron scattering of BeF₂ and LiF crystals on k _eff and spectrum are investigated. The result shows that thermal neutron scattering for bound state of BeF₂ and LiF influence k _eff and spectrum obviously. The elastic scattering cross section for bound state of crystals is smaller than free atom; it makes k _eff decrease (1%–2%) and spectra be hardened. The higher temperature the bound state has, the smaller coherent elastic scattering cross section it gets; therefore, k _eff decreases with temperature. It is suggested that the thermal neutron data of LiF and BeF₂ should be taken into account for molten salt reactor.     

复杂环境条件下脉冲中子散射本底测量方法

介绍了在复杂环境条件下脉冲中子散射本底测量方法和计算公式,并在实验室进行了实验测量,分析了影响实验结果的因素。     

Ab initio computations of one and two hydrogen or deuterium atoms in the palladium tetrahedral site

We report ab initio cluster computations of deuterium atoms in a tetrahedral palladium site, performed at the UHF level with extended basis sets. An interstitial deuteron is found to be energetically favored over an interstitial deuterium atom. Computations with one interstitial deuterium atom or with one deuteron reveal an increase in electron density near the deuteron. Not only valence electrons but also inner core electrons of the palladium atoms are present in the vicinity of the deuteron. A potential energy curve is calculated with two deuterium atoms approaching each other, with one deuterium fixed at the center of a tetrahedral site. With regard to the environment provided by molecular deuterium, our results show that the tetrahedral site does not favor a closer deuteron encounter. No metastable dimer geometry is found. The two deuterons repel each other despite the screening. These computations represent more than 250 CPU h of a new-generation biprocessor vector computer.     

Model independent determination of the elastic incoherent structure factor in neutron scattering experiments

In quasielastic neutron scattering (QENS) the dynamic structure factor S(Q, ω) consists of an elastic central line with area given by the elastic incoherent structure factor (EISF) and a continuous quasielastic broadening. Using only the resolution function of the spectrometer we discuss a method to derive the EISF directly from the raw experimental data points S(Q, ω) without any smoothing or any modelling of the quasielastic broadening. We compare this model to the Fourier transform method, show how it works on actual neutron scattering data and discuss its accuracy for several cases.     

On the neutron diffraction in crystals in the field of a standing sound wave

The diffraction of neutrons is considered in crystals under the influence of a standing sound wave. The scattering probability is calculated for the elastic neutron-crystal interaction, whereas the neutron-standing sound wave interaction can be either elastic and inelastic. The possibility of short-wave (high-energy) neutrons diffraction is illustrated. It is shown that the Debye-Waller factor can be changed and tuned. The analysis of conservation laws are adduced both for thermal and short-wave neutrons. The formation of a “sublattice” is shown in the process of neutrons elastic diffraction with respect to standing sound wave. The analogous to the Kapitza-Dirac effect is considered for neutrons.The problem is solved within the frame of non-stationary S-matrix theory, where the neutron-phonon interaction is described by the Fermi pseudopotential, which is considered as a perturbation.     

利用准弹性中子散射研究金属氧化物团簇内部碳链的动态

中子散射技术是研究凝聚态物质和材料性质的重要手段。通过将碳链植入金属氧化物团簇分子的内部,可以实现对碳链运动的彻底改变。通过分析中子谱得知,这些金属氧化物团簇分子内部的碳链的大尺度空间平移运动几乎被完全限制,碳链上的甲基和亚甲基的转动速度也被大幅度减慢。另外,碳链上的不同碳原子的运动有不均匀性。这些现象都表明,金属氧化物团簇分子对于其内部的高分子运动有强约束效应。该效应在金属氧化物团簇的包裹行为上有决定性作用。     

Stochastic effects of charge transfer on image interaction and plasmon excitation in ion-surface grazing scattering

We model the charge-state evolution of fast hydrogen-like ions in the course of grazing scattering from metal surfaces by a Markov process with the time-dependent jump coefficients. Using the master equation for charge exchange, we show that the process may be treated as quasi-stationary owing to the adiabatically slow ion motion in the direction perpendicular to the surface. We further employ the dielectric formalism within the specular reflection model and the plasmon-pole approximation to evaluate the expectation values for the ion-image interaction with the surface and the rate of surface-plasmon excitation. We find that, close to the surface, both these quantities are significantly affected by the projectile-charge fluctuations. In particular, we find a substantial rate of the plasmon excitation for ion velocities well below the threshold implied by the kinematic constraint.