The application of quasi-elastic neutron scattering techniques (QENS) in surface diffusion studies

Neutron scattering techniques such as quasi-elastic neutron scattering, QENS, have proven to be well-suited tools for studying structure and dynamics of surface adsorbed molecules. In contrast to many more widely used surface science techniques neutron scattering allows the microscopic characterization of samples under a wide range of thermodynamic conditions, as the samples are not constrained to ultra high vacuum environment. Moreover, neutron scattering allows the separation of coherent and incoherent scattering, giving access to different diffusive mechanisms such as single particle diffusion, mass transport, rotations, or vibrations. In this paper we will review recent progress and the state-of-the-art in neutron scattering experiments on surface adsorbed molecules in the sub-monolayer coverage range with a specific emphasis on studies of carbon and other high surface density substrates. We will also cover recent progress in theoretical modeling, since the usefulness of neutron scattering data on surface dynamics can be strongly enhanced by computational modeling, such as molecular dynamics (MD) simulations and the development of analytical models.     

High temperature—high pressure neutron scattering experiments on expanded liquid alkali metals

Abstract

We present neutron diffraction investigation of the static structure factor S(Q) of liquid alkali metals expanded by heating towards conditions close to their liquid-vapour critical point. The experiments were designed to obtain the characteristic changes of the macroscopic structure as a function of temperature and density. The data give information on the density dependent changes in the interatomic forces. as the metal-nonmental transition is approached in the expanded liquid metal.     

Neutron Resonance States in Overdense Crystals

New three-body resonances appeared in the neutron scattering on two nuclei fixed in nodes of overdense crystalline lattice have been calculated for the iron group isotopes. These resonances are strongly dependent on the lattice parameters and characteristics of two-body resonances in neutron–nucleus subsystems. The effective forces created by the neutron resonance rescatterings supplement the Coulomb forces between nuclei of the crystalline lattice. These effective resonance interactions occur only at certain distances between nuclei and are accompanied by gamma radiation.     

Russian measurements of neutron energy spectra on the Mir orbital station

Results of the experiments on neutron energy spectra measurements within broad energy range from 5 x 10(-7) to 2 x 10(2) MeV aboard the Mir orbital station and equivalent neutron dose estimation are presented. Four measurement techniques were used during the experiments. The shape of spectra and their absolute values are in good agreement. According to those experiments, an equivalent neutron dose depends upon effective shielding thickness and spacecraft mass. The neutron dose mentioned is comparable with that of ionizing radiation. Neutron flux levels measured aboard the Mir station have shown that a neutron spectrometer involving broad energy range will be used within the radiation monitoring systems in manned space flights.     

Can quantum gravitational forces stop gravitational collapse?

We consider, in lowest order of the gravitational coupling constant G, the gravitational potential between two neutrons. As we have previously pointed out [1],the quantum (including spin) contributions to the gravitational field dominate for distances smaller than the Compton wavelength of the neutron. At such distances the gravitational force between two neutrons may be repulsive. In particular, the gravitational forces which are analogous to the familiar Darwin and Fermi forces of quantum electrodynamics are capable of stopping gravitational collapse. Our discussion is within the framework of Einstein's theory, but on a microscopic level. We conclude that gravitational collapse may be halted without the necessity of extending Einstein's theory à la Cartan or otherwise.     

The structure of molten germanium

We present a calculation of the structure of molten Ge, based on pseudopotential-derived interatomic forces and the optimized random phase approximation. Good agreement with experiment is achieved. We show that the complex structure of liquid Ge results from the interplay of two characteristic distances: the effective hardcore diameter and the Friedel wavelength of the long range oscillations in the interatomic potential.     

Räumliche Asymptotik der Korrelationsfunktion von VAN HOVE

Asymptotic Behaviour of the Correlation Function of VAN HOVE for Large Distances With the help of the general theory of van-der-Waals forces, given by DZIALOSHINSKI , LIFSCHITZ and PITAIEVSKI [2], the contribution of the long range forces to the space-time-density-correlation function was calculated. This contribution determines the asymptotic behaviour of the correlation function for large distances compared with the interatomic distances without restriction of time. The space and time dependence of that contribution was discussed in the cases of large or small distances compared to the characteristic wave length in the absorption spectrum of the material.     

Larmor spin precession and neutron optics

We consider the neutron-optical phenomena that emerge during the coherent interaction of a neutron with a sample when the neutron spin precesses in a magnetic field. As follows from general considerations, such an interaction gives rise to an extra precession phase, which is added to the Larmor precession phase. This phenomenon can be interpreted as a manifestation of the time delay due to a finite time of the neutron-sample interaction. The Larmor neutron spin precession with a constant frequency serves as a clock for measuring this time delay. We used such a clock to directly measure the difference between the neutron velocity in matter and its vacuum value. We also present the results of the first experiments in which Larmor clocks were used to measure the neutron tunneling time in the resonance of a quasi-bound state and the Bragg diffraction time. Prospects for further applications of the method are discussed.     

Neutron optics of noncentrosymmetric crystals: New possibility of searches for the neutron electric dipole moment and CP-violating forces

New neutron-optics effects, which were predicted and discovered quite recently in noncentrosymmetric crystals and which can be used to study the fundamental properties of the neutron, are discussed. In particular, strong electric fields of strength up to 10⁸ or 10⁹ V/cm may act in such crystals on the neutron, and this provides new possibilities for measuring the neutron electric dipole moment (EDM) by the crystal-diffraction method. It also becomes possible to perform searches for pseudomagnetic forces acting on neutrons and violating CP invariance. For the range of such forces that satisfies the condition λ _A < 10^?5 cm, the best constraints on the product of the scalar and pseudoscalar coupling constants have already been obtained for the interaction induced by the exchange of a light pseudoscalar (axion-like) particle. The present-day status and prospects of neutron-optics crystal-diffraction experiments aimed at searches for the neutron EDM and pseudomagnetic forces are considered.     

中子散射对快中子照相质量影响的分析

通过一内空的圆柱体模型，对在快中子照相时由样品引起的散射中子强度与样品形状和探测距离之间的关系进行模拟，并用^241Am—Be作中子源对散射中子的影响进行实验验证。结果表明，在快中子照相时，由样品引起的散射中子的强度与探测距离以及样品形状有关。对于同一样品，探测距离增加，散射中子的影响则降低。The relationships between intensities of scattered neutrons by specimens and their shapes and detecting distances have been simulated using a hollow cylinder model, and the results were validated by experiments of fast neutron imaging using ^241Am-Be neutron source. The results showed that the intensities of scattered neutrons are closely related to the detecting distances and sample＇ s shapes. The influences of scattered neutronns in fast neutron imaging will be reduced while detecting distances increased.     

Centrifugal Electrical Force

We calculate the force on a point charge moving within a charged spherical shell spinning with ω(t) according to Weber's law. We compare this result with the one given by Lorentz's law and show that Weber's law predicts an extra centrifugal electrical force and an extra inertial electrical force. We give orders of magnitude for these forces and propose some experiments to test the existence of these terms.     

Neutron-mirror-neutron oscillations: how fast might they be?

We discuss the phenomenological implications of the neutron (n) oscillation into the mirror neutron (n'), a hypothetical particle exactly degenerate in mass with the neutron but sterile to normal matter. We show that the present experimental data allow a maximal n-n' oscillation in vacuum with a characteristic time tau much shorter than the neutron lifetime, in fact as small as 1 sec. This phenomenon may manifest in neutron disappearance and regeneration experiments perfectly accessible to present experimental capabilities and may also have interesting astrophysical consequences, in particular, for the propagation of ultra high energy cosmic rays.     

Non-Gaussian nature of the alpha relaxation of glass-forming polyisoprene

We report quasielastic neutron scattering experiments exploring the alpha relaxation in polyisoprene over an unprecedented range in momentum transfer. Corroborating and validating earlier molecular dynamics simulations, the measurements reveal a crossover from a Gaussian regime of sublinear diffusion to a strongly non-Gaussian regime at short distances. We show that a consistent interpretation in terms of a distribution of finite jumps underlying the alpha process is possible. This model leads to a time-dependent non-Gaussian parameter exhibiting all features revealed so far from various simulations.     

Search for long-range forces by means of ultracold neutrons (1)

A method of using a gravitational spectrometer to search for long-range forces between neutrons and atoms is proposed. The constraints on the strength of long range forces within the range of 10⁻¹⁰–10⁻⁴ cm can be obtained from the experiments on measurements of the total cross section of interaction of ultracold neutrons with atoms of noble gases (He, Ne, Ar, ⁸⁶Kr) and the data on the coherent neutron scattering length of the nucleus. The first result of such type analysis is presented.     

Cold neutrons trapped in external fields

The properties of inhomogeneous neutron matter are crucial to the physics of neutron-rich nuclei and the crust of neutron stars. Advances in computational techniques now allow us to accurately determine the binding energies and densities of many neutrons interacting via realistic microscopic interactions and confined in external fields. We perform calculations for different external fields and across several shells to place important constraints on inhomogeneous neutron matter, and hence the large isospin limit of the nuclear energy density functionals that are used to predict properties of heavy nuclei and neutron star crusts. We find important differences between microscopic calculations and current density functionals; in particular, the isovector gradient terms are significantly more repulsive than in traditional models, and the spin-orbit and pairing forces are comparatively weaker.     

Possibility of hypothetical stable micro black hole production at future 100 TeV collider

We study the phenomenology of TeV-scale black holes predicted in theories with large extra dimensions, under the further assumption that they are absolutely stable. Our goal is to present an exhaustive analysis of safety of the proposed 100 TeV collider, as it was done in the case of the LHC. We consider the theories with different number of extra dimensions and identify those for which a possible accretion to macroscopic size would have timescales shorter than the lifetime of the Solar system. We calculate the cross sections of the black hole production at the proposed 100 TeV collider, the fraction of the black holes trapped inside the Earth and the resulting rate of capture inside the Earth via an improved method. We study the astrophysical consequences of stable micro black holes existence, in particular its influence on the stability of white dwarfs and neutron stars. We obtain constraints for the previously unexplored range of higher-dimensional Planck mass values. Several astrophysical scenarios of the micro black hole production, which were not considered before, are taken into account. Finally, using the astrophysical constraints we consider the implications for future 100 TeV terrestrial experiments. We exclude the possibility of the charged stable micro black holes production.     

Neutron experiments to search for new spin-dependent interactions

The consideration is presented of possible neutron experiments to search for new short-range spin-dependent forces. The spin-dependent nucleon-nucleon interaction between neutron and nuclei may cause different effects: phase shift of a neutron wave in neutron interferometers of different kind, in particular of the Lloyd mirror configuration, neutron spin rotation in the pseudo-magnetic field, and transverse deflection of polarized neutron beam by a slab of substance. Estimates of sensitivity of these experiments are performed.     

近距离牛顿反平方定律实验检验进展

为了统一描述自然界的四种基本相互作用,科学家提出了很多理论模型,其中很多理论认为牛顿反平方定律在近距离下会发生偏离,或存在其他的非牛顿引力作用,而理论的正确与否需要高精度的实验检验.国际上很多研究组在不同间距下采用不同的技术对反平方定律进行了高精度的实验检验,本文重点介绍华中科技大学引力中心采用密度调制法分别在亚毫米与微米范围进行的实验研究进展.在亚毫米范围采用精密扭秤技术,在对牛顿引力进行双补偿、抑制电磁干扰后,结合零实验与非零实验结果,在作用程为70—300μm区间对Yukawa形式的破缺给出国际上精度最高的限制.在微米范围采用悬臂梁作为弱力传感器,通过测量金球和密度调制吸引质量间水平力的变化来检验非牛顿引力是否存在,实验结果不需进行Casimir力和静电力背景扣除,是此间距下不依赖于Casimir力和静电力理论计算模型的两个结果之一.     

Current short-range tests of the gravitational inverse square law

Motivated in large part by the possibility of observing signatures of compact extra dimensions, experimental searches for deviations from Newtonian gravity at short distances have improved in sensitivity by many orders of magnitude in the past five years. We review the essential features of the experiments responsible for the current limits on new effects in the range from a few microns to a few centimeters, and discuss prospects for the near future. To cite this article: J.C. Long, J.C. Price, C. R. Physique 4 (2003).     

密闭实验大厅内散射中子注量的模拟研究

为能够快速评判实验测试方案和预估实验结果,建立了中子体通量的快速估算模型。理论上,封闭空间中子平均体通量与特征长度的平方成反比,且中子体通量的大小能反映散射中子注量强弱。采用蒙特卡罗模拟方法,计算得到了密闭实验大厅内中子的体通量,以及不同位置处的散射中子注量,并将模拟得到的体通量和散射中子注量拟合成便于工程实践中应用的解析表达式,拟合结果与模拟结果的相对偏差小于10%。研究结果表明,球形空间内中子的体通量与球半径的1.905次方成反比;密闭实验大厅的中子体通量与大厅横截面宽度的1.948次方成反比,与长宽比的0.775次方成反比;球形空间结构内,每个源中子的平均径迹长度约为半径的5.4倍,而长方体密闭实验大厅内,单个源中子的平均径迹长度为大厅特征尺度的2~3倍。