基于冷中子谱的紧凑型中子自旋倒相器设计

为节省极化中子散射谱仪传输光路的空间,实现特定冷中子谱的极化中子高效率自旋翻转,使用在空间上自然衰减的前端多层膜极化器静磁场作为中子自旋倒相器的导向磁场,在空间上形成了紧凑型冷中子自旋倒相器设计模型。介绍了实际模型物理参数的计算方法。对前端极化器静磁场在空间上的自然衰减进行了实验测试,根据测试结果及拟使用冷中子波段,针对设计的紧凑型中子自旋倒相器的相关参数进行了优化计算。模拟了极化中子在实际复合磁场中的自旋翻转图像,计算了自旋倒相器的翻转效率。对设计的紧凑型中子自旋倒相器进行了翻转效率物理实验测试,测试结果表明：设计的中子自旋倒相器翻转效率可在99.2%以上,达到了预期设计指标,可用于极化冷中子散射谱仪。     

Optimised adiabatic fast passage spin flipping for He neutron spin filters

We describe here a method of performing adiabatic fast passage (AFP) spin flipping of polarized ³He used as a neutron spin filter (NSF) to polarize neutron beams. By reversing the spin states of the ³He nuclei the polarization of a neutron beam can be efficiently reversed allowing for the transmission of a neutron beam polarized in either spin state. Using an amplitude modulated frequency sweep lasting 500 ms we can spin flip a polarized ³He neutron spin filter with only 1.8×10⁻⁵ loss in ³He polarization. The small magnetic fields (10-15 G) used to house neutron spin filters mean the ³He resonant frequencies are low enough to be generated using a computer with a digital I/O card. The versatility of this systems allows AFP to be performed on any beamline or in any laboratory using ³He neutron spin filters and polarization losses can be minimised by adjusting sweep parameters.     

Interaction of neutrons with layered magnetic media in oscillating magnetic field

New experimental possibilities of investigating layered magnetic structures in oscillating magnetic fields are discussed. Spin-flip and nonspin-flip neutron reflection and transmission probabilities show a frequency dependency near the magnetic neutron resonance condition. This allows to increase the precision of the static magnetic depth profile measurements of the magnetized matter. Moreover, this opens new possibilities of measuring the induction of the oscillating field inside the matter and determining the magnetic susceptibility of the oscillating magnetic field. Refraction of neutrons as they pass through a magnetic prism in the presence of an oscillating magnetic field is also investigated. A non-polarized neutron beam splits into eight spatially separated neutron beams, whose intensity and polarization depend on the strength and frequency of the oscillating field. Also, it is shown that the oscillating magnetic permeability of an angstrom-thick layer can be measured with a neutron wave resonator.     

Small-Angle Neutron Scattering Wave Interference on Ferromagnetic Alloys

The detection conditions and features of direct and scattered neutron wave interference are studied on magnetized Co₆₇Fe₃₁V₂ alloy slabs. The angular intensity distributions of neutrons passed through a sample are measured for the opposite polarization directions of the initial neutron beam. The sought-for effect that is induced by the magnetic scattering on crystal structure irregularities in specimens manifest itself by different areas of peaks “without neutron spin flip.” The ratio of these areas depends on the thermal treatment mode, sample thickness and strength of the magnetic field applied to the sample. The peaks “with neutron spin flip” are due to the mechanism of neutron wave passage through magnetononcollinear boundaries. The methods for experimental data acquisition and processing are reported as well.     

Radiation by relativistic dipoles. III

The classical radiation of a point magnetic moment (a magneton) moving at a constant velocity in an arbitrary direction with respect to the field lines of a uniform magnetic field is analyzed. All characteristics of the radiation agree with the Ternov-Bagrov-Khapaev relativistic quantum thoery of the radiation by a neutron. It is thus demonstrated that the classical model of radiation with spin flip is valid. The correspondence principle in the theory of radiation with spin flip will be discussed in more detail in subsequent papers.V. V. Kuibyshev Tomsk State University. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 1, pp. 53–59, January 1994.     

Two examples of in-flight spin flippers

In precise experiments with polarized beam it’s very often appear a necessity to change beam polarization on opposite. If such operation does not change other beam parameters, it helps to avoid or minimize some systematic errors. It is especially important in experiments, where spin dependent effect is small enough. This paper describes two set of equipments, that make spin flip for extracted beams. In both cases, these devices are absolutely distinct, because they are appropriate for different particles and at different energy range. The first of them is intended for future muon (g-2) experiment, which is under preparation now at JPARC. Here, the muon spin flip will be done by chain of electrostatic and magnetic bends at the kinetic energy 340 keV. A beam matching is provided by a number of short solenoids. The other flipper (or Siberian snake) will rotate spin of protons or antiprotons, which come from Λ-meson decay with the energy up to 40 GeV. This experiment (no. 24) is planed at IHEP, Protvino. In this case, two superconducting helical magnets with opposite helicities and magnetic field 4.5 T will be used. To correct beam trajectory, additional dipole correctors are required.     

Transport of polarized neutrons through magnetic noncoplanar layered systems

The transport of neutrons through noncoplanar systems is shown to have a partly unidirectional character. To this end, the transmission of a three-layer magnetic film with a noncoplanar magnetization in the absence of applied magnetic field and the transmission of a two-layer magnetic film with a noncollinear magnetization in the presence of a magnetic field are calculated. The calculations demonstrate that, if the losses in the films are neglected, the transmissions in two opposite directions differ only for processes with and without spin flip. If these losses are taken into account, the total transmissions of a nonpolarized neutron beam in two opposite directions are also different. The consequences of these specific features of noncoplanar systems are discussed.     

Neutron multiwave spin echo

The neutron multiwave interference mode is investigated using the spin echo technique. In this mode a neutron wave repeatedly splits in the magnetic field of resonance coils, which results in the appearance of additional maxima of a constructive interference being absent in the well-known classical and resonance neutron spin echo modes. Simple analytical expressions well describing the experimental data are presented. It is demonstrated that the multiwave part of a spin echo signal appears when the spin flip probability in radiofrequency coils of a resonance spin echo device is ρ < 1. The possibility to use the neutron multiwave spin echo mode for investigation of high-order correlation functions, spatial and time correlations of three and more particles, is discussed.     

装置极化效率对极化中子成像质量的影响及修正分析

极化中子照相技术通过分析极化中子束的自旋相移对样品磁场进行成像, 自旋极化/分析装置是照相系统的主要组成部分. 引入中子自旋极化/分析装置的极化效率参数, 从中子极化矢量与磁场相互作用机理出发, 重新推导探测中子强度与磁场分布的定量关系, 利用谱仪模拟软件VITESS, 选取bender型超镜极化器和 ³He 自旋过滤器作为极化/分析装置, 对量化修正式进行验证, 并综合装置极化效率、单色器能量分辨精度和bender型极化器的几何结构等参数, 初步分析极化中子照相技术的磁场定量检测能力, 相关结果可为极化中子照相的实验数据处理技术研究及装置设计提供参考. 关键词： 极化效率 中子照相 磁场成像     

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.     

磁性纳米结构中由激光引起的超快自旋动力学研究

以单个磁性中心的NiO以及由Co和Ni等元素构成的双磁性中心的纳米结构为例,总结了近年所做的主要工作.为了在理论上实现磁性纳米结构中的超快自旋翻转和转移,提出了一种称为Λ进程(Λ process)的超快自旋转换机理.在实际计算中,首先采用量子化学第一性原理计算得到磁性纳米结构中精确的隙间d电子态,然后考虑外加磁场和自旋轨道耦合分析磁性原子中的自旋局域化程度,最后引入激光脉冲项,研究在其作用下材料的自旋态经由Λ进程实现转换的时间历程.研究结果表明自旋翻转和转移可以在线偏振光的作用下在亚皮秒的时间尺度内完成. 关键词： 超快自旋动力学 第一性原理计算 Λ进程 磁性纳米结构     

Experimente mit dem neutronenspinecho mit langwelligen neutronen aus einem polarisierenden neutronenleiter

We describe an inexpensive method to obtain a highly polarized neutron beam of slow neutrons (λ = 1 – 15 Å) [1]. The high necessary field to saturate the applied soft magnetic material in the surface acting as a polarizing mirror indicates a behaviour of the magnetization in the surface different from that in the bulk material [2]. We also describe the realization of an apparatus proposed by F. Mezei [3], the behaviour of the polarized neutrons in this apparatus, and its use to get a spin echo with a neutron spectrum containing 3 – 9 Å wavelength neutrons. This spin echo is not destroyed by transmission through a nickel sheet, only the number of precessions is increased which appear in a displacement of the echo. This displacement can be used to measure the saturation magnetization of this sheet.     

Development of neutron resonance spin flipper for high resolution spin echo spectrometer

A new type of neutron resonance spin flipper (RSF) with high frequency oscillating magnetic field has been developed for Modulated IntEnsity by Zero Effort (MIEZE) spectrometer at cold neutron beam line MINE1 at JRR-3M reactor in JAEA. Dipole magnets enable us to provide the strong static fields for the RSFs. MIEZE signals have been demonstrated with the effective frequency of 600 kHz by using the new RSFs. The contrast of the signals was 0.58. The MIEZE spectrometer is under final process to practical use. The spectrometers can also be applied to the pulsed neutrons like J-PARC.     

Study of a magnetic Fe/Co multilayer nanostructure via neutron reflectometry

A periodic magnetic Fe/Co multilayer nanostructure is investigated via neutron reflectometry with the help of the time-of-flight technique and the obtained results are presented. The experimental and calculated reflection coefficients of a neutron beam depending on the transmitted pulse are discussed for several magnetic fields and both beam spin components. It is experimentally established and confirmed by calculations that, for the first Bragg peaks and both beam spin components, the reflection coefficients are almost equal to each other and depend slightly on magnetic field.     

Intermediate valence: Phase diagram and Kondo behaviour in a simple model

We have studied an infinitely narrow band version of the Anderson Hamiltonian. Including a Coulomb repulsion between localized and band states in the mean field approximation, we have been able to describe metal-insulator phase transitions and different properties characteristic of intermediate valence in both phases. These include: non-integer occupation, specific heat, saturating static magnetic succeptibility and dynamical properties such as Mössbauer spectra and spin flip neutron spectra. We discuss for what range of values of the parameters some results can be reproduced by a narrow band version of the Kondo Hamiltonian.     

Neutron spin echo: A new concept in polarized thermal neutron techniques

A simple method to change and keep track of neutron beam polarization non-parallel to the magnetic field is described. It makes possible the establishment of a new focusing effect we call neutron spin echo. The technique developed and tested experimentally can be applied in several novel ways, e. g. for neutron spin flipper of superior characteristics, for a very high resolution spectrometer for direct determination of the Fourier transform of the scattering function, for generalised polarization analysis and for the measurement of neutron particle properties with significantly improved precision.     

Magnetic properties of the charge ordered Nd0.75Na0.25MnO3

Nd_0.75Na_0.25MnO₃ polycrystalline ceramic is prepared via sol-gel process and its magnetic properties and electron spin resonance (ESR) spectra have been investigated experimentally. As the compound is cooled from room temperature, a charge-ordered state first develops below 170 K. A high magnetic field melts the charge ordered state and stabilizes a ferromagnetic (FM) state below 170 K. A field induced transition, analogous to a spin flip transition, is observed between 40 and 170 K. The critical temperature for spin flip increases with increasing temperature. Below 130 K, the compound tends to be intrinsically inhomogeneous, i.e. FM clusters and paramagnetic domains coexist in this system at least, which is confirmed by ESR measurements. When the external magnetic field is zero, long range FM interaction is not developed in this system; however, a tendency of re-entrant FM transition is observed in this compound.     

基于金刚石氮-空位色心自旋系综与超导量子电路混合系统的量子节点纠缠

量子纠缠是实现量子计算和量子通信的核心基础,本文提出了在金刚石氮-空位色心(NV centers)自旋系综与超导量子电路耦合的混合系统中实现两个分离量子节点之间纠缠的理论方案.在该混合系统中,把金刚石NV centers自旋系综和与之耦合的超导共面谐振器视为一个量子节点,两个量子节点之间通过一个空的超导共面谐振器连接.具有较长相干时间的NV centers自旋系综作为一个量子存储器,用于制备、存储和发送量子信息;易于外部操控的超导量子电路可执行量子逻辑门操作,快速调控量子信息.为了实现两个分离量子节点之间的纠缠,首先对系统的哈密顿量进行正则变换,将其等价为两个NV centers自旋系综与同一个超导共面谐振器之间的JC耦合;然后采用NV centers自旋-光子混合比特编码的方式,通过调节超导共面谐振器的谐振频率,精确控制体系演化时间,高保真度地实现了两个分离量子节点之间的量子纠缠.本方案还可以进一步扩展和集成,用于构建多节点纠缠的分布式量子网络.     

Neutrino flavor conversion in random magnetic fields

If massive neutrinos possess magnetic moments, a magnetic field can cause a spin flip. In the case of Dirac neutrinos the spin flip converts an active neutrino into a sterile one and vice versa. By constrast, if neutrinos are Majorana particles, a spin flip converts them to a neutrino of a different flavor. We examine the behavior of neutrinos in a random magnetic field as it occurs, for instance, in certain astronomical objects, such as an active galactic nucleus. Both Dirac and Majorana neutrinos behave ergodically: independently of their initial density matrix, they tend towards an equipartition of the helicity states. As a result, about half of the Dirac neutrinos produced becomes sterile. For Majorana neutrinos, there will be an approximate equipartition of flavors, independently of the production mechanism.