Instrumentation with polarized neutrons

Neutron scattering with polarization analysis is an indispensable tool for the investigation of novel materials exhibiting electronic, magnetic, and orbital degrees of freedom. In addition, polarized neutrons are necessary for neutron spin precession techniques that path the way to obtain extremely high resolution in space and time. Last but not least, polarized neutrons are being used for fundamental studies as well as very recently for neutron imaging. Many years ago, neutron beam lines were simply adapted for polarized beam applications by adding polarizing elements leading usually to unacceptable losses in neutron intensity. Recently, an increasing number of beam lines are designed such that an optimum use of polarized neutrons is facilitated. In addition, marked progress has been obtained in the technology of ³He polarizers and the reflectivity of large-m supermirrors. Therefore, if properly designed, only factors of approximately 2–3 in neutron intensity are lost. It is shown that S-benders provide neutron beams with an almost wavelength independent polarization. Using twin cavities, polarized beams with a homogeneous phase space and P>0.99 can be produced without significantly sacrificing intensity. It is argued that elliptic guides, which are coated with large m polarizing supermirrors, provide the highest flux.     

Program for studying fundamental interactions at the PIK reactor facilities

A research program aimed at studying fundamental interactions by means of ultracold and polarized cold neutrons at the GEK-4-4′ channel of the PIK reactor is presented. The apparatus to be used includes a source of cold neutrons in the heavy-water reflector of the reactor, a source of ultracold neutrons based on superfluid helium and installed in a cold-neutron beam extracted from the GEK-4 channel, and a number of experimental facilities in neutron beams. An experiment devoted to searches for the neutron electric dipole moment and an experiment aimed at a measurement the neutron lifetime with the aid of a large gravitational trap are planned to be performed in a beam of ultracold neutrons. An experiment devoted to measuring neutron-decay asymmetries with the aid of a superconducting solenoid is planned in a beam of cold polarized neutrons from the GEK-4′ channel. The second ultracold-neutron source and an experiment aimed at measuring the neutron lifetime with the aid of a magnetic trap are planned in the neutron-guide system of the GEK-3 channel. In the realms of neutrino physics, an experiment intended for sterile-neutrino searches is designed. The state of affairs around the preparation of the experimental equipment for this program is discussed.     

In the wonderland of ultra-parallel neutron beams

Bragg reflections from single crystals yield angular widths of a few arcsec for thermal neutron beams. The Bonse-Hart proposal to attain a sharp, nearly rectangular profile by Bragg reflecting neutrons multiply from a channel-cut single crystal, was realized in its totality three and a half decades later by achieving the corresponding Darwin reflection curves for 5.23 Å neutrons. This facilitated SUSANS (Super USANS) measurements in the Q ～ 10^?5 Å^?1 range. The polarized neutron option was introduced into the SUSANS set-up by separating the up- and down-spin neutron beams by ～10 arcsec with a magnetic (air) prism. The neutron angular width has recently been reduced further by an order of magnitude to ～0.6 arcsec by diffracting 5.3 Å neutrons from a judiciously optimized Bragg prism. This constitutes the most parallel monochromatic neutron beam produced to date. I present the first SUSANS spectra probing the Q ～ 10^?6 Å^?1 domain, recorded with this beam.     

Reflection of neutrons from a magnetic film placed in static and oscillating magnetic fields

Measurements of polarized-neutron reflection from magnetic films placed in a static magnetic field and in an oscillating magnetic field perpendicular to it are described. The process of diffuse neutron scattering involving energy transfer from a RF electromagnetic field to neutrons has been studied. Neutron magnetic resonance has been detected, and splitting of the polarized beam of neutrons reflected from a film has been discovered.     

Larmor labeling development

The first applications of polarized neutrons were to distinguish magnetic from nuclear scattering amplitudes. Much later the first set-ups for measuring the precession phase of the polarization passing a magnetic field were realized. This phase, determined by the field strength and interaction time, could label magnetic fields, wavelength of the neutrons and length over which the field is present. The latter could be used by proper shaping of the magnetic field to label also the direction of transmitted neutrons. The advantage of this labeling is that high precision measurements are possible without strong confinement of the beam by diaphragms. An overview of the use of Larmor labeling of polarized neutrons is given for applications in magnetism, in inelastic neutron scattering and small angle scattering.     

Magnetic layered structure for the production of polarized neutron microbeams

A neutron waveguide is a three-layer structure in which a guiding layer with low optical potential is placed between two cladding layers with high optical potential. Under proper operation conditions, the neutron density is resonantly enhanced inside the guiding layer. In our experimental scheme, the neutron beam enters through the surface of the top layer at glancing angle and goes out from the edge of the guiding layer, with an angular distribution corresponding to Fraunhofer diffraction from a narrow slit. The incident neutron beam is relatively wide (0.1 mm) and highly collimated (0.01°). The outgoing sub-micron beam is extremely narrow at the outlet (0.1 μm) and more divergent (0.1°). So far only the production of unpolarized sub-micron neutron beams has been reported. Here we present first experiments on polarized sub-micron neutron beams. For these studies a polarized incident beam was used and two types of magnetic waveguides were investigated: a polarizing magnetic waveguide Fe(20 nm)/Cu(140 nm)/Fe(50 nm)//glass and a non-polarizing magnetic waveguide Py(10 nm)/Al(140 nm)/Py(50 nm)//glass (Py is permalloy). The waveguide samples were characterized by polarized neutron reflectometry.     

Neutron storage in a magnetic field

The motion of neutrons in magnetic traps is considered for various cases of neutron polarization. The results of implementing such traps in practice and special features of experiments studying magnetic neutron storage are discussed. The problem of neutron losses during injection via magnetic valves can be solved by conjoining a magnetic trap with a converter of cold neutrons into ultracold ones or with a source of ultracold neutrons. Prospects for expanding neutron-storage experiments by invoking a correlation analysis of neutron decay and by using the transport properties of charged particles in a nonuniform magnetic field are analyzed. In such an investigation, the recording of the storage time of neutrons proper can be supplemented with the detection of decay protons and electrons and with a parallel measurement of the asymmetries of proton and electron emission with respect to the magnetic field. A set of relative measurements permits improving the accuracy of an experimental determination of the neutron lifetime and combining this determination with the determination of correlation coefficients. On this basis, it is possible to find directly the ratio of the weak-interaction constants and the constants themselves. The application of the most advanced reactor and accelerator technologies to subcritical electric nuclear devices optimized for generating cold and ultracold neutrons, along with the use of solid deuterium and superfluid helium, creates preconditions for developing a neutron plant and for launching neutron studies at accelerators. Thus, the work that has been done as a development of V.V. Vladimirsky's proposals on magnetic neutron storage is analyzed, and the potential of a further use of ultracold neutrons and magnetic devices for deploying a full-scale precision experiment to study the beta decay of polarized neutrons is demonstrated.     

On the Application of Zeeman Spatial Beam Splitting in Polarized Neutron Reflectometry

Zeeman spatial neutron beam splitting is considered upon reflection from a homogeneous magnetic film placed in an external magnetic field applied at an angle to the surface of a sample. Two ways of applying the Zeeman beam-splitting phenomenon in polarized neutron reflectometry are discussed. One of them is the construction of polarizing devices with a high polarization efficiency. The other is investigations of magnetically noncollinear films at a low spin-flip probability of reflected neutrons. The experimental results are presented for illustration.     

Analysis of PNC neutron resonance experiments with polarized targets

The analysis of parity mixing experiments with unpolarized neutrons on polarized targets is described. Expressions for the neutron total cross section are presented for targets withI=1/2, 3/2, 5/2 and 7/2. These results are compared with the corresponding expressions for polarized neutron beams and unpolarized targets.     

Methods for Probing Magnetic Films with Neutrons

We review various methods in the investigation of magnetic films with neutrons, including those based on the effects of Larmor precession, Zeeman spatial splitting of the beam, neutron spin resonance, and polarized neutron channeling. The underlying principles, examples of the investigated systems, specific features, applications, and perspectives of these methods are discussed.     

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.     

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.     

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。     

In-situ SEOP polarizer and initial tests on a high flux neutron beam

Polarized ³He has shown its unique characteristics in many areas of polarized neutron scattering, its ability to polarize neutrons at short wavelengths, accept wide-angle and divergent beams and low backgrounds enable new classes of experiments. While polarized ³He is not a steady state solution as commonly applied, the benefits have been shown to offset the drawbacks of polarizing and refreshing the polarization in the neutron spin filter cells. As an extension of this work, in-situ polarization using the spin-exchange optical pumping (SEOP) method was explored as a means to construct a system which could be used to polarize ³He in the state used for an effective neutron spin filter to constant polarization while on the neutron beam. An in-situ SEOP polarizer was constructed. This device utilized many devices and principles developed for neutron spin filters which are polarized off the beam line using either SEOP or metastability exchange optical pumping (MEOP) under the same research program. As a collimation of this work effects of extremely high neutron capture flux density incident on the in-situ polarizer were explored.     

The neutron interferometer as a macroscopic quantum device

Neutron interferometry is a unique tool for investigations in the field of particle-wave dualism because massive elementary particles behave like waves within the interferometer. The invention of perfect crystal neutron interferometers providing widely separated coherent beams stimulated a great variety of experiments with matter waves in the field of basic quantum mechanics. The phase of the spatial and spinor wave function become a measurable quantity and can be influenced individually. High degrees of coherence and high order interferences have been observed by this technique. The 4π-symmetry of a spinor wave function and the mutual modulation of nuclear and magnetic phase shifts have been measured in the past. Recent experiments dealt with polarized neutron beams, which are handled to realize the spin-superposition of two oppositionally polarized subbeams resulting in a final polarization perpendicular to both initial beam polarizations. The different actions on the coherent beams of static (DC) and dynamic (HF) flippers have been visualized.     

Dynamic concepts in neutron polarization

A new method of polarizing a neutron beam is proposed, which in contrast to conventional polarizers allows to make use of 100% of the incident intensity. It exploits the fact, that neutrons of different spin states differ in velocity after they have passed through an arrangement of crossed static and time-dependent magnetic fields. Provided the neutrons initially being sufficiently monochromatic, the two spin states can be separated in momentum space and be forced to point into the same spatial direction by means of a momentum-dependent spin rotation procedure. With presently available magnetic field technology (or nuclear pseudomagnetism) this dynamic method is applicable at continuous sources to polarize highly monochromatic thermal neutrons as those obtained by perfect crystal diffraction. The method seems to be particularly powerful, however, in the production of polarized cold and ultra-cold neutrons, where energy spreads of 1% and more are admissible. It is shown that at pulsed sources even polychromatic neutrons can be polarized with this technique.     

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.     

Investigation of neutron emission in the interaction of relativistic protons and deuterons with lead targets

Measurements of neutron double-differential cross sections and yields in time-of-flight experiments with various lead targets and beams of protons and deuterons with an energy of about 2 GeV are discussed. Space and energy distribution of neutrons in an extended lead target is studied by threshold detectors in the proton energy range of 1–3.7 GeV. The average multiplicity of neutrons, neutron energy, and neutron multiplication in lead as functions of target dimension, type, and energy of the beam particle are analyzed.     

Facility for parity and time reversal experiments with intense epithermal (eV) neutron beams

A facility for polarized epithermal neutrons of high intensity is set up at the Los Alamos National Laboratory for parityviolation and time reversal experiments at neutron resonances over a wide range of neutron energies. The beam is polarized with the aid of a polarized proton target used as a neutronspin filter. Total cross section measurements as well as capture gamma-ray experiments will be carried out. The main features of this system will be discussed.     

重离子加速器示范装置被动式束流配送系统次级中子特征的蒙特卡罗研究

在碳离子放射治疗中,碳离子束在剂量配送过程中会与束流输运线相互作用,形成以中子辐射为主的外辐射场.由于中子是高LET射线,具有较高的相对生物学效应,减少碳离子放疗中产生的次级中子有助于降低放疗后正常组织并发症几率及二次肿瘤风险.利用蒙特卡罗方法对保守情况(能量为400 MeV/u,多叶光栅完全闭合)下碳离子治疗被动式束...