Neutron laue diffraction in a weakly deformed crystal at the Bragg angles close to π/2

An essential magnification of an external force acting on a diffracting neutron for the Bragg angles θ_B close to the right one is observed. Any external action (caused by either crystal deformation or external force affected the neutron) results in a bend of the so called “Kato trajectories” inside the crystal and, for the case of a finite crystal, gives considerable variation of the intensities of both diffracted neutron beams (direct and reflected). It is shown that the magnification factor is proportional to tan² (θ_b) and can reach (10²−10³) for Bragg angles surfficiently close to 90°. The text was submitted by the authors in English.     

Depolarization of a neutron beam in Laue diffraction by a noncentrosymmetric crystal

The depolarization of a neutron beam executing Laue diffraction in a thick (～3.5 cm) noncentrosymmetric α-quartz crystal is observed. This effect was predicted by us earlier and suggested for measuring the electric dipole moment (EDM) of a neutron. The effect is due to an interaction of the magnetic moment of a moving neutron with a strong crystal electric field, as a result of which the neutron spin rotates in opposite directions for waves of two types excited in the crystal. The effect is studied for neutron diffraction by a system of crystallographic (110) planes at Bragg angles close to π/2, up to 87°. It is shown that, for a crystal of thickness L=3.5 cm, a direct beam initially polarized along the reciprocal lattice vector becomes depolarized upon diffraction, irrespective of the value of Bragg angle, whereas the beam polarized perpendicular to the diffraction plane retains its polarization. The Eτ value determining the sensitivity of the method to EDM is experimentally estimated.     

Direct measurement of the delay time for a neutron in a crystal in the case of the Laue diffraction

The dynamical Laue diffraction has been studied for a direct beam diffracted from a thin (～3.5 cm) α-quartz crystal at the Bragg angles close to 90°. It is shown that diffraction occurs at Bragg angles up to 87°. The time-of-flight method is used to measure the time of the neutron presence in the crystal under the diffraction conditions. The time delay for a scattered neutron inside the crystal predicted earlier for the Bragg angles close to 90° confirmed experimentally. The effective velocity of the neutron propagation in a crystal measured at the incident-neutron velocity of 810 m/s and the diffraction angle of 87° equals (43±1) m/s. The result obtained confirms the possibility of increasing, by an order of magnitude, the sensitivity of the diffraction method of determining the dipole moment of a neutron at Bragg angles close to 90° predicted earlier theoretically.     

Diffraction enhancement effect and new possibilities of measuring the electric charge of the neutron and its inertial-to-gravitational mass ratio

Diffraction enhancement of small effects affecting a neutron undergoing Laue diffraction at Bragg angles θ _B close to 90° is predicted and experimentally observed. The enhancement is due to the delay of the neutron inside the crystal during diffraction and is proportional to tan² θ _B. As a result, the diffraction enhancement factor may be as large as ～10⁸–10⁹. On this basis, a new method is proposed for searching for the electric charge of the neutron and for measuring the ratio of its inertial mass m _i to the gravitational mass m _G . It is shown that the accuracy of the neutron charge measurement can be improved by more than two orders of magnitude in relation to the present-day accuracy and that the ratio m _i /m _G can be measured to an precision of σ(m _i /m _G ) ～ 10^?6.     

Anomalous behavior of the dispersion of a neutron transmitting through a crystal at nearly Bragg energies

The features of the propagation of a neutron through a crystal at nearly Bragg energies has been studied within the framework of the preparation of an experiment on the search for the electric dipole moment of a neutron by the crystal diffraction method. The time of passage of the neutron through the crystal has been studied as a function of the deviation from the Bragg condition. The anomalous behavior of the dispersion of the neutron, i.e., the energy dependence of its average velocity, has been observed. It has been shown that the derivative dv/dE for the diffracting neutron at nearly Bragg energies can be three or four orders of magnitude larger than this derivative for a free neutron. This opens new possibilities in precision neutron spectroscopy.     

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.     

Program of Fundamental-Interaction Research for the Ultracold-Neutron Source at the the WWR-M Reactor

The use of ultracold neutrons opens unique possibilities for studying fundamental interactions in particles physics. Searches for the neutron electric dipole moment are aimed at testing models of CP violation. A precise measurement of the neutron lifetime is of paramount importance for cosmology and astrophysics. Considerable advances in these realms can be made with the aid of a new ultracold-neutron (UCN) supersource presently under construction at Petersburg Nuclear Physics Institute. With this source, it would be possible to obtain an UCN density approximately 100 times as high as that at currently the best UCN source at the high-flux reactor of the Institute Laue–Langevin (ILL, Grenoble, France). To date, the design and basic elements of the source have been prepared, tests of a full-scale source model have been performed, and the research program has been developed. It is planned to improve accuracy in measuring the neutron electric dipole moment by one order of magnitude to a level of 10^?27 to 10^?28e cm. This is of crucial importance for particle physics. The accuracy in measuring the neutron lifetime can also be improved by one order of magnitude. Finally, experiments that would seek neutron–antineutron oscillations by employing ultracold neutrons will become possible upon reaching an UCN density of 10³ to 10⁴ cm^?3. The current status of the source and the proposed research program are discussed.     

High-resolution gamma-ray spectroscopy

To obtain the highest possible resolution in a measurement has always been one of the major challenges for experimental physicists because increased resolution generally results in progress. At the Institut Laue-Langevin, gamma rays emitted after neutron capture can be recorded with parts-permillion resolution. This is achieved by diffracting the gamma rays on highly perfect Sior Ge crystals. Precise measurement of the Bragg angles and the crystal lattice spacings permits the determination of wavelengths or energies. This outstanding resolving power allows the measurement of extremely small Doppler effects caused by the emission of primary gamma rays. These so-called gamma-ray-induced Doppler broadening measurements have given rise to applications in both nuclear and condensed matter physics.     

静电场作用下KDP和TGS单晶中子衍射增强现象的物理机理

本文提出了KDP和TGS单晶在静电场作用下中子衍射增强现象的物理机理。KDP和TGS都是质子导电晶体。外加直流电压后,晶体中电场有一空间分布。由于压电效应,相关的衍射晶面间距是空间坐标的函数,造成中子衍射增强。对衍射晶面间距为空间坐标函数的情况,导出了透射方式和反射方式的中子衍射强度公式,估计了非均匀的压电效应引起的中子衍射增强的量级,和实验相符。并对进一步验证上述机理提出了新的实验建议。顺便指出,对于α-LiIO₃,在静电场作用下其中子衍射增强,非均匀压电效应不是主要因素。并提出其可能的微观机制是:在非均匀电场作用下,缺陷(包括杂质)在晶体中有一空间分布,引起中子衍射增强。 关键词：     

An X-ray interferometer with Bragg case beam splitting and beam recombination

A new type of X-ray interferometer which uses Bragg case transmission for beam splitting and for beam recombination is described. The principles and special problems of this interferometer are discussed and a strict plane-wave treatment of the interference phenomena is presented. The feasibility of such a device is demonstrated by some interference patterns, obtained with an instrument made from a silicon crystal for use with copperKα radiation and the 220 Bragg reflection. Since, for the operation of the Bragg case interferometer, strong absorption is not essential, such devices may also be more suitable than the previously reported Laue case interferometer^1,2 for use with neutrons.     

On measuring the neutron coherent scattering length with ultrahigh precision

We propose an order of magnitude improvement in the present five parts in 10⁵ precision of a nondispersive interferometric measurement of the neutron coherent scattering lengthb _c. For this purpose we make a judicious selection of the Bragg angle for the interferometer and the sample thickness. The precision is further improved by an optimal choice of the Bragg reflection (and a consequent neutron wavelength). By performing the experiment in vacuum, errors arising from possible variations in the pressure, composition or humidity of the ambient air can be eliminated. On attaining such precision, we ought to account for the neutron beam refraction at the sample-ambient interfaces, to infer the correctb _c from the observed phase. The formula for the phase used hitherto is approximate and would significantly overestimateb _c. The refractive index for neutrons can thus be determined to a phenomenal precision of a few parts in 10¹².     

Antiferromagnetic domains in nickel oxide by magnetic neutron laue diffraction

Two kinds of domains had been observed in antiferromagnetic nickel oxide: T-domains, associated with the four 〈111〉 propagation vectors and S-domains, associated with the three equivalent antiferromagnetic directions within a T-domain. Taking advantage of the direct interaction of neutrons with the arrangement and direction of magnetic moments, the T and S antiferromagnetic domain distributions are investigated in several single crystal samples by magnetic neutron Laue diffraction (MNLD). The T-domain behaviour when a stress is applied is qualitatively investigated. In white beam a direct topographic observation and identification of T-domains are presented. By simultaneously measuring the relative intensities of magnetic Bragg reflections from a sample with preferential domain population it is possible to confirm the direction of sublattice magnetization which had been obtained by an optical method. The S-domain distribution is studied as a function of the magnetic field on the one hand, and as a function of time on the other. This last phenomenon is very similar to the well-known magnetic after-effect which occurs in most ferromagnets. It is concluded that MNLD could be a very valuable technique to investigate many topics of magnetism.     

Spectrometric pairs on a unique single-crystal basis in experiments on small-angle neutron scattering

Spectrometric pairs have been fabricated on a unique basis in various combinations of scattering geometry—according to Bragg, Laue, and their combination—to develop and improve the technique for applying a double-crystal spectrometer from a highly perfect single-crystal Si ingot. Possible schemes of their using in combination with a crystal-analyzer in experiments on small-angle neutron scattering have been proposed.     

Studies of shrinkage as a result of holographic recording in acrylamide-based photopolymer film

We studied the shrinkage in acrylamide-based photopolymer by measuring the Bragg detuning of transmission diffraction gratings recorded at different slant angles and at different intensities. Transmission diffraction gratings of spatial frequency 1000 lines/mm were recorded in an acrylamide-based photopolymer film having 60±5 μm thickness. We have obtained the grating thickness and the final slant angles from the Bragg curve and hence calculated the shrinkage caused by holographic recording. The shrinkage of the material was evaluated for three different recording intensities 1, 5 and 10 mW/cm², while the total exposure energy was kept constant at 80 mJ/cm². From the experimental results it can be seen that the shrinkage of the material is higher for recording with lower intensities and the corresponding values are 1.9%, 1.3% and 1%.     

X-ray point- and line-projection microscopy and diffraction

A theoretical framework is presented to treat both imaging and diffraction experiments performed with point-focus and line-focus X-ray sources, with particular emphasis on two-dimensional and planar X-ray waveguides. In particular, point-projection and line-projection microscopy has been approached within the Huygens-Fresnel formalism; point-projection and line-projection diffraction, such as spatially-resolved Bragg/Laue diffraction of crystalline samples in a regime of dynamical scattering, has been treated both by means of the Huygens-Fresnel formalism and of the Takagi-Taupin dynamical theory. Both in point- and line- projection geometry, simply rotating the investigated crystalline samples, it is possible to switch from Fresnel self-imaging to Bragg/Laue diffraction conditions. This means to image, within the same experiment, either morphological features, with a sub-micrometric resolution, out of the exact diffraction condition, or the structure order on an atomic scale if placing the sample in diffraction.     

Comparison of two experiments on radiative neutron decay

Over 10 years ago we proposed an experiment on measuring the characteristics of radiative neutron decay in papers [1, 2]. At the same time we had published the theoretical spectrum of radiative gamma quanta, calculated within the framework of the electroweak interactions, on the basis of which we proposed the methodology for the future experiment [3, 4]. However, because we were denied beam time on the intensive cold neutron beam at ILL (Grenoble, France) for a number of years, we could only conduct the experiment in 2005 on the newly opened FRMII reactor of Technical University of Muenchen. The main result of this experiment was the discovery of radiative neutron decay and the measurement of its relative intensity B.R. = (3.2 ± 1.6) × 10⁻³ with C.L. = 99.7% for radiative gamma quanta with energy over 35 kev [5, 6]. Over a year after our first announcement about the results of the conducted experiment, “Nature” [7] published a letter asserting that its authors have also measured the branching ratio of radiative neutron decay B.R. = (3.13 ± 0.34) × 10⁻³ with c.l. = 68% and gamma quanta energy from 15 to 340 kev. This article aims to compare these two experiments. It is shown that the use of strong magnetic fields in the NIST (Washington, USA) experiment methodology not only prevents any exact measurement of the branching ratio and identification of radiative neutron decay events, but also makes registration of ordinary neutron decay events impossible.     

Determining the magnetic ground state of TbNi5 single crystal using polarized neutron scattering technique

The TbNi₅ compound shows an interesting magnetic phase transition with an incommensurate structure below 23 K, whose true nature remains unresolved. In order to solve this question, we have carried out polarized neutron diffraction experiments by measuring temperature and field dependence of the intensities of satellites and Bragg reflections. From the temperature dependence of both satellite peaks and Bragg reflection, we demonstrated that it has only one magnetic structure at a given temperature. Furthermore, unlike previous reports, we found that both ferromagnetic and modulated components of the Tb ion magnetic moments should be collinear to each other. Our data also show strong depolarisation effects that are most likely to arise from domain structure of ferromagnetic component. A critical field, which destroyers a modulated magnetic structure is found to be lower than a field value to saturate the ferromagnetic component, in which the intensity of Bragg ferromagnetic reflections reaches saturation.     

Polarized neutron diffraction from dynamically and statically polarized protons

Polarized neutron diffraction studies of Nd³⁺ doped La₂Mg₃(NO₃)₁₂·24H₂O single crystal with satically and dynamically polarized protons were made. The observed flipping ratios of Bragg reflections for each polarization method agree with the calculated ones from the X-ray determined atomic positions and the proton polarizations determined by enhancement factors from NMR monitoring.     

Spectral beam combining of fiber lasers based on a transmitting volume Bragg grating

Based on the principle of spectral beam combining using a transmitting volume Bragg grating, a simple experimental setup was designed, in which an approach to control the incidence angles of beams using a focusing lens is presented. The experimental results for the spectral beam combining of two fiber lasers are reported. The gain media used in this experiment were large-mode area Er³⁺/Yb³⁺ co-doped double-clad fibers, pumped by semiconductor lasers. The output powers of the two fiber lasers were 0.39 and 0.53 W, respectively. With a grating diffraction efficiency of about 60%, a maximum combined power of 0.64 W with an absolute combining efficiency of about 69.6% was achieved.     

Ternary fission yields of 241Pu(nth,f)

Ternary events in the thermal neutron induced fission of ²⁴¹Pu(n,f) were studied with the recoil separator LOHENGRIN at the high-flux reactor of the Institut Laue Langevin in Grenoble. Yields and energy distributions could be determined for most isotopes of the elements hydrogen to oxygen. Also several heavier nuclei up to ³⁰Mg could be observed. Yields were measured for 42 isotopes, for further 17 isotopes upper limits could be deduced. For the first time the halo nuclei ¹¹Li, ¹⁴Be and ¹⁹C were found in neutron induced fission with yields of some 10⁻¹⁰ per fission.