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.     

Research on glass cells for He neutron spin filters

Glass cells play an important role in polarized ³He neutron spin filters. To evaluate the scattering and absorption contribution from glass cells during neutron scattering experiments, we measured small-angle scattering and neutron transmission in GE180 and other glasses. The small-angle neutron scattering measurements revealed that the glasses used for ³He spin filters have acceptably lower scattering: dΣ(Q)/dΩ=4-7×10⁻⁴ cm⁻¹ at Q=0.03-0.12 Å⁻¹. The transmission measurement was performed at J-PARC. Neutron transmission of about 92% through empty GE180 cells was observed over a wide wavelength range 0.014-7.0 Å. To pursue the possibility of being a structural influence on ³He spin relaxation in GE180 glass cells, we performed precise X-ray diffraction measurement using synchrotron radiation at SPring-8. From these measurements, a structural difference was observed among GE180 glasses with different thermal treatments.     

α-碘酸锂单晶在外电场作用下中子衍射增强的各向异性

在四圆中子衍射仪上对α-碘酸锂单晶在外加直流电场作用下的中子衍射强度增大和各向异性作了系统的测定。对测定结果的分析表明:在场强不大时,衍射的增强及其各向异性可以用经典的各向异性消光理论得到较好的说明。 关键词：     

Refinement of hydrogen positions in (NH4)2SeO4

The crystal structure of ammonium selenate has been studied by means of single crystal x-ray and neutron diffraction with the purpose of the refinement of hydrogen positions. The refined hydrogen positions obtained by single crystal neutron diffraction show that N-H bond lengths form a regular tetrahedron in an ammonium ion. The single crystal x-ray diffraction data show that N-H bond lengths are shorter than those obtained by neutron diffraction and are not equal between themselves. Thus, the comparison of the results of x-ray and neutron diffraction allows one to suggest that the shorter N-H bond lengths obtained by x-ray diffraction reflect the distribution of the electron charge density of ammonium ions within the (NH₄)₂SeO₄ crystal lattice. The text was submitted by the authors in English.     

Neutron scattering length of lithium and boron and their isotopes

Coherent neutron scattering lengths and free cross sections were measured in order to determine the spin dependent scattering lengths of the isotopes of Li and B. The transmission measurements on Li at neutron energies of 0.51 meV, 1.26 eV and 5.19 eV delivered data for the⁶Li-abundances in the samples and for the free scattering cross section of⁷Li. By means of experiments on 15 different samples of lithium compounds we obtained the complex spin state scattering lengths for the bound atoms to be: (ie81-01) Measurements on 3 boron samples with different enrichments led to (ie81-02) A review on slow neutron scattering and resonance data shows an over all consistency of all values.     

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.     

Polarised neutron diffraction study of Mn1.09Sb

In order to study the influence of interstitial site occupancy on the electronic structure of MnSb, a polarised neutron diffraction experiment was performed on Mn_1.09Sb. The composition of the sample was obtained from the refinement of single crystal neutron diffraction data. The polarised neutron diffraction experiment yielded unpaired electron populations of the levels a^t_1g, e_g^t and e_g as follows: μ_x0 = 0.57 μ_B, μ_x± = 1.60 μ_B, μ_u± = 1.28μ_B. The comparison of these numbers with results of Yamaguchi et al shows that the spin density for this composition is reduced for the low energy level a^t_1g. This finding is interpreted by an occupation of minority spin states by electrons of the interstitial Mn atoms. In agreement with this, increasing Mn excess leads to a decreasing magnetic moment magnitude/Mn-atom and to a contraction of the lattice parameter c, indicating the strengthening of the Mn-Mn bond in [001].     

Effect of neutron spin rotation at Laue diffraction in a deformed transparent crystal with no center of symmetry

The effect of neutron spin rotation at Laue diffraction in a noncentrosymmetric weakly deformed neutron-transparent crystal has been studied experimentally. The effect appears because of two phenomena: (i) the interaction of the magnetic moment of a moving neutron with the crystalline electric field and (ii) bending of trajectories of the neutron in a deformed crystal; as a result, the amplitudes of two waves excited at Laue diffraction, which propagate in the crystal in opposite electric fields, at the output of the crystal are not equal.     

Test of time reversal invariance in 2-MeV neutron scattering from aligned165Ho

We describe a search for the time reversal non-invariant five-fold correlation (s·Ixk)(I·k) in the scattering of 2 MeV polarized neutrons from an aligned¹⁶⁵Ho target. Here s is the neutron spin,I is the target spin, andk is the neutron momentum vector. Differences in transmission were sought when s was reversed. The target was a single crystal of holmium, cryogenically aligned with a specially built dilution refrigerator. Rotating the target axis of alignment while cold permits the parity-even, five-fold correlation to be identified among other spin-dependent correlations in the forward elastic scattering amplitude. An analyzing power was found to be (1±6)×10⁻⁴, consistent with time reversal invariance.We obtain a bound of 5×10⁻³ for α_T, the ratio of T-odd to T-even couplings in the effective nucleon-nucleon interaction.     

Goldanskii-Karyagin effect in α-SnF2: A neutron diffraction and Mössbauer absorption study

Single crystal neutron diffraction data have been obtained at several temperatures for α-SnF₂. These data were used to evaluate the Goldanskii-Karyagin effect expected for the¹¹⁹Sn transition, and compared with the effect measured directly by means of the Mössbauer absorption in powdered samples, free of preferential orientation. These results provide further support for Karyagin's theory. The values of the anisotropy parameterg ₁₁ obtained from the neutron diffraction data are somewhat lower than those obtained from the Mössbauer absorption.     

Method of spin assignment of bound levels populated by (n, γ) reactions

The conditions are examined under which the spin of low-lying levels may be deduced from a comparison of the probabilities of populating such levels with γ-cascades starting from neutron capture states with different spins. To test this spin-assignment method low-energy γ-ray spectra from neutron capture in single resonances of¹⁰⁵Pd and ¹⁷⁷Hf have been measured, and the relative intensities of γ-transitions from low-excitation levels have been determined. For each level the ratio between its populations from two different initial spin states is obtained and is found to provide a firm indication of the level spin. The experimental values of the population ratios are compared with the values obtained by a general formula based on simple statistical assumptions and by more realistic Monte Carlo calculations of the γ-cascade. The perspectives of the method are discussed.     

Edge localized modes of cold neutrons in periodic condensed media

It is found that for certain energies of discreet cold neutrons, quasi-stationary eigen solutions of the corresponding Schrodinger equation, which are localized in the layer of a periodic medium, exist. The localization time of these solutions is strongly dependent on the layer thickness, being finite for a finite layer thickness and increasing indefinitely upon a infinite growth of the layer thickness as the third power of the layer thickness. The problem has been solved in the two-wave approximation of the dynamic diffraction theory for the neutron propagation direction coinciding with the periodicity axes (normal incidence of the neutron beam on the layer). The expressions for neutron eigenwave functions in a periodic medium, the reflection and transmission coefficients, and the neutron wavefunction in the layer as a function of the neutron energy incident on the layer have been determined. It turns out that for the certain discrete neutron energies, the amplitudes of the neutron wavefunction in the layer reach sharp maxima. The corresponding energies are just outside of the neutron stop band (energies forbidden for neutron propagation in the layer) and determine the energies of neutron edge modes (NEMs) localized in the layer, which are direct analogs of the optical edge modes for photonic crystals. The dispersion equation for the localized neutron edge modes has been obtained and analytically solved for the case of thick layers. A rough estimate for the localization length L is L ~(db N)^–1, where b is the neutron scattering length, d is the crystal period, and N is the density of nuclei in the crystal. The estimates of the localized thermal neutron lifetime show that acheaving of a lifetime close to the free neutron lifetime seems nonrealistic due to absorption of thermal neutrons and requires a perfect large size crystal. Nevertheless, acheaving the localized neutron lifetime exceeding by ~10⁴ times the neutron time of flight through the layer appears as experimentally attainable. The perspectives of the NEM observation are briefly discussed. It is proposed to use NEM for ultrahigh thermal neutron monochromatization by means of NEM excitation in perfect single crystals.     

Single crystal diffraction with X-rays and neutrons: High quality at high pressure?

Abstract

Single crystal X-ray and neutron diffraction is essential for determining occupancies, positional as well as static and dynamic displacement parameters in crystalline matter by measuring Bragg, satellite or diffuse reflections. In our contribution a new low-temperature high pressure cell for neutron single crystal diffraction will be presented. It is designed to operate from a few K to ambient temperature in “orange” cryostats at pressures up to at least 3 GPa. We will present first neutron diffraction results obtained at E4/HMI Berlin and discuss the quality of the data and the significance of the results. A software package has been written (as a part of the PROMETHEUS system), which deals with the data reduction for both X-ray and neutron high pressure cell single crystal data. Likewise a data collection program has been developed for single crystal data collection on four-circle diffractometers using Merrill-Bassett cells. A series of single crystal experiments on H₂O and D₂O ice VI and KDP (KH₂PO₄) show that results of very high quality can be obtained routinely including even higher order terms in the atomic displacement parameters.     

Spin distribution factors for formation of metastable states of 90Y and 91Y through (n,p) reactions over neutron energies 5–15 MeV

The variations in the isomer ratio with the spin distribution factor were studied to obtain the cross-sections for formation of metastable states of ⁹⁰Y and ⁹¹Y nuclei formed respectively through ⁹⁰Zr(n,p)⁹¹Y^m and ⁹¹Zr(n,p)⁹¹Y^m reactions over the neutron energy range 5 to 15 MeV. The theoretical values of the cross-sections could be obtained very close to the respective literature experimental values by varying the spin distribution factor from 1.5 to 2.75. The limiting value of the cross-section was derived from the maximum value of the isomer ratio at that neutron energy, and for this the spin distribution factor was found to vary from 2 to 3.5 over the neutron energies 5 to 15 MeV. Using enriched zirconium targets, cross-sections for formation of ⁹⁰Y^m and ⁹¹Y^m nuclei at 14.8 MeV neutron energy were also measured in the laboratory; the values obtained are 17 ± 2 mb and 22 ± 2 mb respectively for ⁹⁰Y^m and ⁹¹Y^m. Both these values are close to the corresponding limiting cross-sections at 14 MeV neutron energy.     

Studies of parity and time reversal symmetries in neutron scattering from165Ho

We describe searches for parity and time reversal violations in the scattering of polarized neutrons from polarized and aligned¹⁶⁵Ho targets. We have completed a search with 7.1 and 11.0 MeV neutrons for P_oddT_odd terms in the elastic scattering forward amplitude of the form s. (I×K), wheres is the neutron spin,I is the target spin andk is the neutron momentum vector. The target was a single crystal of holmium, polarized horizontally along itsb axis by a 1 Tesla magnetic field. The neutrons were polarized vertically. Differences in the neutron transmission were measured for neutrons with spins parallel (antiparallel) toI×k. The P,T violating analyzing powers were found to be consistent with zero at the few 10⁻³ level: ρ_P,T(7.1 MeV)=−0.88 (±2.02) x 10⁻³, ρ_P,T(11.0 MeV)=−0.4 (±2.88) x 10⁻³. We have also attempted to find enhancements with MeV neutrons in P-violation due to the term s\k. We are preparing an aligned target cryostat for investigations of P_evenT_odd terms {bd(I\k)(I×k)\s} in neutron scattering. The target will be a single crystal cylinder of¹⁶⁵Ho cooled to 100 mK in a bath of liquid helium and rotated by a shaft from a room temperature stepping motor. The cylinder will be oriented vertically and the alignment (c) axis oriented horizontally. Warming or rotation of the sample allows one to separate effects that mimic the sought-after time reversal violating term.     

Applications of He neutron spin filters at the NCNR

At the NIST Center for Neutron Research (NCNR), we have applied ³He neutron spin filters (NSFs) to the instruments where ³He NSFs are advantageous, such as thermal triple-axis spectrometry, small-angle neutron scattering, and diffuse reflectometry. We present the status of our development and application of this method, including polarized gas production by spin-exchange optical pumping, magnetostatic cavities for storage of the polarized gas on the beam line, and nuclear magnetic resonance (NMR)-based, on-line monitoring and reversal of the ³He polarization. We present the status of developing user-friendly interfaces incorporated into the instrument software to handle these ³He neutron spin filters while taking data and performing data analysis. Finally we discuss the status of development of a polarization capability on the multi-axis crystal spectrometer, which requires polarization analysis over a 220° angular range.     

Inhomogeneous lattice distortions in the Zn0.99Cd0.01Se crystal

The structural state of a Zn_0.99Cd_0.01Se crystal at temperatures of 78 and 300 K has been investigated using thermal neutron diffraction. It has been found that the diffraction patterns of the crystal contain diffuse scattering regions in the vicinity of the basic structure reflections, which are independent of temperature. It has been shown that the effects of diffuse scattering are caused by transverse displacements of lattice atoms, which are induced by impurities of the Cd²⁺ ions whose sizes substantially exceed the sizes of the Zn²⁺ ions. The obtained results have been used to construct a picture of the influence of cadmium ions on the structural state of the crystal under study.     

Magnetic structure of a CaMnO2.75 crystal with ordered oxygen vacancies

The magnetic state of a CaMnO_{3 ? δ} crystal with ordered oxygen vacancies (for δ = 0.25, when the numbers of Mn⁴⁺ and Mn³⁺ ions in the manganite are equal to each other) is studied using neutron diffraction. Magnetic scattering in the CaMnO_2.75 crystal in the ground state is determined by the wave vector (1/2, 1/2, 1/2)2π/a _c (G-type antiferromagnetic order). In the crystal, long-range magnetic order disappears at the temperature T _N = 116 K, whereas short-range magnetic order is retained up to 240 K. It is shown that the instability of the G-type structure in the temperature range 60 K < T < T _N is associated, in many respects, with the formation of the C′ antiferromagnetic phase in the bulk of the crystal. The structure of the C′ antiferromagnetic phase involves chains with Mn³⁺-Mn⁴⁺ ferromagnetic interaction. A comparison of the results of the neutron diffraction investigations with the experimental data on the magnetic characteristics and electrical resistivity demonstrates that the specific features revealed in the spin system of the CaMnO_2.75 crystal are governed directly by the competition of the Mn³⁺-Mn⁴⁺ ferromagnetic double exchange with the antiferromagnetic superexchange between manganese ions.     

T-violation neutron experiment and polarized3He

An experimental scheme of the spin detailed balance in polarized neutron transmission through a polarized nuclear target is discussed for the T-violation test. The value of the spin detailed balance is estimated by using the data of the scattering amplitudes. The nuclear polarizations of proton,³He and¹³⁹La are discussed for the T-violation experiment.     

Magnetic and crystal structure of copper(II) fluoride

A powder sample of the monoclinic, weak ferromagnet CuF₂ was investigated by neutron diffraction. Using the profile method, the crystal structure was refined and the spin configuration determined. The layer type structure with planar quadratic fluorine coordination of Cu²⁺ (3d⁹) and the magnetic structure are remarkably similar to those of AgF₂. The spin configuration is however different from the magnetic structures of other 3d-fluorides; the magnetic unit cell is doubled with respect to the chemical cell (a_m = 2a, P2₁/c). The ordered magnetic saturation moment corresponds to quenched orbital momentum.