Neutron Spin Echo Spectrometer Research Articles

Optimal Larmor precession magnetic field shapes: application to neutron spin echo three-axis spectrometry

Resolution characteristics of neutron spectrometers using Larmor precession of the neutron spin are limited by magnetic field homogeneities of a special type. The line integral of the modulus of the magnetic induction along a neutron trajectory of length L is a measure of the amount of precession performed by the neutron. Hence it should be precisely the same for all neutron trajectories in a diverging beam. We present an analytical solution to the variational problem , for the case of cylindrical magnets (better than any lower symmetry geometry) coaxial to the beam axis. This solution describes the best irrotational (rot B = 0) field shape along the beam axis z. It can be obtained in practice by superposing a number of solenoids of different lengths. The optimal homogeneity is significantly better than for a simple solenoid of comparable dimensions, the only magnets used until now. For realistic lengths L and beam radii r, it is however not good enough for very high-resolution measurements. We therefore introduce a technique to correct both for the residual inhomogeneities of optimized cylinder magnets and the line integral variations due to path length differences resulting from finite angular beam divergence. Such corrections can only be done by introducing current distributions in the beam. Their optimal distributions can also be calculated analytically. Until now only the residual inhomogeneities have been corrected by in beam currents. With the two concepts of optimal field shape (OFS) and path length corrections described here, the resolution properties of Larmor precession techniques can be pushed to their intrinsic limits. As a further result of the correction technique introduced here, wider angular divergences can be used, for example using multi-detectors, resulting in substantially improved neutron economy. Several neutron spin echo (NSE) spectrometers based on above ideas have in the meantime been constructed by different working parties. The experimental results confirm the calculations reported here; no significant polarization drop is observed at maximum field. For the investigation of dispersive elementary excitations in solids where the neutron energy change depends on the momentum transfer, a special type of gradient coils is needed. We describe their design for OFS precession magnets.

A neutron spin echo spectrometer with two optimal field shape coils for neutron spin precession

We have designed and have been constructing at the C 2−2 cold neutron guide port of JRR-3M, JAERI, a neutron spin echo spectrometer (NSE) which is equipped with two optimal field shape (OFS) coils for neutron spin precession with the maximum field integral of 0.22 T m, an assembly of position sensitive detectors (PSD), a converging polarizer and a wide area analyzer. The dynamic range of scattering vector Q covers from 0.005 Å −1 to 0.2 Å −1 and that of energy ħω from 10 neV to 30 μeV. Performance tests of the OFS coils show that the inhomogeneity of the magnetic field integral in the OFS coils with the spiral coils is so small that the NSE signal amplitude decreases little even for the neutron cross section of 30 ⊘mm diameter as the Fourier time t increases up to 25 ns, though the precession coils are close to iron covers of the neighboring neutron guide. This verifies that the OFS precession coils are appropriate for this NSE spectrometer. Another test experiment shows that the homogeneity condition of the precession magnet is loosened by use of PSD.

Neutron spin-echo spectrometer at JRR-3M

We have designed and have been constructing at the C 2−2 cold neutron guide port of JRR-3M, JAERI, a neutron spin echo spectrometer (NSE). It is equipped with two optimal-field-shape coils for neutron spin precession, a position-sensitive detector, a converging polarizer and a wide-area analyzer. The dynamic range of scattering vector Q covers from 0.005 Å −1 to 0.2 Å −1 and that of energy E from 10 neV to 30 μeV. In a performance test of the precession coils, the NSE signal amplitude P NSE decreased slightly for the neutron beam cross-section of 30 φ mm as the Fourier time t increases up to 25 ns. We have carried out NSE experiments on complex fluid systems and obtained their diffusion coefficient D eff.

A time-of-flight neutron spin echo spectrometer

At present a new high-resolution neutron spin echo spectrometer, IN15, installed at the horizontal Cold Source of the HFR reactor at the Institut Laue-Langevin (ILL) in Grenoble, is tested with neutrons, being in its static mode. For a time-of-flight (TOF) operation of the spectrometer the velocity selector for the static mode will be replaced by an arrangement of four choppers, producing neutron pulses of variable time width [1]. Neutron tests for the TOF mode will start early in 1991. As can be seen from eq. (1), even for a relatively small wavelength band, )tmax/Ami n between 2 and 3, a large dynamic range is covered. Another aspect of this TOF mode is generally to gain experience for pulsed neutron sources. A broad time width ~of the pulses in the TOF mode corresponds to the broad incident wavelength band AA/A in static spin echo spectroscopy, which can be used because of the decoupling to energy resolution. This means a higher echo signal.

Low-field neutron spin echo observation and development of a neutron spin echo spectrometer

The results of neutron spin echo (NSE) experiments conducted in the direct beam geometry using the BeO filtered incident spectrum have been discussed in the light of the results published in the literature. The design details of the NSE spectrometer to be installed at the Dhruva reactor are reported.

Construction of optimized superconducting spin precession magnets for neutron spectroscopy

The design and construction of a superconducting magnet system for a high-resolution neutron spin echo spectrometer. The principal solution for the field shape of optimal precession magnets is B/sub 0/cos/sup 2/( pi z/L). In practical precession magnets, this field shape is approximated by 30 superimposed concentric solenoids with a bore of 80 mm. The required field integral of 1 Tm, providing 10/sup 4/ precession turns, is achieved in a magnet with a length of about 1.5 m. The field in the center is 1.5 T maximum. The relative line integral inhomogeneity of about 10/sup -3/ obtained with such a coil is improved to less than 10/sup -6/ by two in-beam correction coils. The advanced homogeneity level means that after 10000 precession turns the precession angle remains still well defined without using tedious correction procedures.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Inelastic and spin-echo neutron scattering studies of the dynamics in the quasi 2d planar antiferromagnet BaNi 2(PO 4) 2

We have performed accurate inelastic neutron scattering experiments on the quasi-2d planar antiferromagnet BaNi 2(PO 4) 2 using both conventional triple-axis and neutron spin-echo spectrometers. The behaviours of spin-wave and quasi-elastic contributions have been studied in the vicinity of the phase transition.

The multidetector neutron spin echo spectrometer IN11B

The first multidetector neutron spin echo spectrometer IN11B has been designed, constructed and measurements performed. The general features of the spectrometer are presented and the performance is illustrated by some measurements. It was shown that it is possible to acquire the real-time dynamic neutron scattering cross-sections simultaneously, for a wide range of momentum transfers, using a multidetector bank and an especially designed wide angle Larmor precession field magnet. This provides a method for following the temporal evolution of a scattering process remarkably rapidly with energy resolution comparable to that of the IN11 single detector spectrometer. The experimental results obtained with the IN11B spectrometer on the critical dynamics of the compound TbAl 2 in the vicinity of the Curie temperature are presented and discussed.

A neutron spin echo spectrometer using superconducting magnets

A neutron spin echo spectrometer using superconducting magnets has been built and tested for operation. The effect of the inhomogeneity of the magnetic field integral on the observability of the spin echo signal has been discussed on the basis of matrix analysis of neutron spin behavior and compared with the experimental results. In order to achieve an energy resolution of 10 neV, it is shown to be necessary to realize a relative inhomogeneity of the magnetic field integral as low as 10 −6.

A neutron resonance spin echo spectrometer for quasi-elastic and inelastic scattering

We propose a modified neutron spin echo (NSE) spectrometer using high frequency spin flippers to replace the static spin flipper and the long magnetic precession field in each arm of the classicalspin echo setup. The spectrometer is suitable for quasi-elastic as well as inelastic scattering and can work with an arbitrary magnetic field on the sample. The restrictions on sample size and scattering angle can be relaxed, in comparison with the classical NSE method.

Neutron spin echo: Ten years after

The principle of neutron spin echo (NSE) spectroscopy was discovered just 10 years ago — in April 1972 in Budapest — and the first fully fledged NSE spectrometer, IN11, has been in routine user operation since 1978 at the ILL in Grenoble. By now a whole variety of different types of applications have been successfully explored and well established. The resolutions achieved, such as 4 neV in quasielastic scattering and μ eV in phonon linewidth studies, represent more than an order of magnitude improvement with respect to other neutron scattering methods. In the present lecture the fundamental conceptual and practical aspects of NSE will be considered, together with a few representative experimental examples.

The neutron spin-echo spectrometer: A new high resolution technique in neutron scattering

Abstract The neutron spin-echo (NSE) spectrometar provides the highest energy resolution available in neutron scattering experiments. The article describes the principles behind the first NSE spectrometer (at the Institute Laue-Langevin, Grenoble, France) and, as an example of one of its applications, some recent results on polymer chain dynamics are presented.

Correlations and dynamics of polyelectrolyte solutions

The dynamics of polyelectrolyte solutions have been studied using a neutron spin echo spectrometer for wave vectors q around the maximum of the static structure factor (qm). The q dependent diffusion coefficient D(q) decreases sharply below q = qm, and is nearly constant above qm. All these features can be understood in terms of a new picture for the correlations, based on : i) the notion of a correlation hole, of diameter ξ = aO-1/2 ; ii) locally rigid chains with a persistence length b = aO-1 (where O is the polymer volume fraction and a the monomer size).

Neutron spin-echo and three-axis spectrometers

A detailed calculation is presented of the resolution properties of a three-axis neutron spectrometer equipped with a neutron spin-echo device. The calculation shows that the Fourier transform of the spectral density of an elementary excitation may be measured with a resolution which is substantially better than that available with a conventional three-axis spectrometer. In contrast to conventional spectrometers the spin-echo technique allows focusing of both longitudinal and transverse phonon modes.