W.H. Kraan

Spin-echo small angle neutron scattering in Delft

We describe two spin-echo instruments for neutron small angle scattering, which have been installed at the reactor institute in Delft. The first setup is using a monochromatic beam and magnetized foils as spin flippers, while the second uses resonant spin flippers in a pulsed neutron beam. The components that play an essential role for operation are described in some detail. Each setup has specific advantages in its range of spin-echo lengths that covers the range of correlation lengths that could be measured. This is demonstrated in a comparative measurement, the setup with magnetized foils measuring at spin-echo-lengths up to 20 μm and the setup with resonant flippers measuring in the range up to 0.5 μm.

Development of spin-echo small-angle neutron scattering

A polarised neutron spin echo technique is used to build a novel kind of small angle neutron scattering (SANS) instrument. The basis of this instrument is a symmetric set-up with a spin flipper in the centre, which creates a spin echo, even with a divergent beam. The precession regions on either side of the spin flipper are shaped such as to produce a very sensitive relation between the vertical angle of the neutron path and the total precession angle. Any SANS of a sample placed in the instrument reduces the symmetry of the neutron path and therefore decreases the echo. Magnetised foils define the precession regions by rotating the neutron spin from being parallel to the magnetic field to perpendicular to the field, to start the precession. These foils and the flipper were built and tested. A spin echo SANS signal is measured with the complete set-up . It should be possible with this technique to measure within minutes a full correlation function in samples over distances from 5 to 1000 nm.

Construction and testing of a multichannel polariser for thermal neutrons

A curved polarising multichannel neutron guide with total beam cross section 26 × 56 mm2, containing 20 channels was produced at PNPI. The concave sides have a supermirror coating consisting of the alloys Co64Fe34V2 and Ti60Zr40, giving θcλ = 34(±0.8) mrad/nm. The convex side is a natural Ni layer with θcλ = 17.5 mrad/nm. Both sides have an antireflecting underlayer based on an alloy of TiZr and Gd. All coatings were produced by sputtering. The characteristic bending angle for which the system is designed is 5.79 mrad giving a characteristic wavelength equal to 0.17 (±0.05) nm. The transmission and polarisation as functions of λ were measured at IRI by means of “Lannor precession spectroscopy” and the “3 polariser 2 shim method”, respectively. The transmission is equal to the theoretical value within 10%; the polarisation is 0.98 at λ = 0.16 nm decreasing to 0.75 at λ = 1.0 nm.

Elastic Neutron Scattering Measurements Using Larmor Precession of Polarized Neutrons

An overview will be given of new instruments using Larmor precession of polarized neutrons in precession regions with inclined front and end faces. These instruments concern small angle scattering, neutron reflectometry and high-resolution diffraction. The advantages of the first application, spin echo small angle scattering (SESANS), with respect to conventional SANS, is the range of applicability and orders of magnitude higher available intensity. The reflectometry application makes it possible to measure the momentum dependent intensity without hindrance of the waviness of the sample also with the high intensity of SESANS. The high resolution application enables one to measure very high resolution diffraction (10−5 relative in momentum space) without angular of wavelength confinement of the beam, thus with very high intensity.

Magnetized foils as π flippers in neutron spin-echo spectrometry

In neutron spin-echo spectrometry Larmor precession regions are used, in which the homogeneity of the field line integrals along the neutron paths play an essential role. In the application of this technique for small-angle scattering, the transmission angle encoding happens by strong precession gradients in a certain direction in a magnetic field. Here the use of magnetized foils as π flippers will be discussed. They appear to compensate very efficiently for field line inhomogeneities and moreover they create an effectively strong gradient in Larmor precession, needed for the small-angle scattering application. The π flippers consist of magnetized foils of the proper thickness, magnetized by the field of the precession devices itself. Application of such π flippers in a spin-echo small angle neutron scattering setup with fields perpendicular and parallel to the beam direction is discussed. It appears that such flippers avoid corrections for the inhomogeneity of the field line integrals for the main part ...

Larmor precession applications: magnetised foils as spin flippers in spin-echo SANS with varying wavelength

A new development in SESANS instrumentation will be discussed. Magnetised foils as π-flippers for neutron polarisation were discussed in previous papers as powerful tools for constructing a strong gradient in Larmor precession in some direction. Up to now, such foils could only be used with limited flipping power and only for monochromatic neutrons. In this paper, it will be shown that it is possible to use foils also as nearly perfect flippers and moreover that such foils can be used also for varying neutron wavelength.

Adiabatic rotators for 3-D neutron polarisation analysis

Abstract A set of two polarisation rotators designed according to the principle of adiabatic rotation of the polarisation vector of a polychromatic thermal neutron beam is described. The adjustment of the polarisation vector along the axes of the laboratory coordinate system is examined by 3-dimensional polarisation analysis over the λ range of 0.15–0.55 nm. The spectra needed for this analysis are obtained by means of the method of Larmor precession followed by Fourier transformation.

Neutron Larmor precession and energy analysis

Abstract Some applications of neutron Larmor precession as an energy analyser are presented. The spectral analysis of a white beam demonstrates the use as a transmission spectrometer. Higher order contamination in a monochromatic beam can be analysed by measuring only a few field periods. Larmor precession in combination with an other energy analysing system seems to be an attractive combination for an inelastic spectrometer of the inverted geometry type. The possibility of measuring neutron polarization in parallel and antiparallel alignment of the polarizer and analyser gives an effective intensity gain up to a factor 4 in comparison with mechanical chopper systems.

Spin-echo small-angle neutron scattering to study particle aggregates

A new method to measure spin-echo small-angle neutron scattering using π-flipping magnetised foils has been realised. This method makes it possible to measure in real space structural feature up to length scales of 2.5 μm. Measurements on concentrated colloidal systems reveal directly the evidence for short-range ordering.

Separating the polarising power from depolarisation in a set-up with 3 neutron polarisers

Abstract A method of separating the polarising power of neutron polarisers from the depolarisation in a set-up composed of three neutron polarisers, the so-called “three polariser two shim method”, is described. It appears that, using this method, the net polarising power of the second polariser, i.e. without any depolarisation effects between the polarisers, can be determined. Also the application to magnetic Bragg scattering experiments is discussed.