O. Uca

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.

Model calculations for the spin-echo small-angle neutron-scattering correlation function

Spin-echo small-angle neutron scattering (SESANS) is a new kind of SANS technique enabling measurements to be made directly in real space from a range of a few nanometres up to micrometres. In this paper it is shown by calculations on models that SESANS measures correlations directly. Furthermore, the effect of polydispersity and structure factor has been studied. An exact expression for the correlation function has been derived in the case of random systems, such as fractal systems.

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.

Technical Aspects of Larmor Precession with Inclined Front and End Faces

Some technical and physical features of Larmor precession techniques will be discussed. Various options to encode the transmission angle of the neutron beam by inclined front and end faces using DC fields are considered, under which magnetized foils and wedge shaped precession regions. The use of shaped pole faces as precession regions to avoid material in the transmitted beam are considered together with correction methods for the inhomogeneous field line integrals accompanied by those magnetic fields. It appears that the use of π flippers as occurring in the resonance method are of great advantage.

Line integral corrections in spin-echo small angle neutron scattering instrument

A good polarization of the neutron beam is required for the spin-echo small-angle neutron scattering (SESANS) instrument. SESANS is a novel kind of small-angle neutron scattering (SANS) technique. The measured quantity in a SESANS experiment is the depolarization of the neutron beam due to elastic scattering by a sample. The amount of precession of a neutron along a path is proportional to the line integral of the magnetic field intensity along this path. The inhomogeneities in the magnetic field intensity due to fringing of the magnetic field lines near the magnet edges give rise to line integral differences. This causes extra depolarization that decreases the sensitivity of the instrument. The requirement on the homogeneity of the line integral is calculated. In first order, the line integral increases quadratically towards the magnet poles. This quadratic increase is compensated by correction coils which have an opposite quadratic dependence.

An analysis of magnetic field inhomogeneities in a spin-echo small-angle neutron scattering instrument

Abstract In a spin-echo small-angle neutron scattering instrument homogeneity of the magnetic field is of great importance. The maximum allowable variation of the magnetic field is calculated using a model whereby an inhomogeneity only in the vertical direction is taken into account.

Magnetic design of a spin-echo small-angle neutron-scattering instrument

In a spin-echo small-angle neutron scattering instrument dipole magnets and guide field coils are used. The homogeneity of the fields should be sufficient to have linear labeling of the height with precession. Furthermore, the instrument must have a homogenous line integral over the beam cross-section. It is shown that line integral inhomogeneities are directly connected to field components perpendicular to the main field. The design parameters of these magnetic units of the setup are calculated.