Neutron spin echo
The spin echo spectrometer possesses an extremely high energy resolution (roughly one part in 100,000).
Other neutron scattering techniques measure the dynamic structure factor S(Q,ω), which can be converted to F(Q,t) by a Fourier transform, which may be difficult in practice.
Because of its extraordinary high effective energy resolution compared to other neutron scattering techniques, NSE is an ideal method to observe[2] overdamped internal dynamic modes (relaxations) and other diffusive processes in materials such as a polymer blends, alkane chains, or microemulsions.
The time differences occur due to a velocity change acquired by non-elastic scattering at the sample.
The distribution of these time differences is proportional (in the linearization approximation which is appropriate for quasi-elastic high resolution spectroscopy) to the spectral part of the scattering function S(Q,ω).
The time parameter depends on the neutron wavelength and the factor connecting precession angle with (reciprocal) velocity, which can e.g. be controlled by setting a certain magnetic field in the preparation and decoding zones.
It is important to note: that all the spin manipulations are just a means to detect velocity changes of the neutron, which influence—for technical reasons—in terms of a Fourier transform of the spectral function in the measured intensity.
The main reason for using NSE is that by the above means it can reach Fourier times of up to many 100ns, which corresponds to energy resolutions in the neV range.
The closest approach to this resolution by a spectroscopic neutron instrument type, namely the backscattering spectrometer (BSS), is in the range of 0.5 to 1 μeV.
The spin-echo trick allows to use an intense beam of neutrons with a wavelength distribution of 10% or more and at the same time to be sensitive to velocity changes in the range of less than 10−4.
[citation needed] In soft matter research the structure of macromolecular objects is often investigated by small angle neutron scattering, SANS.
Many inelastic studies that use normal time-of-flight (TOF) or backscattering spectrometers rely on the huge incoherent neutron scattering cross section of protons.
The scattering signal is dominated by the corresponding contribution, which represents the (average) self-correlation function (in time) of the protons.
However, in pure cases, i.e. when there is an overwhelming intensity contribution due to protons, NSE can be used to measure their incoherent spectrum.
For systems containing hydrogen, contrast requires the presence of some protons, which necessarily adds some amount of incoherent contribution to the scattering intensity.
Fully protonated samples allow successful incoherent measurements but at intensities of the order of the SANS background level.
