Spin-echo spectrometer with time-of-flight option and focusing option
The neutron spin echo spectroscopy uses the Lamor precession of the spin of the neutron in a magnetic field as un extremely accurate stopwatch. Counting the number of precessions made by the neutron before and after the sample is a measure its velocity (energy) change inside the sample.
The animation shows the various steps of the measurement process, first for elastic scattering (no neutron-sample energy transfer), then for inelastic scattering (with energy transfer).
- A velocity selector selects a rather broad Δλ/λ bandwidth (typically 15%) from the incident polychromatic neutron beam.
- The monochromatic beam is then polarised by a supermirror polariser which stops the neutrons with the "wrong" spin component.
- The Larmor precession is initiated by passing through a "π/2 flipper" which flips the spins perpendicular to the main magnetic field.
- The neutrons are scattered by the sample. Elastic sample: neutron spins are not modified ; inelastic sample: neutron spins are more or less altered.
- Close to the sample, a "π flipper" transforms the precession angle α into -α.
- The Larmor precession goes on, and is finally stopped by a second "π/2" flipper.
- In the two precession areas defined on each side of the π flipper the field integrals are equal. Thus, the measure of the difference of precession angle in the two precession areas gives a measure of the inelasticity of the scattering process.
Compared to other neutron techniques, spin echo generally measures at lower energy transfer, making it possible to study slow dynamical processes in soft and condensed matter.