Neutron spin echo

Neutron spin echo spectroscopy is an inelastic neutron scattering technique invented by Ferenc Mezei in the 1970's, and developed in collaboration with John Hayter. In recognition of this work and in other areas, Mezei was awarded the first Walter Haelg Prize in 1999.

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The technique of neutron spin echo exploits the neutron's intrinsic angular momentum, or spin, to access extremely high-resolution inelastic scattering windows.

The core of a neutron spin echo instrument is a symmetric field integral around the sample position, and a spin flipper (or the sample itself) to reverse the spin direction, so that any loss in polarisation at the detector position can be directly attributed to inelastic scattering processes in the sample.

Because of the interference of the wavevectors associated with the spin up and spin down quantum states of the neutron, the measured neutron polarisation is proportional to the sample's correlation function in real time. This makes it a very useful and intuitive technique for the high-resolution study of low-energy excitations in materials.

In soft matter research the structure of macromolecular objects is often investigated by small angle neutron scattering, SANS. The exchange of hydrogen with deuterium in some of the molecules creates scattering contrast between even equal chemical species. The SANS diffraction pattern --if interpreted in real space-- corresponds to a snapshot picture of the molecular arrangement. Neutron spin echo instruments can analyze the inelastic broadening of the SANS intensity and thereby analyze the motion of the macromolecular objects. A coarse analogy would be a photo with a certain opening time instead of the SANS like snapshot. The opening time corresponds to the Fourier time which depends on the setting of the NSE spectrometer, it is proportional to the magnetic field (integral) and to the third power of the neutron wavelength. Values up to several hundreds of nanoseconds are available. Note that the spatial resolution of the scattering experiment is in the nanometer range, which means that a time range of e.g. 100ns corresponds to effective molecular motion velocities of 1 nm/100ns = 1cm/s. This may be compared to the typical neutron velocity of 200..1000 m/s used in these type of experiments.

Existing Spectrometers

• IN11 (Institut Laue-Langevin,ILL, Grenoble, France)
• IN15 (Institut Laue-Langevin,ILL, Grenoble, France)
• J-NSE (Juelich Centre for Neutron Science JCNS, Juelich, Germany, hosted by FRMII, Munich (Garching), Germany)
• NG5-NSE (NIST CNRF, Gaithersburg, USA)
• V5/SPAN (Hahn-Meitner Institut, Berlin, Germany)
• C2-2 (ISSP, Tokai, Japan)

See also

• neutron scattering
• neutron resonance spin echo