A new proof-of-concept neutron backscattering spectrometer is so sensitive to tiny changes in energy that even the neutrons’ potential energy due to gravity matters. The spectrometer, located at Institut Laue–Langevin in Grenoble, France, was devised by ILL scientist Bernhard Frick, Andreas Magerl of Friedrich–Alexander University of Erlangen–Nuremberg in Germany, and colleagues. Its energy resolution, 78 neV, is an order of magnitude sharper than a typical instrument of its type and a factor of four sharper than any currently available.
Neutron backscattering spectroscopy measures the minute shifts in energy imparted to neutrons when they scatter off a sample. Those shifts can result from sample resonances, such as phonons or hyperfine excitations, or from random molecular motions such as diffusion.
To select neutrons with a particular kinetic energy, neutron backscattering spectrometers use Bragg reflection off a crystalline surface. As shown in the figure, incoming neutrons first reflect off the temperature-controlled monochromator, then scatter off the sample, and finally reflect off the analyzers, which are made of the same crystalline material as the monochromator. Neutrons reach the detector only if their energies satisfy the Bragg condition at both the monochromator and the analyzer—that is, if the energy gained or lost in the sample matches the energy difference between the monochromator and analyzer’s Bragg peaks. Tuning the monochromator’s temperature allows researchers to select for neutrons with different energies: Warmer crystals have larger lattice spacing, so they reflect less energetic neutrons.
Ever since the advent of the technique in 1969, neutron backscattering spectrometers have used the (111) surface of silicon for their monochromators and analyzers, and their energy resolution has been limited by the Si(111) reflection’s intrinsic linewidth. The (200) reflection of gallium arsenide has a 10-fold narrower linewidth, and GaAs fabrication is now sufficiently advanced for a large-area perfectly crystalline analyzer to be feasible.
Building a GaAs(200) spectrometer is a lot more involved than piecing together some commercially available GaAs wafers. The researchers had to control and account for several other resolution-limiting effects, including facet alignment, beam geometry, crystal strain, and the effect of gravity. All else being equal, neutrons scattered from the sample toward the top of the analyzer end up with measurably less energy than those scattered toward the bottom. To compensate for gravity’s influence, the researchers introduced a thermal gradient such that the bottom of the analyzer is 10 K cooler than the top. (K. Kuhlmann et al., Rev. Sci. Instrum. 90, 015119, 2019.)
