Neutron backscattering spectroscopy with sub-μeV energy resolution has profited over recent years from intensity gains enabled by a phase space transformation (PST) chopper, which is a fast-moving neutron optical component first proposed by Schelten and Alefeld (Internal Report No. Jül1954, KFA Jülich, 1984). Here, we present its principle, the considerations for our technical layout, the related challenges, the mechanical and neutron optical aspects, and tests related to the graphite mosaic crystals, moving with a center velocity of 243 m/s in the scattering plane perpendicular to the reciprocal lattice vector of the reflection. The reported tests of the graphite crystal quality are informative for other neutron optical applications. Our mechanically innovative, most compact PST chopper layout has proven its reliability during user operation in the backscattering spectrometer IN16B, and certain aspects of its design have already been adopted for another backscattering spectrometer. We report the relative intensity gain measured on the backscattering spectrometer IN16B, ILL.

We present new developments in neutron backscattering spectroscopy from two major neutron scattering centers. The first example is the reactor backscattering project 1N16B, which is under construction at the Institut Laue-Langevin (ILL), Grenoble, France, the second is the spallation source near-backscattering spectrometer BASIS at the spallation neutron source SNS, Oak Ridge National Laboratory, USA, in operation since 2007. After explaining the energy resolution contributions to neutron backscattering and presenting the current status, we describe the layout, performance and the complementarity of the two most advanced backscattering projects. A few examples should illustrate the status and new experimental possibilities.