Backscattering Spectrometer

View inside the BAIS tank

View inside the BASIS tank.

BASIS is a near-backscattering, crystal-analyzer spectrometer that provides very fine energy resolution, as low as 3.0 to 3.5 µeV at the elastic peak (depending on sample size). This requires a long initial guide section of 84 m from moderator to sample in order to achieve the timing resolution necessary for obtaining the desired energy resolution. BASIS provides an excellent dynamic range near the elastic peak of about plus and minus 100 µeV in the standard high-intensity operation regime, which, if needed, could be extended to plus and minus 200 µeV and beyond. The spectrometer is optimized for quasielastic scattering but provides about 0.1% resolution in energy transfers up to ~40 meV; the inelastic excitations need to be very sharp in order to be measurable at high energy transfers.

For detailed information about BASIS, please refer to "A time-of-flight backscattering spectrometer at the Spallation Neutron Source, BASIS," Mamontov, E., and Herwig, K. W., Review of Scientific Instruments 82, 85109 (2011).


BASIS can be used to probe dynamic processes in various systems on the pico- to nanosecond time scale. It is well suited for probing diffusive and relaxational motions but can also be effectively used for studying some types of collective excitations in condensed matter. Applicable fields of study include, but are not limited to, biology, polymers, small molecules, complex fluids, magnetism, materials science, ionic conductors, catalysts, H storage materials (functional energy-related materials), and low-energy spin excitations.