Filter Difference Spectrometer | FDS

Chemical Spectroscopy and Protein Structures

The Filter Difference Spectrometer (FDS) is used for molecular vibrational spectroscopy by inelastic neutron scattering. The instrument is designed for high count rates by use of large solid-angle detectors for the neutrons scattered by the samples. It is therefore most useful for measurements requiring high sensitivity; for example, very dilute systems or molecules adsorbed on surfaces such as in catalysts. Other applications of this instrument include hydrogen-bonded molecular crystals, biomolecular model systems, and some high explosives. While most experiments are done on hydrogen-containing molecules, it is also possible to study molecular vibrations involving other atoms; for example, O in CMR (colossal magnetoresistance) materials.

The figure below shows a polarized neutron reflectometry measurement taken from a 1 cm² sample using a Displex cryostat.

Vibrational spectra are obtained by neutron energy loss. Incoming neutrons excite molecular vibrations by giving up energy upon scattering from the sample.

The FDS determines the change in energy by the use of low-energy band-pass filters (Be or BeO) between the sample and detector. These filters will only allow neutrons of a certain energy to reach the detector.

The scattered neutrons are sorted by their total time of flight as they journey from the source to the detector. From this information, their incident energies can then be determined. Data reduction is accomplished by either variable numerical deconvolution of the spectrometer response function or maximum-entropy methods. he instrument's energy resolution can thereby be varied as needed and ranges from 2% to 5% of the energy transfer.