Introduction to QENS

Quasi-elastic neutron scattering (QENS) is a technique which is used to measure dynamic processes in solid materials such as plastic, ceramics, gels and many more. In QENS we achieve information about atoms moving in the sample (via e.g. diffusion) through incoherent scattering. This is in contrast to diffraction where we achieve information about the atomic (static) structure of the sample through (elastic) coherent scattering.

The name quasi-elastic refers to scattering events within materials where the energy of the neutron changes only slightly by delivering (or receiving) energy to/from the sample. If the energy change of the neutron is bigger, the process is called inelastic scattering. Sometimes the neutrons scatter from collective movements (as opposed to random movements) of atoms in the sample. This type of scattering is called coherent inelastic scattering, but we will not go in detail with that here.

The neutron energy losses (negative energy transfers) we observe in a QENS experiment occur because the neutrons push atoms in the sample which then start to move and gain momentum, but at the same time the neutron loses momentum and thus energy. On the other hand, neutrons can also gain momentum if pushed by atoms in the sample which we see as a positive energy transfer (neutron energy gain). This means we can measure the movement of atoms or molecules in a sample by looking at the change in energy of the neutrons.

In order to find the energy change of the neutrons during scattering at the sample we of course need to know the energy of the neutron before and after the sample scattering. You can learn more about how this is done in practice with monochromators, analysers and choppers by following the links.

In the present QENS experiment the neutron energy after the sample is selected by an analyser to a fixed energy. The range of energies in the neutron beam before the sample is selected by a chopper-system. Some of those neutrons will be scattered quasi-elastically by the sample and thereby have or get the correct energy to pass the analyzer and finally reach the detector.

The chopper-system is also used to set the energy resolution of the instrument which is needed for each particular experiment. In general terms you need good energy resolution to observe slow movements in the sample whereas you can have a relatively poor energy resolution and still observe rapid movements of atoms in the sample