Quasielastic neutron scattering: An advanced technique for studying the relaxation processes in condensed matter

Definition of the Topic Quasi-elastic neutron scattering (QENS) is a part of more general inelastic neutron scattering and is a very powerful technique to explore the motions in biomolecules, polymers, simple liquids, alloy melts, and soft matter in general. In this chapter, we will discuss basic theoretical aspects of QENS, instrumentation, types of motion in liquids and solids, understanding these motions from the QENS data, and recent studies using QENS. Overview The QENS is a versatile technique to study the relaxation processes in the condensed materials, particularly in liquids. This technique typically uses cold neutrons as the energy of the cold neutrons is in a similar range with relaxation process in liquids and the wavelength is of the order of the interatomic distance in condensed materials. The QENS techniques can provide much more information on the relaxation process (motions) as compared to other techniques, e.g., dielectric spectroscopy, nuclear magnetic resonance, and tracer diffusion measurements. The QENS technique is particularly helpful for understanding the microscopic dynamics of soft and biomaterials since these materials contain a large number of hydrogen atoms, and the hydrogen atom is very sensitive to neutron scattering due to a large scattering cross section. Neutron scattering offers not only QENS but also other scattering techniques like diffraction, inelastic, reflectometry, triple axis, small-angle scattering, and neutron imaging. However, facilities available for neutron scattering research are very few in the world. The production of neutrons is very expensive and typically requires either a nuclear reactor or a target-based source and also detection of neutrons need materials like³He, which is rare on earth, are the reason for it. Over the last few decades, few neutron scattering facilities like Spallation Neutron Source, National Institute of Standards and Technology (NIST) Center for Neutron Research (NCNR), Australian Neutron Scattering Facility, Forschungsreaktor Munchen (FRM)-II Germany, ISIS-UK, and JAPARK at Japan have been built. Also, there has been tremendous improvement in neutron scattering instrumentations, particularly building of new backscattering and spin echo spectrometers in the abovementioned facilities. At present relaxation process in a time scale of 0.1 ps-350 ns can be measured using neutron scattering. In the beginning, QENS was mainly used to study the diffusion in simple liquids and hydrogen diffusion, in metals, etc. At present the QENS technique has been used in many fields of science ranging from colloids, polymers, ionic liquids, hydrogen storage, food processing, biotechnology, and environments. In this chapter we will provide an overview of basic theoretical understanding of QENS, concepts of instrumentation, data analysis, identification of transport mechanism from QENS data, and recent research results using QENS.