The past, present and future of inelastic neutron scattering
25 Sep 2017
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- Emma Cooper

 

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The use of electron-volt neutron spectroscopy in materials research is a growing area of neutron science, capitalizing upon the unique insights provided by epithermal neutrons on the behaviour and properties of an increasing number of complex materials.

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​​The VESUVIO detectors in front (d-m) and backscattering (c), incident (a) and transmitted (b) monitors. The sample is the blue slab at the centre. Credit: VESUVIO webpage.
 

​Professor Carla Andreani, recipient of the 2016 Giuseppe Occhialini Medal and Prize for her “outstanding contributions to novel experimental techniques and methods in neutron spectroscopy and her tireless commitment to fostering the British–Italian collaboration in neutron science" has published two reviews in Advances in Physics, which capture the rapid progress in the field.

 

Prof. Andreani published her first review in 2005, giving a detailed description of the theoretical foundations of electron-volt neutron spectroscopy and their application to fundamental systems. It noted that the technique has undergone a remarkable development since the first intense fluxes of epithermal neutrons were made available from pulsed neutron sources in the mid-1980s, introduced the theoretical framework for the interpretation of deep inelastic neutron scattering experiments, and reviewed experimental studies performed on a variety of condensed matter systems, ending with a brief consideration of foreseeable developments.

 

Catering for both experts and novices, this year's review starts by presenting the general principles underpinning the technique and discussing recent conceptual and methodological developments. For example, the technique is increasingly used as a non-invasive spectroscopic probe with intrinsic mass selectivity, and is being used concurrently with neutron diffraction. Recent case studies have been chosen to highlight the use of electron-volt neutron spectroscopy across physics, chemistry, biology, and materials science. As in the first review, the important contributions of VESUVIO are prominently highlighted.

 

Understanding and exploiting the properties of new and increasingly complex functional materials is an intrinsically interdisciplinary effort across physics, chemistry, biology, materials science, and engineering. Neutron scattering using thermal and cold neutrons is a well-established non-invasive technique, used extensively to provide direct and unique information on the physical and chemical properties of materials at the atomic level. It can offer important insights into global challenges, including finding clean and sustainable sources of energy. Neutrons are particularly well suited for studying the transport and binding of energy- and charge-carrying atoms and molecules, both in situ and in operando. Neutron-based analysis is now a routine technique for the characterization of hydrogen-storage, fuel cell, catalysts and battery materials.

 

The review focuses on neutron-scattering experiments using epithermal neutrons of up to hundreds of eV for Deep Inelastic Neutron Scattering (DINS). At ISIS Neutron and Muon Source these studies began on the pioneering eVS spectrometer, which has been superseded by VESUVIO in the last decade. VESUVIO is an inverted-geometry neutron spectrometer,  designed to measure Nuclear Momentum Distribution (NMD) using eV neutrons. VESUVIO has been in operation since 2001, and offers neutron energies up to two orders of magnitude higher than any other neutron instrument currently supporting a science programme.

 

DINS offers direct access to the quantum character of atomic motions in a mass-resolved manner, which distinguishes it from other neutron techniques at low energies. The final section of the review is devoted to an intense area of DINS research - water and aqueous media. Water exhibits a number of unique and anomalous physical and chemical properties in both bulk states and under confinement. The structural and dynamical properties of water play a central role in many phenomena of relevance to fundamental and applied science, and theoretical studies of aqueous media constitute one of the main fields of interest in physics and chemistry.

 

The future for DINS is bright, with the possibilities for the technique remaining practically boundless, and not limited by the complexity of target materials. There is much work to be done in exploring the correlations between DINS observables and other material properties, for which the ability to perform parametric DINS studies is a prerequisite.

 

Prof. Andreani wraps up her review by observing that further progress in the discipline will require continued developments in instruments, and supports the development of an experimental station for Epithermal and Thermal Neutron Analysis, ETNA. Alongside other enhancements, ETNA could provide a quantum leap in capacity and capability, allowing epithermal neutrons from short-pulse spallation sources such as ISIS Neutron and Muon Source ​to be exploited at their full potential for years to come.

 

 Further information

Andreani, C. et al. Electron-volt neutron spectroscopy: beyond fundamental systems. Advances in Physics 66 (2017). DOI:10.1080/00018732.2017.1317963

 

Andreani, C. et al. Measurement of momentum distribution of light atoms and molecules in condensed matter systems using inelastic neutron scattering. Advances in Physics 54, 5 (2005). DOI: 10.1080/00018730500403136

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