Page 69 - Neutrons for Sciences and Society

Chapter 3 - Why invest so much money in a source of neutrons?
E as a function of wave vector Q, which need to be analysed
in each direction in the sample crystal. Measuring the neutron energy change after interaction with phonons can explore these dependences. There are numerous ways of performing these measurements which I will briefly describe a little further on. When the sample is a liquid the internal motions are described
by a probability distribution G(r,t) which gives the probability
of finding an atom at time t at a distance r from its position at time zero. In 1954, Léon Van Hove showed55 that the angular distribution and energy of neutrons scattered by a liquid is related simply to G(r,t). This theoretical work had a major impact on the use of neutrons for studying what is now known as condensed matter.
The simplest way to measure inelastic neutron scattering, in principle, is to use a monochromator crystal to select a neutron beam of a given energy. Neutrons of a specific energy are reflected in a direction defined by Bragg’s Law:
λ = 2d.sinθ
where d is the distance between the selected lattice planes and
λ a certain wavelength (hence specific energy). After scattering
by the atoms of the sample under study (also usually in the form of a single crystal) the final neutron energy is selected by a last reflection from an analyser single crystal. This method, known as triple axis spectrometry (Fig. 3.1 and Fig. 3.2) was developed by Bert Brockhouse56. He showed that it was possible to measure the neutron intensity scattered as a function of the energy transfer E
55 Leon Van Hove, Phys. Rev. (1954), 95, p 249-262, DOI 10.1103/PhysRev.95.249
56 B.N. Brockhouse (1955) Phys. Rev., 98, 1171, M7, DOI 10.1103/PhysRev.98.1144. This very short note gives
what is probably the very first description of a three-axis-spectrometer.
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