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1. Introduction

Neutrons used in diffraction work are usually produced in a nuclear reactor by the fission of some heavy nucleus, such as ${}^{235}_{\phantom{0}92}$U. Neutrons released in this reaction have a kinetic energy of about 5 MeV, corresponding to a de Broglie wavelength of about $1.26 \times 10^{-4}$ Å. To make these fast neutrons suitable for diffraction work, they have to be slowed down until their de Broglie wavelength becomes of the same order as the separation of atoms in condensed matter, i.e. about 1 Å. This is achieved by letting the neutrons pass through a moderator in which they gradually lose energy through a series of elastic collisions with the nuclei of the moderator. If the moderator is sufficiently thick, the neutrons emerging from it will have a Maxwellian energy distribution, their average kinetic energy being ${3\over 2}KT$ where K is the Boltzmann constant and T is the absolute temperature of the moderator. For a moderator at room temperature--i.e. for $T \approx 300$ K--this gives an average kinetic energy of about 0.04 eV, corresponding to an average neutron wavelength of about 1.5 Å (thermal neutrons).

Since the thermal neutrons emerging from the moderator form a divergent beam with a continuous wavelength distribution, whereas diffraction studies require parallel beams of neutrons with a well-defined single wavelength, the thermal neutrons have to be monochromatized and collimated first before they can be allowed to fall on the specimen under study.

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