Monochromatic neutron beam

The collimated, monochromatic neutron beam is delivered to the sample on a diffractometer, and diffraction is detected by an area detector (Chapter 4, Section III.C). The most common type is a multiwire area detector that uses helium-3 as the active gas, according to this reaction ... 

Inelastic neutron scattering, on the other hand, usually employs a monochromatic neutron beam and records the intensity of the scattered neutron beam as a function of neutron kinetic energy. Such inelastic collision spectra are monitored as a function of the applied field and the (usually low) temperature. The observed peaks then represent the energy differences of thermally populated and excited unpopulated multiplet states. Inelastic neutron scattering experiments can be conducted using triple-axis, backscattering, or time-of-flight spectrometers. 

Historically, all the early neutron sources were of the conventional reactor type, and a large monochromating crystal was commonly employed (for single-crystal diffraction experiments) to select neutrons with a small wavelength spread in order to produce a monochromatic neutron beam. The rest... 

In a SANS experiment a monochromatic neutron beam, intensity lo, is directed on the sample and scattered intensity I(Q) is measured as a function of angle 20, to the incident direction. Here Q is the momentum transfer (Q = 47Tsin0/ ). An important and recent development of the SANS technique concerns the investigation of materials which contain an oriented porous texture, such as fibres and layer-like materials [11-13]. 

In their mode of operation a monochromatic neutron beam, of wavevector k, is selected fi om the white incident beam, using Bragg reflection firom a single crystal, the monochromator. This beam illuminates the sample. In a selected direction from the sample a second single crystal, the analyser, is oriented such that only a given final wavevector, kf, is reflected onto a single detector. The choice of monochromator and analyser crystals depend on the energy transfer... 

Some of the alternative TOF instrument designs involve replacing the beryllium filter with either a crystal or a mechanical chopper to monochromate the incident beam. With this change, the spectrometer can be used with a higher incident neutron energy (typically E 50 meV) so that a smaller momentum transfer Q is possible for 5 the same energy transfer (21,22). With a monochromatic incident beam, a beryllium filter is sometimes substituted for the chopper after the sample in order to increase the scattered intensity but with a sacrifice in the,minimum Q attainable. Energy transfers up to 100 meV (800 cm" ) can be achieved with TOF spectrometers at steady state reactors before the incident neutron flux is limited by the thermal spectrum of the reactor. (With hot moderators such as at the Institut Laue-... 

Triple-crystal Spectroscopy (Fig. 2) — A collimated beam of thermal neutrons impinges on a monochromating crystal, iif- This crystal selects monochromatic neutrons of wavelength, by Bragg reflection... 

Using a monochromatic incident beam, the intensity for neutron scattering measured at each frequency (energy transfer ho = hu) depends on the orientation and magnitude of the final wave vector. In one dimension, the scattering function at momentum transfer Qx and energy transfer fuolj for a transition between states j(a )) and f(x)) can be written as ... 

In INS a beam of monochromatic neutrons is fired at the sample under the study. The magnetic interaction between the neutrons (spin 1/2 particles) and the sample leads to scattering of the neutrons, with energy gain and loss, inducing transitions within the sample with the selection rules A5 = 0, 1 and AMs = 0, 1. Thus, INS can give direct information not only on ZES interactions (as in EPR) but also inter-multiplet splittings,... 

In the time-of-flight spectrometer, shown schematically in Figure 8.12, the incident neutron beam is converted into pulses and, at the same time, monochromatized by... 

Crystal monochromators are also used to obtain monochromatic neutrons as well as to analyze the spectrum of electromagnetic radiation or neutron beams of unknown spectral composition. 

The Laue method, in contrast, applies a white beam of neutrons, containing a continuous spectrum of wavevectors the sample orientation is kept fixed. Bra reflections may be detected at several Bragg angles simultaneously. For quite a long period of time, Laue diffraction has mainly been used for the monochromatization of neutron beams (see O Sect. 29.5) lately, however, single-crystal diffractometers have been built that exploit this method. 

The simplest equation for the reaction rate produced by a parallel beam of monochromatic neutrons in an ideally thin and homogeneous sample can be written as follows ... 

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