Neutron velocity selector

X-Rays have the same velocity as light irrespective of the wavelength. The velocity of a neutron, on the other hand, depends on its wavelength and varies as hlmX, as given by Equation (1.8). Monochromatization of neutrons can therefore be achieved 

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V. I. Mostovoi, M. I. Pevsner, and A. P. Tsitovich, A Mechanical Neutron Velocity Selector, USSR, Proc. International Conference on the Peaceful Uses of Atomic Energy 4, 12 (1955). 

In neutron TOF experiments, the neutron source is a burst of neutrons generated either by a velocity selector (chopper) or by an ion beam, as explained later in this section. The time-of-flight t is the difference between the time of production of the neutron burst and the time of neutron detection. 

Pile fast choppers. Typical of such devices are the velocity selector systems for use with the Brookhaven, Argonne, and M.T.R. reactors. The high neutron... 

The polychromatic beam from the cold source is monochronaated by a helical slot velocity selector which selects neutrons of +1- 10% about a mean wavelength determined by the rotation speed of the drum. Typically, a mean wavelength of -0.525 nm is used. The neutrons are then collimated by a series of movable guides. Guide sections are inserted into, or removed from, the beam depending on the incident beam divergence required. A position-sensitive neutron detector is located inside an evacuated tube and can be moved to any distance between 1.1 and 35 m from the sample. 

Initial wavelength selection is done by a mechanical (rotating) velocity selector, since energy resolution is not an important issue here. Polarized neutrons may be produced by reflection by a magnetic multilayer mirror. Essential parts are the so-called flippers. These devices, consisting of flat coils, can change the direction of the neutron spin (which is extremely sensitive to external magnetic fields). 

We have performed preliminary measurements of the neutron flux at the end of the guide with a gold-foil technique. Measurements were made with the velocity selector set at three wavelengths, A = 6, 9, and 12 A with AA/A = 10% FWHM the results are shown in Figure 2, where the agreement between the measured results and the calculations is good at 9 A however, the difference be-... 

Fig. 6. Principle of pinhole SANS a neutron beam entering the instrument from the left is the subject of monochromation (done by the velocity selector), coUimation over a variable distance (achieved by using a set of adaptive system of apertures), scattering on the sample and detection on a two-dimensional position sensitive detector over long distances airning for detecting neutrons at small scattering angles the SANS instruments are very long.

Thermal neutrons from reactors have Maxwellian velocity distribution and need to be monochromatized before being allowed to hit scattering samples. Monochromatization is performed with (1) phased chopper velocity selectors, (2) helical slot velocity selectors, (3) crystal... 

The preceding theory is a special case of a general quantum-mechanical treatment of neutron cross sections. With high neutron-flux sources (reactors), and the development of velocity selectors (choppers, spectrometers) which permit measurements at chosen velocities, the cross-section values obtained are more accurate, but the methods much more complex. The above theory shows how the first cross-section measurements were made. The presentation of this simple method is further justified by the fact that many cross sections have been measured as averages in the same way, and for some materials this simple method is still very useful. 

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