Neutron chopper

At Brookhaven National Laboratory and at Kernforschungszentrum Karlsruhe, small computers performed data-handling chores on a timesharing basis. The Brookhaven system provided data collection and sorting functions for two independent neutron time-of-flight experiments 44). At the BNL Graphite Research Reactor in 1963, an SDS 910 computer with a 4096 24-bit memory, paper tape equipment, typewriter, and priority-interrupt system served as both a 1024 channel time analyzer for fast-neutron chopper total and partial cross-section experiments and a 256-channel time analyzer for slow-neutron chopper experiments. Control... 

The extrapolation length (in the diffusion approxima-tiem) for polyethylene has been determined directly from asymptotic neutron-spectrum measurements using the pulsed-etectron-linac phase-neutron-chopper technique. The neiitrpn spectrum is sampled 3.4 mean life times (344 p sec) after ibe initial burst of neutrons from the linac to insure that the dominant mode is the time asymptotic mode. 

The TOF method is applied to elastic and inelastic scattering instruments. In both cases, neutron choppers are used to produce pulsed beams for time of flight experiments. TOF instruments use time of flight to give a continuous wavelength scan at one or many fixed angles. 

The experiment discusses the time-of-flight technique for the selection of neutrons of definite energy ranges from a neutron spectrum. The Fermi slow neutron chopper is described and is used to measure the reactor beam spectrum, as well as the total neutron cross section of several materials as a function of velocity. 

If the neutron bursts are produced by a rotating shutter of a neutron chopper (Fig. 30.1), the velocity ranges in each channel depend on three variables ... 

The Fermi slow-neutron chopper with its electronics, which is used in this experiment, utilizes the time-of-flight technique for selection of neutrons of small energy ranges out of a beam of neutrons of all energies. The rotating shutter of the chopper, shown in cross section in Fig. 30.11. 

In the case of a fast neutron chopper, an additior of a chopper over the crystal monochromator is the constam ing rate with energy for the former in the region of dE/E sp Reference 8). Also, neutron resonances are measured main chopper (see Reference 4). 

In order to determine the dynamics of atoms we have to carry out an inelastic neutron scattering measurement. With a reactor source this can be done with a triple-axis spectrometer, which has an analyzer crystal. Tripleaxis refers to the three axes for the monochromator, sample, and analyzer, all moving independently and controlled by a computer. With a pulsed source we use a mechanical chopper, which is a rotating cylinder with a hole perpendicular to the rotating axis that allows neutrons with a chosen range of velocity to go through. The neutrons scattered by the sample are detected... 

Neutrons with a wavelength of 5 to 10 A are often used. A narrow range of wavelengths is usually selected to give a quasi-monochromatic beam. This can be achieved by a crystal monochromator or, more usually, by a velocity-chopper system of rotating discs which select neutrons of a given velocity. 

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-... 

To obtain S(Q,a>) from the measured data it is necessary to know the instrumental flight paths, the energy of the incident neutrons and the angles of the detectors. The incident energy is found from the beam monitors. The first monitor is normally placed before the Fermi chopper to monitor the incident flux for the purposes of normalisation. The... 

M. Arai, M. Kohgi, M. Itoh, H. Iwasa, N. Watanabe, S. Ikeda Y. Endoh (1989). ICANS X Proceedings of the Tenth International Collaboration On Advanced Neutron Sources Held at Los Alamos 3-7 October 1988, Institute of Physics Conference Series, Institute of Physics, Bristol, 97, 297-308. Development of a chopper spectrometer at KENS... 

The full spectral form is seen in the data obtained on a chopper spectrometer, see Figs. 9.18 and 9.19 [68]. The continuum is a very broad response that tracks the unit-mass recoil line and is by far the strongest spectral component. Especially since the unbound scattering cross section of hydrogen, 20 bam ( 2.1), should be used in calculations of this effect. Analysis of this spectmm has proved very difficult because the width of the response. Fig. 9.19, far exceeds conventional predictions. Since neutrons cannot determine the electrical nature of the scatterer directly H" ", H , or H are all possibly present. 

Figure 2.12 (a) Schematic diagram illustrating the principle of neutron wavelength selection on a continuous neutron source by use of mechanical choppers and the neutron time of flight, (b) Curved slot rotor another design for neutron velocity selection. 

Figure 8.13 Schematic view of a back-scattering spectrometer. The neutrons incident from the neutron guide are back-scattered by the monochromator mounted on a Doppler drive, deflected by a graphite crystal to the sample, scattered to the analyzers and then back-scattered again to the detectors located close to the sample. The chopper interrupts the beam and makes it possible to discriminate the neutrons scattered directly into the detectors.

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. 

Fermi s chopper was a slow chopper, the word slow referring to the speed of the neutrons, and was used for neutrons up to 1 eV. Fast choppers have also been developed for use with neutron energy up to the keV range, with the rotating cylinders of the chopper having different design, depending on the requirements of the measurement. 

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