New Opportunities in Materials Research With Pulsed Neutrons
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laser sources. The second is that to produce neutrons a reactor or spallation source is needed, and these are both expensive and complex. Researchers must therefore travel to a central facility to conduct experiments. The first disadvantage, while still a considerable problem, is being addressed both by constructing more intense sources and by the design of new equipment. For example, position of sensitive detectors, focusing monochromators, and the advent of "insertion devices,"cold and hot moderators that tailor the neutron spectrum, have enormously enhanced the versatility of neutron research. Experiments can now be done on single crystals of a few milligrams and polycrystalline samples of perhaps 0.1 g. Larger samples are needed for inelastic scattering, but the information content may well be worth the effort. The second disadvantage has played a major part in making the neutron community (defined as the number of people actually participating in at least one neutron experiment per year) in the United States relatively small, approximately 600 scientists in 1985. However, once good equipment is provided and an open "user" system established, the number rapidly grows, as has been demonstrated by the European community, which now aproaches 1,400 scientists. The growth in demand for neutron facilities has also been demonstrated in this country. Almost from the startup of the Intense Pulsed Neutron Source (IPNS) in late 1981, its beam time has been oversubscribed. Nevertheless, this should not discourage the newcomer, as the program committees of all establishments routinely set aside beam time for both "test" experiments and innovative new ideas. Attendance at schools and user meetings is another way to become familiar with the possibilities of the different neutron facilities. Pulsed Sources A spallation source such as IPNS produces short bursts of neutrons with a pulsewidth of 10-100 jus, depending on the neutron energy, at an interval of 30 ms. The wavelength of each neutron is determined by its time-of-flight (tof) from the source to the detector. The longer this distance the better the separation of wavelengths, i.e., the resolution. At IPNS, distances of up to 20 m are used. This gives a resolution of Ad/d ~ 3 x 10~4 in powder diffraction, where d is the d-spacing between atomic planes. Descriptions of how a spallation source works are given by Carpenter etal., 1 and will not be repeated here. In any case, the materials scientist need not be
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concerned with this aspect. Experimental methods are also described in Ref. 1, and in a book by Windsor. 2 In this article, a few examples of science performed at IPNS are described. They have been chosen to illustrate the power of both neutrons and the new spallation sources, 3 and it is hoped that they will interest a number of materials scientists in using these sources. Diffraction for Structural Studies Experiments that determine the average spatial position of atoms (or molecules in the case of polymers) are elastic i
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