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COPYRIGHT 2002 Science Reviews Ltd.
The last century has seen a large development in diffraction techniques. The time-of--flight neutron diffraction method is now so advanced that it provides high precision results for position and thermal parameters, which are complementary to other diffraction results from X-ray sources. Here we review the history of neutron sources, the difficulties encountered with the time-of-flight technique and an outlook for applications. In this context, we will show the limitations of existing neutron sources and the expected advantages of new spallation neutron sources. An overview of all corrections to be taken into account with the wavelength--sorting technique will be presented as well as actual results, how to overcome such problems, and the special difficulty of integration of three-dimensional Bragg peaks.
Introduction
In 1994, Clifford G. Shull was awarded the Nobel prize for the development of the neutron diffraction technique in the late 1940s at the first nuclear reactor in the USA. Together with Bertram N. Brockhouse he was honoured for helping answer the question of where the atoms "are" and what the atoms "do" (Nobel prize citation). Neutron scattering techniques have since developed considerably and in the past few years neutrons have been used to an increasing extent for studying the structure (arrangement) and dynamics (movement) of solid and fluid matter. Studies at the high flux reactor of the Institut Laue Langevin (ILL), for instance, include both the structure and the dynamics of the new ceramic superconductors (Nobel Prize 1987 to Bednorz and Muller), molecule movements on surfaces of relevance to catalytic exhaust cleaning, the structure of viruses and how these defend themselves against dehydration, and the connection between the ordered and the non-ordered structures of polymers and their elastic properti es (Nobel Prize 1991 to de Gennes).
Research with neutrons became so important due to the unique properties of the neutron: The wavelengths are similar to atomic spacings. Neutrons allow us to see light atoms (e.g. hydrogen) in the presence of heavier ones, and to distinguish neighbouring elements. The neutron's magnetic moment is ideally suited to the study of microscopic magnetic structure. Neutrons are non-destructive, even to complex, delicate biological materials. Besides this, they are highly penetrating, allowing the non-destructive investigation of the interior of materials.
For a long period, about 30 years, the ILL (Institut Laue Langevin) steady state research reactor (Grenoble/France) has been producing the highest neutron flux in the world ([PI] = 1.2 X [10.sup.15] n/[cm.sup.2]/s) by fission of heavy nuclei, and it is equipped with an important number of the world's best instruments. Although permitting an impressive quantity of noteworthy scientific results, some experiments remained not feasible, because of the limitation in neutron intensity. Indeed, X-ray sources deliver fluxes much higher than steady state reactor fluxes. The intensity from synchrotron sources brought an additional boost of ten decades in brilliance together with an excellent focussing of the beam, which corresponds to a factor of about [10.sup.4] in a monochromatic flux. Another comparable step is expected with the advent of the free electron laser (FEL). With respect to such intensities, neutron sources will never be competitive, but they give access to a complementary and equally important domain of physics.
There have been proposals, for instance in the USA (Advanced Neutron Project at Oak Ridge National Laboratory), to build a new high power steady state reactor (350 MW in comparison to the 57 MW reactor of the ILL), but even under the best conditions it would never reach fluxes more than five times the ILL flux and, in addition, the costs would escalate to astronomical sums. For these reasons this project was cancelled in 1995. In fact, problems occur on the technological side: Fission of nuclei is accompanied by an intense heating of the reactor seed (e.g. 180 MeV per produced neutron), which makes a stable cooling system necessary. From a certain fission rate upwards, the fast removal of heat becomes technologically very difficult and thus puts limits on the enhancement of the flux (see Figure 1).
More recently, in the last 20 years, another technique of producing neutrons has been investigated and applied to a similar field of research: Neutrons can be obtained by spallation of nuclei by protons. If the protons are released from an accelerator periodically with a certain repetition rate and impinge on a heavy atom target, this gives rise to a pulsed neutron source. This technique could lead to a peak flux of two orders of magnitude higher and an average flux comparable to the ILL's with a heating of only 30 MeV per produced neutron, as proposed for the European Spallation Source (1) (ESS) with a power of 5 MW. Pulsed neutron spallation sources are already operational at several sites: at the Intense Pulsed Neutron Source (IPNS) in Argonne/USA, at the Los Alamos Neutron Science Center (LANSCE) in the USA, at the Neutron Scattering Facility KENS at KEK/Japan and, actually the world's best neutron spallation source, at ISIS (156 kW) in the Rutherford Appleton Laboratory/ UK. Two other projects are under construction: The Spallation Neutron Source (SNS, 1-2 MW) in Oakridge/USA and the Japan Spallation Neutron Source (JSNS, 1 MW). In the long-term, these pulsed spallation sources seem very promising because of their high intensities and efficiencies combined with reduced technological difficulties.
Taking single crystal diffraction as an example, we would like to illustrate some typical research with neutrons, advantages and disadvantages of both neutron sources and show that a high intensity pulsed source is able to allow experiments never previously performed with neutrons.
Neutron single-crystal Laue fiffraction
Subatomic particles, such as neutrons, can also act as waves, their wavelengths being related to their energy. Experimentally, the wave nature of a particle can be shown by using...
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