Pulsed spallation neutron sources

Powder diffraction studies with neutrons are perfonned both at nuclear reactors and at spallation sources. In both cases a cylindrical sample is observed by multiple detectors or, in some cases, by a curved, position-sensitive detector. In a powder diffractometer at a reactor, collimators and detectors at many different 20 angles are scaimed over small angular ranges to fill in the pattern. At a spallation source, pulses of neutrons of different wavelengdis strike the sample at different times and detectors at different angles see the entire powder pattern, also at different times. These slightly displaced patterns are then time focused , either by electronic hardware or by software in the subsequent data analysis. 

The types of radiations that are used in structural crystallography are mainly x-rays, neutrons, and electrons. The use of electrons is still difficult for structure determination but can be a useful tool for the detection of structural transitions (see Section X). White or monochromatic x-ray beams can conveniently be obtained from sealed tubes, rotating anode generators, or synchrotron sources [5], with relative flux magnitudes on the order of 1, 10, >100, respectively. The first two x-ray sources are continuous and are generally designed to produce almost monochromatic beams, while synchrotron radiation is pulsed and white. Neutron sources are comparatively much weaker and are either continuous (nuclear reactor) or pulsed (spallation source [6]). 

CCD = charge-coupled device IHIs = interligand hypervalent interactions ILL = Institut Lane-Langevin INS = inelastic neutron scattering IPNS = intense pulsed neutron source LINAC = linear accelerator MaNDi=macro-molecular neutron diffractometer NiMH=nickel-metal OPAL = open pool Australian light-water reactor hydride SANS = small-angle neutron scattering SNS = spallation neutron source. 

One of the brightest operational pulsed neutron sources (ISIS) is located in the UK (http //www.isis.rl.ac.uk/). In the US, the construction of the spallation source at the Oak Ridge National Laboratory (http //www.sns.gov/) is scheduled for completion in 2006. 

Early studies (1936-1950) of neutron scattering used radium-beryllium neutron sources but their low neutron flux prevented exploitation of neutron scattering as a spectroscopic technique [4]. Today neutrons are either extracted from a nuclear reactor or generated at a pulsed, accelerator-based spallation source. The exploitation of neutrons from nuclear reactors in structural studies and spectroscopy dates from the 1950s and from pulsed sources from the 1970s. A useful summary of the development of neutron sources is given in [5]. 

Stream of new pulsed neutron sources in Japan (J-PARC), the United States (SNS), and the proposed new European spallation source (ESS). 

Unlike X-rays, which can be produced in a laboratory, neutrons are only available in sufficient quantities at large facilities. Two main types of such facilities must be distinguished, reactor sources and spallation sources, because they have completely different characteristics in high-pressure experiments. The principle of reactor sources need not be described since they are by far the most common sources and have been used for several decades for solid-state studies. The flux from a reactor is usually continuous— the pulsed DUBNA source is an exception—and reactor fluxes are used mainly in the monochromatic angle dispersive mode (as ADXD) for X-ray diffraction. 

As shown above, neutrons are in particular important for studies of hydride materials. Neutrons can be produced either in nuclear reactors or by pulsed (spallation) neutron sources. In the research nuclear reactors neutrons are produced by fission processes based on U-235 (which is 0.7% in natural uranium, but usually emiched as fuel for reactors). Since the released neutrons from these processes are very energetic, the required chain reaction for continuous production of neutrons requires moderation (to reduce the energy)... 

The general characteristics of neutron diffraction as applied to studies of condensed matter are summarized, along with their means of production in steady-state reactor and pulsed spallation sources. Techniques for single-crystal neutron diffraction are discussed, and the application of these techniques in the study of molecular systems is summarized. A short review of applications in the field of molecular crystals is given. A more detailed account of this area was recently published. ... 

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