Neutron Powder Diffraction
- PDF / 6,184,352 Bytes
- 8 Pages / 604.8 x 806.4 pts Page_size
- 42 Downloads / 243 Views
TIN/NOVEMBER1990
netons, and an additional scattering mechanism arises when éléments présent within the sample hâve unpaired spin density. This magnetic scattering provides a well-exploited way to study magnetic structures in solids. The absolute values of the neutron scattering cross sections are (except for the light éléments) much smaller than for x-ray or électron scattering. Neutrons therefore easily probe the interiors of bulk samples comprised of ail but a few éléments that hâve large neutron absorption cross sections. In addition, it is straightforward to perform diffraction experiments on samples contained in spécial environment apparatus such as cryostats, furnaces, or pressure cells, since the neutrons can easily penetrate the walls of such equipment. 4 A possible disadvantage, exacerbated by the relatively modest fluxes provided by today's neutron sources, is that neutron diffraction samples must be significantly larger than those for x-ray diffraction; typical sample sizes range from 1 to 10 grams. However, successful experiments hâve been performed on samples of less than 50 mg. Neutron Powder Diffractometers Instrumentation for performing neutron powder diffraction experiments is available at essentially ail the world's research reactors and pulsed neutron sources. The intensity scattered from a sample is measured as a function of d-spacing which, through Bragg's law, d = \/2 sin0, détermines the position of diffraction events. At a reactor, a fixed-wavelength (\) is selected by a monochromator crystal, and the measured variable is the scattering angle, d, i.e., one-half the angle between incident and scattered beams. In gênerai, the diffractometer design is analogous to that used for conventional x-ray diffraction. At a pulsed neutron source, the data are accumulated at a fixed Bragg angle as a function of neutron wavelength. The wavelength is determined by measuring
the neutron flight time from the source to the detector. Although time-of-flight and fixed-wavelength diffraction techniques each hâve their own advantages for particular experiments, the overall performance of the two classes of instruments is comparable. The modem era of neutron powder diffraction began with Rietveld's 5 development of a full profile analysis code in 1967 and with the design and construction of high-resolution powder diffractometers at both reactors and pulsed neutron sources. Perhaps the first of the modem génération of instruments was the D1A diffractometer which began opération at the Institut Laue Langevin (ILL) reactor in Grenoble, France in 1975.6 Hewat demonstrated that a welldesigned powder diffractometer could almost perfectly track the resolution required to résolve peaks in a typical structure over a wide range of d-spacings. He further argued that the resolution could be improved to the limits imposed by particle size broadening. The resulting D1A instrument design used 10' Soller collimators to attain the desired resolution. The count rate lost in achieving
650 mm
Figure la. Schematic diagram of the D1A
Data Loading...
