Monochromators powder neutron diffraction

Neutron powder diffractometers that exploit a monochromatic beam are normally situated at reactors, which have a steady-state output of neutrons, though the SINQ at the Paul Scherrer Institute in Switzerland is a continuous spallation source. As with synchrotron X-rays, neutrons of a particular wavelength are selected from the polychromatic beam using a single-crystal monochromator, and the diffraction pattern is measured as a function of angle. 

Neutron diffraction was carried out on the liquid samples on the D4c diffractometer [6] at the ILL, using neutron wavelengths of 0.5, 0.33, and 0.29 A. A Cu(220) monochromator was used for 0.5 A and a Cu(331) monochromator was used for both the 0.33 and 0.29 A neutrons. In order to avoid harmonic contamination a rhodium filter was inserted between the monochromator and the sample when measuring at 0.5 A. Diffraction measurements were taken at the three wavelengths on the empty furnace, the empty container, the liquid sample, nickel powder in a vanadium can for calibration, a vanadium bar for normalisation, and powder in a silica container for a self absorption correction. All measurements were taken at ambient temperature, except the sample measurement, where a vanadium furnace was used to heat the sample to 800 °C. The essential diffraction parameters are shown in Table 7.1. Significant care was taken in minimising the background to avoid the possibility of an A1 peak coincident with the InSe first peak [13]... 

There are two types of neutron sources available for powder diffraction. One is the nuclear reactor, which provides a monochromatic beam of wavelength 1.0 A, selected by means of a crystal monochromator from the continuous wavelength spectrum of thermalized neutrons [481. The diffraction experiment uses the Bragg method as in X-ray single crystal diffractometry. 

This section is restricted to the description of an angle dispersive device (see Sect. 29.3.1.4). The instrument that is capable of recording neutron powder diffraction patterns is also called two axis neutron spectrometer, after the two principle axes monochromator-sample and sample-detector the original design is due to Hewat and Bailey (1976). 

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