The recently constructed GEneral Materials diffractometer, GEM, at the ISIS pulsed neutron source, offers a number of novel features of advantage in the study of the structure of materials. The most important feature is the detector array, which covers a very large area with state-of-the-art scintillator neutron detectors. Eight detector banks cover a range in scattering angle, 2θ, from 1.1 to 169.3 deg., and azimuthal angles from -45 to +45 deg. These allow the neutron diffraction pattern to be measured with a high count rate over a very wide dynamic range.
The 17 metre incident flight path is relatively long, whilst the 5mm detector width is relatively narrow, and these lead to a good resolution which is best for back-scattering with a typical value 0.34% in the 160 deg. detector bank. The high resolution of GEM is well suited to structural studies of crystalline materials. The highest count rate is for the 90 deg. detector bank, which covers a very large solid angle and has a very low background, making it ideal for the study of time-dependent phenomena, such as crystallisation and phase transitions.
The high count rate of the 90 deg. detector bank also provides high quality data up to very high momentum transfers, Q, of about 60 Angstrom^-1 in favourable cases. For studies of non-crystalline materials this yields an excellent real-space resolution for the correlation function. A novel design has been developed for the lowest angle detector bank which operates inside the sample tank vacuum, extending the minimum Q down to below 0.1 Angstrom^-1 in favourable cases. An important area of growth for the scientific program on GEM has been in the overlap between crystallography and non-crystalline materials, for example the study of the structure of carbon nanotube materials. Pair correlation function methods, originally developed for the study of glasses and liquids, are now being applied to the study of disorder in crystalline materials.

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