In this chapter we shall describe the main features in the procedure for measuring the set of relative integrated intensities ρhkl of the crystal. We shall assume that the difTractometer has been correctly positioned and alined with respect to the incident beam. The methods of making these adjustments are fully described by Furnas (1957).

Choice of crystal size and shape

To minimize errors arising from the effects of absorption, extinction and simultaneous reflexions, the crystal must be as small as possible. The ultimate limit to the specimen size is determined by counting statistics (see §10.7), and, because of the greater intensity of primary X–ray beams compared with monochromatic neutron beams, the crystal size tends to be much larger in neutron diffraction. In the early period of neutron diffraction the linear dimensions of the crystal were measured in centimetres, but now, with the availability of high–flux reactors and with the use of automatic methods to speed up the collection of data, the size of crystal is down to 1 or 2 mm.

On p. 240 we gave the criterion μR ∼ 1 as a rough indication of the optimum size of the crystal, where μ is the linear absorption coefficient and R the average crystal radius. From Table XIX, this corresponds to an average radius for X–ray work of about 0.1 mm. The diffracted intensity decreases with the size of the unit cell, so that larger specimens are used when the cell size is large and the absorption is relatively low.