Section 9.1.5.3. Single-counter diffractometers

A great advance in automation came with the development of single- and later three- and five-counter diffractometers. The most common type was the four-circle diffractometer (Arndt & Willis, 1966). Single-scintillation-counter detectors are capable of measuring the intensity of only one individual reflection at a time. Therefore, in this technique, it is necessary to set the counter at the appropriate 2θ angle and to orient the diffracting plane so that the vector normal to it bisects the angle between the source and the detector. This can be achieved by the use of three axes of the Eulerian ω, χ, φ cradle or of the ω, κ, φ cradle. Such systems lent themselves readily to automated computer control, with accurate intensities and standard uncertainties output directly to storage devices at the rate of one reflection every one to five minutes. A full discussion of four-circle diffractometers and their associated geometry is given in IT C (2004).

Single-counter diffractometers are still widely used for small molecules. They were also applied in the 1960s and 1970s to the first protein structures, albeit at limited resolution. Their use is greatly limited for macromolecules since only a single reflection can be collected at a time, despite the fact that many simultaneously lie in a diffracting position. The overall exposure time is very large and the radiation damage is likely to be considerable.

Single-counter diffractometers are so rarely used in present-day macromolecular crystallography that they are not discussed further here. Their applications are limited to specialist techniques, such as multibeam methods for direct phase determination.