Eulerian Cradle Research Articles

Apparatus for Measuring Complete Single-Crystal X-Ray Diffraction Data by Means of a Geiger Counter Diffractometer

A device for measuring x-ray diffractions from a single crystal is described. The heart of the apparatus is an ``Eulerian Cradle,'' so named because it permits rotation of the crystal about each of the Eulerian axes. It is used for orienting single crystals in an x-ray beam, in order to bring each reciprocal lattice point into the equatorial plane for Geiger counter measurement of the intensity. It eliminates the need for raising the Geiger tube out of the equatorial plane and suggests that the reciprocal lattice be surveyed by either ``zone'' or ``cone'' schemes. These are described, and the ``cone'' scheme is shown to permit a complete hemisphere of the reciprocal lattice to be surveyed with only a single mounting of any crystal. Fine points concerning the use of the apparatus in the precise orientation of a crystal, and for the measurement of integrated intensities at single settings, are given. All angular motions are controlled by worm wheels and worms with dials for direct reading to 0.01°. Data collection is further facilitated by computing appropriate angular coordinates for reciprocal lattice points from an accurate set of lattice parameter measurements (readily obtained with the same instrument) and setting the various circles accordingly. The long spacings, characteristic of protein crystals, require both the high precision of this instrument and computed coordinates for the many thousands of reflections that must be measured.