Introduction

There is no one ideal X–ray or neutron diffractometer which is equally well suited to the many different investigations which have to be carried out. The principal factors in the performance of a diffractometer are:

1. Accuracy of intensity measurement

2. Speed of operation

3. Number of accessible reflexions

4. Amount of manual intervention required

5. Accessibility of specimen

6. Availability of a computer

7. Versatility

8. Reliability

9. Cost

These varying requirements frequently conflict with one another and the choice of instrument for a given application will depend on the relative importance attached to them. We shall compare later the performance of two specific instruments under the headings listed above.

As we have seen in Chapter 2, diffractometers can be constructed according to two different geometrical arrangements, leading to a division into equatorial and inclination instruments. All diffractometers suitable for the collection of three–dimensional intensity data must have a number of shafts capable of being set independently: in the two arrangements the rotational degrees of freedom are allocated differently between crystal and detector shafts.

The general design of the diffractometer is dictated primarily by the type of geometrical arrangement, and by the method adopted in setting the shafts. The collimator, the goniometer head which supports the crystal, alinement aids such as viewing telescopes and even the detector itself can be thought of as exchangeable attachments which can be selected in accordance with the particular investigation in hand, and are of secondary importance in influencing the design of the instrument.