Introduction

Before describing the measurement of the coherent Bragg reflexions, we shall discuss the various sources of background scattering occurring with the reflexions. The background scattering introduces both systematic and random errors in the determination of the diffracted intensity. Systematic errors arise from those components of the background scattering which have a non–linear dependence on scattering angle in the neighbourhood of the Bragg peak: the contribution of the background to the peak cannot then be estimated by simply extrapolating background measurements taken on either side of the peak. The random or statistical error associated with the presence of the background is discussed later in Chapter 10: we shall note there the importance of reducing the background as much as possible in order to improve the statistics of counting, especially in measuring weak reflexions.

Thus the conditions under which the reflexions are measured must be chosen with two points in mind: first, we must obtain a good estimate of the background under the Bragg peak, in order to make a valid background subtraction; secondly, the background must be as small as possible, so as to enhance the signal–to–background ratio.