Experimental determination of the instrumental transparency function of texture goniometers

The instrumental transparency functions of five commercially available text ure goniometers were measured experimentally with six monocrystalline sampl es cut in different orientations from a large highly perfect silicon crysta l with a rocking curve of less than 0.01 degrees. Transparency functions we re measured in steps of 0.02 to 0.2 degrees in the pole-figure angles alpha , beta. The window size Delta alpha depends on the Bragg angle theta in the form 1/sin theta; the window size Delta omega is constant for each goniome ter. The dominant instrumental parameter determining the long axis Delta al pha of the pole-figure window is the axial width of the detector entrance s lit. This parameter is smallest for area detectors (smaller by more than an order of magnitude compared with conventional scintillation detectors as w ell as one-dimensional position-sensitive detectors). The main features of the pole-figure window and their dependence on the instrumental parameters can be deduced fairly well from a simple geometrical model. The particular shapes of the transparency functions of the studied goniometers are markedl y different. Particularly, they are not very well represented by Gauss func tions. The two-dimensional transparency function can be fairly well charact erized by its alpha and beta profiles. The normalized profiles are virtuall y independent of the goniometer angles 2 theta and alpha. The increasing si ze of the pole-figure window with decreasing theta puts a lower limit on th e Bragg angle below which pole-figure measurement ceases to be meaningful.