STUDY OF LOSS IN ABSORPTION OF X-RAYS IN THIN NATURAL DIAMOND CRYSTALS AROUND LAUE DIFFRACTION MAXIMA. Krishan Lal, S. Niranjana, N. Goswami, A. R. Verma, National Physical Laboratory, New Delhi-110012, India

Significant loss in absorption of X-rays in thin diamond crystals,with ut << 1, and having varying degrees of crystalline perfection has been observed at and near Laue diffraction maxima by using a high resolution X-ray diffraction technique. The specimen crystals were {111} platelets. A Five Crystal X-ray Diffractometer was used in three crystal configuration with Mo K[[alpha]]₁ exploring X-ray beam. The specimens were oriented in Laue geometry for diffraction from (220), (440), (111), (113) and (224). Three beams are observed when the crystal is oriented for diffraction [e.g. Lal et al, Solid State Commun. 96 33 (1995)]. The X-ray intensity measurements were made by using a scintillation counter coupled to a timer counter. It was mounted on a track and positioned to alternatively receive forward diffracted beam, plus the residual direct beam and the diffracted beam. Total transmitted intensity was obtained by adding intensities of all the three beams. Plots of total transmitted intensity as a function of glancing angle showed peaks at the diffraction peak positions. This enhancement in the transmitted intensity is due to a loss in the absorption of the exploring beam when diffraction takes place. This is identical to the Borrmann Effect. However, in the present case the crystals were thin with ut ~ 0.3 only. Also, their degree of perfection could be rather low. In a typical crystal, with ut = 0.29, the value of u dropped from the usual 0.219 mm^-1, to 0.075 mm^-1 [(111)]; 0.136 mm^-1 [(220)]; 0.117 mm^-l[(113)]; 0.16 mm^-l [(224)] and 0.198 mm^-1 [(440)]. The half width of its diffraction curve being 100 arc sec[for (220)], which is 87 times the theoretical half width for a perfect diamond crystal. Nearly perfect diamond crystals give a half width of <= 10 arc sec. So far as known to us this is the first time that such a loss in absorption at diffraction peaks has been reported for such thin and not so perfect crystals. These results indicate a coupling between the diffracted and the forward diffracted beams.