77TH ANNIVERSARY ADDRESS - Indian National Science Academy

SpecimencrystalBForwarddiffracted X-raybeamAExploring X-raybeam, Kα 1CDiffracted X-raybeam (normal)tFig. 16: A schematic diagram showing the presence of a forward diffracted beam when a thick(mt = 10 or more) and perfect crystal is oriented for diffraction in Laue geometry. ABC is theBorrmann triangle. Length AB = t sec qperfection are not essential. Some typical results of these investigationsare described in the following.Diamond is known to have low absorption for X-rays. For Mo Karadiation the value of its linear absorption coefficient m is 0.22 mm -1 .Therefore, for a 1 mm thick diamond platelet the value of mt is 0.22.We had investigated diamond crystals with varying degree of perfectionand having thickness of about 1.3 mm. All the diamond specimens wereborrowed from Raman Research Institute, Bengaluru through the courtesyof Late Professor V. Radhakrishnan and Late Professor S. Ramaseshan.Figure 17 shows schematically the experimental arrangement developedfor these studies. The exploring X-ray beam is highly monochromatic andcollimated, Mo Ka 1in these studies. The specimen crystal is oriented fordiffraction in the Laue geometry. In the forward direction there are twobeams, namely the forward diffracted beam and the residual direct beam.20