ABSTRACT
This is a picture of a modern x-ray tube. Most x-rays used in crystallography are produced by similar tubes. Synchrotrons also produce x-rays used in some protein studies. Nuclear bombs produce x-rays, which caused much damage to human tissue in Hiroshima. The sun produces x-rays, as do many other stars. X-ray astronomy is an important branch of star study. Certain radioactive materials decay to produce x-rays. For example, the low intensity x-rays from radioactive 55Fe are used to test x-ray detectors. (Photograph by Maureen M. Julian.)
6.1 INTRODUCTION Chapters 1 through 5 have provided an introduction to the principles of symmetry in crystals. From the 1600s a connection between the characteristic interfacial angles in crystals and an underlying periodicity was hypothesized. The 230 space groups were worked out almost simultaneously in Russia, Germany, and England in the 1890s. What was needed was proof of the existence of the unit cell, including measurements of its size. The missing link was made by x-ray diffraction in 1912.
