A short history of x-rays

by Arne Hessenbruch

• Abstract
• The new radiation: what is it good for?
• The medical market
• X-ray diffraction: a tool for probing the atomic scale
• Protection: an infrastructure
• The marriage of the computer with the x-ray
• Controlling and quantifying: Sources and detectors

X-ray diffraction: a tool for probing the atomic scale

Because of the centrality of radiotherapy in the marketplace during the interwar period, much effort had gone into measuring dose, the intensity of x-ray radiation. Physicists were more interested in x-ray wavelength. A crucial finding was that x-rays actually did produce interference patterns when they impinged upon and were diffracted by a crystal and historical reviews often assert that the 1912 x-ray diffraction experiment proved beyond doubt the electromagnetic wave nature of x-rays. However, Max von Laue, who received a Nobel Prize for the discovery, in fact explained the evidence using both models of x-rays: EM radiation and the bunched-up, particle-like theory mentioned above [10] . It was the Braggs, father and son, who homed in on x-rays as a crystallographic tool, eventually concluding that they were EM waves. Their research corroborated that x-rays have a wavelength of atomic dimensions, some 1000 times smaller than visible light, which enables them to interfere with lattices and yield information about crystal structure that visible light could never do. This was the first tool to yield images from which atomic structures of the solid state could be inferred and has been widely used in the 20^th century [11] . The Braggs [12] succeeded in describing very simple structures, such as that of rock salt (NaCl). Furthermore, analysis of the intensities of x-rays reflected by crystals led to corroborations of Niels Bohr's atomic model and of a concept of chemical bonding [13] . The Braggs wrote a most successful textbook on x-rays and crystal structure, trained many people and established an extensive network of collaboration.

However, because the inference from x-ray evidence to the structure of matter was complex, the progress from simple to more complex structures was laborious. In the 1920s, molecules of inorganic chemistry were examined and in the 1930s Desmond Bernal and coworkers, such as Dorothy Crowfoot (later Hodgkin) [14] , determined the structure of sterols, and in the following decades that of other biologically interesting molecules, including penicillin and vitamin B12, were also determined [15] . X-ray analysis always required much effort and ingenuity [16].

However, even today [1964] structure determination by X-ray methods does not yield a direct route from the experimental data to the structure. In complicated cases the scientist only obtains a result after considerable mental effort, in which chemical knowledge, imagination and intuition play a significant part. In addition, the experimental data often have to be processed using different mathematical treatments, which must be varied according to the circumstances. Add to this the fact that the more complicated the structure, the greater becomes the volume of experimental data which must be amassed and processed. For relatively simply built compounds it was possible to carry out the calculations with pencil and paper. Nowadays it is nearly always necessary to use electronic computers, and their arrival has made an enormous difference to the possibility of carrying out structure determinations. However, it is not usually possible to just feed in the experimental data, and get out the figures which give the final structure; the scientist's ability to handle the data is still of vital importance.

X-ray analysis techniques were adopted in other life science sub-fields, such as virology, where crystalline forms of plant viruses were being prepared in the 1930s, rendering x-ray analysis feasible. By 1956, Crick and Watson concluded from such analysis that a small virus contains identical subunits, packed together in a regular manner. During the same period they worked on the structure of DNA for which they received the Nobel Prize in 1962 along with Maurice Wilkins. [17] Famously, Rosalind Franklin, who had died in 1958, did not receive due credit for her contribution to the DNA research but later historians have made up for it [18] .