Z. Kristallogr. 227 (2012) 27–35 / DOI 10.1524/zkri.2012.1445 27 # by Oldenbourg Wissenschaftsverlag, Mu¨nchen Disputed discovery: The beginnings of X-ray diffraction in crystals in 1912 and its repercussions1 Michael Eckert* Deutsches Museum, Forschungsinstitut, Museumsinsel 1, 80538 Munich, Germany Received August 22, 2011; accepted September 22, 2011 M. von Laue / A. Sommerfeld / W. H. Bragg / W. L. Bragg Crystallography, and other celebratory reviews of the events in 1912, according to Forman, served the purpose Abstract. The discovery of X-ray diffraction is reviewed of maintaining a disciplinary identity among the crystallo- from the perspective of the contemporary knowledge in graphers. “This circumstance, and its evident social func- 1912 about the nature of X-rays. Laue’s inspiration that tion of reinforcing a separate identity, strongly suggests led to the experiments by Friedrich and Knipping in Som- that the traditional account may be regarded as a ‘myth of merfeld’s institute was based on erroneous expectations. origins’, comparable to those which in primitive societies The ensuing discoveries of the Braggs clarified the phe- recount the story of the original ancestor of a clan or nomenon (although they, too, emerged from dubious as- tribe” (Forman, 1969, p. 68). In turn, Ewald regarded this sumptions about the nature of X-rays). The early misap- interpretation as “the myth of the myths”. Forman failed, prehensions had no impact on the Nobel Prizes to Laue in in Ewald’s view, “to appreciate the vagueness of the phy- 1914 and the Braggs in 1915; but when the prizes were sical information confronting Laue before the experiment. finally awarded after the war, the circumstances of ‘Laue’s To make his interpretation plausible, the author repeatedly discovery’ gave rise to repercussions. Many years later, aggrandizes statements taken from the literature, especially they resulted in a dispute about the ‘myths of origins’ of from Laue’s Nobel Lecture and from the Festschrift for the community of crystallographers. the semicentennial of the discovery. These unjustified ac- cents, needed in support of his main thesis, show that his scheme is pre-conceived, artificial, and unnecessary” 1. Introduction (Ewald, 1969, p. 81). From these passages it is apparent that a review of the The discovery of X-ray diffraction in crystals a hundred discovery of X-ray diffraction in crystals in a short article years ago, and the ensuing birth of the new specialities of cannot cover the disputed issues in a comprehensive man- X-ray spectroscopy and X-ray crystallography, have been ner. The complexity is due to historical as well as scien- praised and reviewed on numerous occasions, most exten- tific aspects. The latter are concerned with the nature of sively half a century ago in P. P. Ewald’s Fifty Years of X-rays and the space-lattice hypothesis of crystals as known X-ray Diffraction (Ewald, 1962). The pioneers were awarded before the discovery; the former concerns the lack of doc- with the Nobel Prize in physics as early as 1914 and umentary evidence – letters, diaries, manuscripts – from 1915: Max von Laue, who had suggested in spring 1912 the crucial period in spring 1912. The Nobel speeches of the – now famous – experiments performed by Walter the discoverers, Ewald’s voluminous Festschrift for the Friedrich and Paul Knipping, ‘for his discovery of the dif- semicentennial, Forman’s critical scrutiny together with fraction of X-rays by crystals’; William Henry Bragg and Ewald’s refutation, and further accounts (see Wheaton, William Lawrence Bragg, father and son, ‘for their ser- 1983, pp. 199–220) provide insights from a variety of dif- vices in the analysis of crystal structure by means of ferent perspectives – and yet there remain questions that X-rays’ (Nobel Prizes, 1914–1915). Few other discoveries cannot be resolved unequivocally from the available archi- received such swift recognition and widespread praise. val sources. We restrict this study to Laue’s initial idea The reconstruction of the events that led to Laue’s idea about the nature of the interference effect. We will not in 1912 have provoked critical scrutiny. The science his- discuss Laue’s views concerning the space-lattice hypoth- torian Paul Forman challenged the ‘clan of X-ray crystal- esis, which have also been disputed in the course of the lographers’ with a ‘critique of the myths’ that he dis- Forman–Ewald controversy (Gasman, 1975; on the early cerned in their accounts of the discovery. Ewald’s Fifty history of crystallography, see Kubbinga, 2012). Central to Years Festschrift, dedicated to the International Union of the following review is the knowledge on X-rays in Ar- nold Sommerfeld’s Institute for Theoretical Physics at * e-mail: [email protected] Munich University, the site of the discovery, where Laue 1 This Laue centennial article has also been published in Acta spent the early period of his career as Privatdozent and Cryst. (2012). A68, 30–39. where Walter Friedrich was Sommerfeld’s assistant. 28 M. Eckert 2. The quest for the nature of X-rays Bragg, in a response to the Bremsstrahlen theory, pre- sented Sommerfeld with an alternative view which be- Since his call to Munich in 1906 to one of the very few came known as the neutral-pair hypothesis (Stuewer, 1971; chairs of Theoretical Physics, Sommerfeld had struggled Wheaton, 1983, pp. 87–90). Instead of assuming an elec- hard to meet the high expectations of Ro¨ntgen concerning tromagnetic interaction, Bragg speculated that an electron, the theoretical explanation of X-rays. “Isn’t it a shame that on encountering an atom, may neutralize its charge and ten years after Ro¨ntgen’s discovery one still does not know “takes the form of the X-ray or the g-ray as the case may what is going on with X-rays”2, Sommerfeld alluded to this be: it may again lose the neutralising complement and be- expectation shortly before his call to Munich in a letter to come a secondary cathode or b-ray, the double transfor- Wilhelm Wien, who had become Ro¨ntgen’s successor at mation being accompanied by no very great change of Wu¨rzburg and pursued fundamental experimental investi- speed” (W. H. Bragg, 1910). In another letter he referred gations about the nature of X-rays (Sommerfeld, 1905). On to photographs in the recently invented cloud-chamber: Wien’s X-ray investigations see Pohl (1912). Sommerfeld “Have you seen any of C. T. R. Wilson’s pictures of the had made a mark in this quest with an interpretation of ear- fog formed instantly after the passage of ionizing rays lier experimental observations by Hermann Haga and Corne- through a gas?” (W. H. Bragg, 1911). It seemed impossi- lis H. Wind about the passage of X-rays through narrowing ble to interprete these images other than by assuming a slits. Haga and Wind (1899) interpreted a diffuse broadening particle nature of the ionizing radiation. at the narrower end of the slit as a diffraction phenomenon. The quest about the nature of X-rays, therefore, cannot Sommerfeld regarded X-rays as a shower of electromag- be isolated from the riddles about radioactivity in the early netic square pulses and estimated from the experiments of years of the twentieth century. In a sequel to his Brems- Haga and Wind (1899) that the order of magnitude of the strahlung paper, Sommerfeld attempted to explain g-radia- width of an X-ray pulse is about one a˚ngstrom (¼ 0.1 nm). tion in a similar way to the X-ray Bremsstrahlung:He But he did not perceive such pulses as a superposition of perceived the emission of b-rays from a radioactive sub- waves; nor was the experimentally observed broadening stance as the ejection of electrons that become accelerated conclusive enough to justify the assumption of a diffrac- within short distances to almost the speed of light. The tion effect. The conclusion that X-rays were indeed elec- electromagnetic radiation caused by this acceleration tromagnetic waves with a wavelength of about 1 A˚ , there- would be radiated away in a similar manner to the X-ray fore, was far from persuasive (Wheaton, 1983, pp. 35–40). Bremsstrahlung caused by the deceleration of electrons. If By 1908, Charles Glover Barkla had provided new evi- an electron is accelerated to almost the speed of light dence about the nature of X-rays. He arrived at the con- within a very short distance, the intensity of the electro- clusion that X-rays appear in two varieties. One sort of magnetic radiation would be distributed within narrow for- X-rays was independent of the material from which they ward-directed lobes. Couldn’t this be the g-radiation ob- emerged and could be polarized; the other was like fluor- served together with b-rays? This “very strange view escence radiation and could not be polarized. The latter about the structure of g-rays”, Sommerfeld admitted, was was entirely dependent on the irradiated material and de- “the ultimate consequence of my view about the structure signated as ‘fluorescence’ or ‘characteristic’ radiation. Only of X-rays” (Sommerfeld, 1911a, p. 3). the former could be perceived in terms of the electromag- If g-radiation and X-ray Bremsstrahlung are electro- netic pulse hypothesis. Sommerfeld (1909) elaborated this magnetic radiation produced by the acceleration of elec- hypothesis so that he could account for the spatial distri- trons, then the energy of these radiations is related to the bution of X-ray intensity due to the deceleration of elec- energy of the electron. In the case of X-ray Bremsstrah- trons (‘Bremsstrahlen’) on their impact upon the anode lung, therefore, there should be a relation between the ki- material of an X-ray tube.