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Home , Lawrence Bragg, William Henry Bragg

www.mrs.org/publications/bulletin HISTORICAL NOTE

Lawrence Bragg’s “Brainwave” Drives Father-Son Collaboration In 1912, some 17 years after the serendip- and quickly began to learn what he could itous discovery of x-rays by Wilhelm on the subject. Röntgen, a debate raged as to the wave or Until this point in his life, at age 42, particle nature of this radiation phenome- William later recalled, “It had never non. Bragg, a 50-year-old entered my head that I should do any professor of at Leeds University in research work.” His curiosity aroused by England, came down firmly on the side of his reading on radiation, he soon obtained particles, citing the bullet-like nature of the some radium samples and began the rays, and how they were preferentially experiments that were to make him a lead- scattered in the forward direction when ing figure in radiation theory in a few colliding with matter. of years’ time. He quickly developed novel Germany, having produced elegant spot- hypotheses about the nature of radioactiv- diffraction photographs of CuS by aiming ity. The penetrating power of x-rays, and x-rays at crystal samples, used the diffrac- the fact that they are not deflected by a tion behavior as evidence for the wave magnetic field, were accounted for by the argument. Experiments by Charles G. “neutral pair hypothesis,” which stated Barkla that demonstrated the polarization that x-rays consisted of “an electron which of x-rays confirmed the wave theory in the has assumed a cloak of darkness in the minds of many scientists. Ironically, it was form of sufficient positive electricity to Bragg’s son William (who neutralize its charge.” Many of his ideas went by the name of Lawrence) whose were initially proposed in letters to British burst of insight—a “brainwave,” as he , a correspon- called it—settled the argument, at least for dence that sustained him throughout the time being. Unfortunately for his father, what historians now call the “Bragg– Sir . Copyright it was in favor of von Laue and Barkla. ©1915 The Nobel Foundation. Barkla Controversy.” “When I was walking one day along the Lawrence Bragg was only 22 at the time Backs at Cambridge—I can remember the of his “brainwave.” He had studied mathe- place behind St. John’s College,” Lawrence matics at Trinity College and again at Bragg recalled in The Start of X-ray Analysis, University before his father con- “I suddenly realized that von Laue’s spots vinced him to add some formal physics were the reflections of the x-rays in the study to his curriculum. Returning to sheets of atoms in the crystal.” Prior to this Trinity College upon his father’s transfer to realization, von Laue’s interpretation was Leeds, Lawrence worked in the Cavendish that the patterns were due to a complex set Laboratory under J.J. Thomson and lis- of wavelengths coming from the x-ray tened as the eminent physicist lectured on tube. Lawrence Bragg announced his the wave nature of x-rays. A fellow stu- insight in the Proceedings of the Cambridge dent in the scientific society at Cambridge Philosophical Society () in a read a paper concerning the packing of paper titled “The Diffraction of Short atoms as spheres in crystal structures, Electromagnetic Waves by a Crystal,” which led Lawrence to the realization that deliberately avoiding the mention of x-rays atoms in crystals were arranged in parallel to allow his father’s theory some hope of planes. It was the combination of these surviving. But instead of driving father ideas during his walk in 1912 that gave and son apart, the discovery prompted a him the insight into von Laue’s x-ray rare father–son collaboration that resulted photographs and led to the determination in the awarding of a joint in of the crystal structures of KCl and NaCl. Physics for the pair in 1915. For their He soon developed the mathematical efforts, von Laue was awarded the Nobel expression that defines the conditions Prize in 1914, and Barkla in 1917. under which reflected waves interfere con- X-ray diffraction has proven to be a structively, aptly named Bragg’s Law: powerful analytical technique for the Sir William Lawrence Bragg. Copyright nλ = 2dsinθ, (1) determination of the atomic structure of a ©1915 The Nobel Foundation. . While the symmetry of crystals where n is an integer, λ is the wavelength found in nature was recognized long William Henry Bragg began his career of the incident radiation, d is the inter- before the discovery of this technique, the in Australia as the Professor of Mathe- planar spacing, and θ is the angle of inci- detailed nature of the positions of atoms matics and Physics at Adelaide Univer- dence of the radiation. The schematic or ions—their relative locations and spac- sity in 1894, with an excellent back- illustration of these factors can be ings—remained a mystery. ground in mathematics but none in accessed at Web site www.britannica. spoke of molecules of NaCl in a solid as if physics. In 1904, having been elected com/eb/art?id=30656&type=A. the sodium and the chlorine ions bonded president of the physics section of the In order to confirm that the diffracted in one-to-one correspondence; indeed, Australian Association for the Advance- waves were indeed x-rays, William Bragg some were upset by the true ionic solid ment of Science, he chose the topic of constructed the first x-ray spectrometer picture when it emerged. radioactivity for a speech to the members with his assistant Jenkinson at Leeds.

MRS BULLETIN/JULY 2001 577 HISTORICAL NOTE

Radiation from an x-ray tube was colli- Bragg and Charles Barkla were right in marine detection during , and mated by passing through a slit, then their early dispute; their differences arose took up investigations of organic crystals through the sample crystal, which was from choices in experimental techniques after the war. He was knighted in 1920, mounted on a pedestal. The ionization and their own preconceptions. Barkla, con- made director of the in chamber that served as the detector was vinced of the wave nature of x-rays, set up 1923, and president of the Royal Society in mounted on an arm that could rotate polarization experiments that confirmed 1935. He died in London in 1942. Follow- about the crystal, and the angles of reflec- his theories. Bragg, having experimented ing his father’s path, Lawrence Bragg was tion were read from a scale on the sample with α particles, conducted experiments knighted in 1941, and elected director of pedestal. A Wilson gold-leaf electroscope designed to detect the preferred forward- the Royal Institution in 1954. He retired in measured the charge of the ionization scattering effect of particles. Both found 1966 and died in Ipswich in 1971. chamber. Whereas the von Laue photo- what they were looking for. Albert TIM PALUCKA graphs could take up to 20 h to produce, Einstein’s paper on the photoelectric effect with frequent stops to cool the x-ray in 1905, with its characteristic equation E = FOR FURTHER READING: G.M. Caroe, William tube, William Bragg’s ionization tech- hν (where E is the energy of a photon, h is Henry Bragg, 1862–1942 (Cambridge nique made the analysis faster and more Planck’s Constant, and ν is the frequency University Press, Cambridge, 1978); Sir accurate, since the angles between the of the radiation) predicted this dual Lawrence Bragg, The Development of X-Ray crystal and the diffracted beam could be wave–particle nature of radiation and mat- Analysis (Hafner Press, New York, 1975); measured for each reflection. Lawrence ter, but it was not generally understood in and Sir Lawrence Bragg, The Start of X-Ray had been trying to deduce the structure of 1912. Later in life, William Bragg frequent- Analysis (published for the Nuffield diamond at the time from the von Laue ly explained to listeners, “ use Foundation by Longmans, London, 1967). photographs, with no luck. However, the wave theory on Mondays, Wednes- As mentioned in the article, a schematic when analyzed in the x-ray spectrometer, days, and Fridays, and the particle theory illustration of Bragg’s Law can be accessed the diamond quickly yielded its secrets. on Tuesdays, Thursdays, and Saturdays.” on the World Wide Web at www.britannica. It was later concluded that both William William Bragg devoted himself to sub- com/eb/art?id=30656&type=A.

578 www.mrs.org/publications/bulletin MRS BULLETIN/JULY 2001