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     Early History of X Rays

     The discovery of X rays
     in 1...

                                         Wilhelm Conrad Roentgen (1845–1923).


refused the calls until the Universi-    centimeters that Lenard had found
ty of Würzburg o...

                                                    effects of the rays in air or in a sec-...

Henri Poincaré, 1854–1912. (Courtesy of
       AIP Emilio Segrè Visual Archives)



velocity of the electrons excited by
X rays is greater than could be giv-
en to them by a w...

                                                     White radiation Laue diffraction pattern...

Top right: Sir William Henry Bragg, 1862–1942. Lower right: Sir William Lawrence
Bragg, 1890–...


  Charles G. Darwin, L. M. Thomas, and
         Gregory Breit. (Courtesy of the

                                                                  Arthur Holly Compton, 1892–...

 Niels Bohr, 1885–1962. (Courtesy of the
                  AIP Niels Bohr Library

   Few ...

                                              Louis de Broglie, 1892–1987.
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Early history of x rays


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Early history of x rays

  1. 1. Early History of X Rays by ALEXI ASSMUS The discovery of X rays in 1895 was the beginning of a revolutionary change in our understanding of the physical world. IN THE WINTER of the year of his fiftieth birthday, and the year following his appointment to the leadership of the University of Würzburg, Rector Wilhelm Conrad Roentgen noticed a barium platinocyanide screen fluorescing in his laboratory as he generated cathode rays in a Crookes tube some distance away. Leaving aside for a time his duties to the university and to his students, Rector Roentgen spent the next six weeks in his labora- tory, working alone, and sharing nothing with his colleagues. 10 SUMMER 1995
  2. 2. Wilhelm Conrad Roentgen (1845–1923). (Courtesy of AIP Emilio Segré Visual Archives) Three days before Christmas he that jolted the fin- brought his wife into his laborato- de-siècle disci- ry, and they emerged with a photo- pline out of its graph of the bones in her hand and of mood of finality, the ring on her finger. The Würzburg of closing down Physico-Medical Society was the first the books with to hear of the new rays that could ever more precise penetrate the body and photograph measurements, of its bones. Roentgen delivered the losing itself in de- news on the 28th of December 1895. bates over statistical Emil Warburg relayed it to the Berlin mechanics, or of try- Physical Society on the 4th of Janu- ing to ground all ary. The next day the Wiener Press physical phenomena in carried the news, and the day fol- mathematically precise lowing word of Roentgen’s discovery fluctuations of the ether. began to spread by telegraph around All three discoveries, X rays, the world. uranium rays, and the elec- On the 13th of January, Roentgen tron, followed from one of the presented himself to the Kaiser and major experimental traditions in the was awarded the Prussian Order of second half of the nineteenth the Crown, Second Class. And on the century, the study of the discharge 16th of January the The New-York of electricity in gases. All three Times announced the discovery as contributed to a profound transfor- a new form of photography, which mation of physics. In the 20th cen- revealed hidden solids, penetrated tury, the discipline has been ground- wood, paper, and flesh, and exposed ed in the study of elementary Forms of tube used by Roentgen the bones of the human frame. “Men particles. in 1895–1896 for the production of science in this city are awaiting As with the invention of in- of X rays. with the utmost impatience the candescent light arrival of English technical journals bulbs, the study which will give them the full par- of electrical dis- ticulars of Professor Roentgen’s dis- charge through covery of a method of photographing gases was made opaque bodies,” The New-York possible by the Times began, and it concluded by pre- development of dicting the “transformation of mod- improved vacu- ern surgery by enabling the surgeon um technology to detect the presence of foreign in the 1850s. Ear- bodies.” (Jan. 16, 1896, p. 9) ly on, English The public was enthralled by this scientists were new form of photography and curi- investigating the German Museum, Munich ous to know the nature of the new patterns of light rays. Physicians put it to immediate and dark that ap- use. Physicists sat up and took no- peared in sealed tice. The discovery of X rays was the lead-glass tubes. first in a series of three discoveries The patterns in BEAM LINE 11
  3. 3. Roentgen’s apparatus for studying the ionization of air by X rays, 1906. transparent to ultra-violet light. When Heinrich Hertz found that he could pass the rays through metal foil, a fellow German scientist, Philip German Museum, Munich Lenard, began to study them more carefully. Lenard designed a tube with a thin aluminum window through which the rays could emerge, and he measured how far they could travel and still induce these partially evacuated tubes were fluorescence. Defined in this way, stimulated by a voltage drop between the range of the cathode rays was six a cathode and an anode: typically to eight centimeters. Lenard’s ex- there was a dark space, called periments inspired Roentgen to won- Crookes’ dark space; then a glow, der if the rays in an attenuated form called negative light; then another really traveled farther, and he dark space, this one called Faraday’s; planned experiments to see if a and a final glow of positive light. If sensitive electroscope would mea- the air in the tube was exhausted un- sure a discharge at four times the til the first dark space expanded to distance Lenard had identified. fill the entire tube and all glows dis- This line of work was outside appeared, then the rays emitted from Roentgen’s usual research pursuits, the cathode could be investigated. which had by this time gained him The rays cast shadows, and were great stature in German science. Son deflected by magnetic fields, but of a cloth manufacturer and mer- appeared to be immune to the ef- chant from the Rhine province, fects of static electric forces. Roentgen was not a particularly As was to be characteristic of the diligent student in his youth. He new ray physics to come—the phys- eventually made his way to the ics of cathode rays, X rays, alpha rays, Polytechnic in Zurich, where he beta rays, gamma rays, and N rays— obtained a diploma in mechanical the nature of the cathode rays was in engineering in 1868 and a doctor- dispute, the British favoring a stream ate one year later. In Zurich he of particles, those on the Continent became an assistant to August Kundt preferring to think of them as some and moved along with him to the sort of disturbance of the ether. (The University of Würzburg, and then on British position, and the research pro- to the Physical Institute at Stras- gram developed by J.J. Thomson at bourg. His first move on his own was the Cavendish Laboratory to study to the chair of physics at Giessen ionization in gases, would result in in Hesse in 1879, from which he the discovery of the electron. But our received many offers to go elsewhere. story does not take us that way). The path upward in the German A strong reason for believing that university system was to follow calls the cathode rays were particles was to universities of higher and higher Sir Joseph John Thomson, 1856–1940. the observation that they would stature, and finally to obtain an (Courtesy of the AIP Niels Bohr Library) not pass through matter that was institute of one’s own. Roentgen 12 SUMMER 1995
  4. 4. refused the calls until the Universi- centimeters that Lenard had found ty of Würzburg offered him the to be the maximum distance for Directorship of their Physical Insti- which cathode rays maintain their tute. In 1894 he was elected Rector power to induce fluorescence. Roent- at Würzburg. In his inaugural ad- gen recognized the effect as wor- dress, given the year before his dis- thy of his undivided attention and covery of X rays, Roentgen stated devoted the next six weeks to its that the “university is a nursery of uninterrupted study. scientific research and mental edu- Historians have speculated about cation” and cautioned that “pride in why Roentgen was the first to rec- Phillip Lenard, 1862–1947. (Courtesy of one’s profession is demanded, but not ognize the significance of this effect. Ullstein Bilderdienst and the AIP Niels professional conceit, snobbery, or The equipment, a cathode ray tube Bohr Library) academic arrogance, all of which and a fluorescing screen, had been in grow from false egoism.”* use for decades. In 1894 J.J. Thomson Roentgen’s pride could rest in the had seen fluorescence in German- over forty papers he had published glass tubing several feet from the from Strasbourg, Giessen, and discharge tube. Others had noted Würzburg. These early interests fogged photographic plates. But ranged widely—crystals, pyroelec- before Lenard’s work, the object of trical and piezoelectrical phenomena, study was always the effects inside and the effects of pressure on liquids the tube itself, and stray ultra-ultra- and solids—but did not yet include violet light could be used to explain electrical discharges in gases. He had the fogging of photographic plates. taken his turn at measuring the Lenard’s great interest was in prov- Demonstration by Crookes that cathode specific heat ratios of gases using a ing, in contradiction to the British, rays travel in straight lines: a) cathode; sensitive thermometer of his own the ethereal nature of cathode rays, b) aluminum cross and anode; d) dark making. He was an exact experi- and he was the first to study the shadow; c) fluorescent image. menter who often made his own apparatus—a skill learned during his training as an engineer in Zurich— and he was able to measure ex- tremely small effects, surpassing even Faraday’s measurement of the rotation of polarized light in gases. Roentgen turned to a new interest in October of 1895: the study of cath- ode rays. In the course of repeating the experiments of Hertz and Lenard, he happened to notice a glowing flu- orescent screen set off quite some distance from the Crookes’ tube he was operating. The screen sat much farther away than the six to eight *Quoted in “Wilhelm Conrad Roentgen,” Dictionary of Scientific Biography (New York: Scribner’s, 1975), p. 531. BEAM LINE 13
  5. 5. effects of the rays in air or in a sec- shadowy pictures they produce: ond glass tube into which he directed bones in a hand, a wire wrapped them. around a bobbin, weights in a box, Roentgen, a meticulous and ob- a compass card and needle hidden servant experimenter, made the away in a metal case, the inhomo- obvious tests on the new X rays: geneity of a metal. The ability of the Were they propagated in straight new rays to produce photographs lines? Were they refracted? Were they gave them great popular appeal and reflected? Were they distinct from brought Roentgen fame. Many arti- cathode rays? What were they? Like cles appeared in photography jour- Heinrich Rudolf Hertz, 1857–1894. the cathode rays, they moved in nals, and The New-York Times in- (Courtesy of Deutsches Museum and straight lines. Roentgen was unable dexed the new discovery under AIP Emilio Segrè Visual Archives) to refract them with water and car- photography. Since the rays exposed bon bisulphide in mica prisms. Nor photographic plate, the public as- could he concentrate the rays with sumed they were some form of light. ebonite or glass lenses. With ebonite The physicist Roentgen concurred. and aluminum prisms he noted the Accepting Lenard’s claim that cath- possibility of refracted rays on a pho- ode rays were vibrations of the ether, tographic plate but could not observe Roentgen compared the new rays to this effect on a fluorescent screen. them and forwarded the opinion that Testing further, he found that X rays the two were ethereal, although dif- O, Röntgen, then the news is true, could pass freely through thick lay- ferent from visible, infra-red and And not a trick of idle rumour, ers of finely powdered rock salt, ultra-violet light in that they did not That bids us each beware of you, electrolytic salt powder, and zinc reflect or refract. He suggested that And of your grim and graveyard humour. dust, unlike visible light which, cathode rays and X rays were longi- because of refraction and reflection, tudinal vibrations of the ether rather We do not want, like Dr. Swift, To take our flesh off and to pose in is hardly passed at all. He concluded than transverse ones. Our bones, or show each little rift that X rays were not susceptible to Now that their existence was And joint for you to poke your nose in. regular refraction or reflection. established, it was easy enough to Roentgen found that the X rays experiment with the new X rays. We only crave to contemplate originate from the bright fluores- Roentgen himself published only Each other’s usual full-dress photo; cence on the tube where the cathode three papers on the subject, but oth- Your worse than “altogether” state Of portraiture we bar in toto! rays strike the glass and spread out. ers jumped quickly into the field. The point of origin of the X rays And not just physicists. Thomas The fondest swain would scarcely prize moves as the cathode rays are moved Edison used modified incandescent A picture of his lady’s framework; by a magnetic field, but the X rays light bulbs to produce the new rays. To gaze on this with yearning eyes themselves are insensitive to the He boasted to reporters that any- Would probably be voted tame work! magnet. Roentgen concluded that one could make photographs of No, keep them for your epitaph, they are distinct from cathode rays, skeleton hands; that was mere child’s these tombstone-souvenirs unpleasant; since Lenard’s work had shown that play. Within a month of Roentgen’s Or go away and photograph cathode rays passing through the announcement doctors were using Mahatmas, spooks, and Mrs. B-s-nt! tube maintained their direction the X rays to locate bullets in human but were susceptible to magnetic flesh and photograph broken bones. —Punch, January 25, 1896 deflection. Dr. Henry W. Cattell, Demonstrator Roentgen justified calling the new of Morbid Anatomy at the Univer- phenomena rays because of the sity of Pennsylvania, confirmed their 14 SUMMER 1995
  6. 6. Henri Poincaré, 1854–1912. (Courtesy of AIP Emilio Segrè Visual Archives) importance for the diagnosis of kidney stones and cirrhotic livers and commented that “The surgical imagination can pleasurably lose it- self in devising endless applications of this wonderful process.” (The New-York Times, Feb. 15, 1896, p. 9). In the first six months after their dis- covery Viennese mummies were un- dressed, doctors claimed to have pho- tographed their own brains, and the transverse ethereal vibrations: light, human heart was uncovered. By 1897 uranium rays, X rays. Uranium rays the rays’ dangerous side began to were given off by certain minerals, be reported: examples included loss and they needed no apparatus to pro- of hair and skin burns of varying duce them, but they shared certain severity. properties with X rays. They exposed Electricians and physicists specu- photographic plates and they caused lated on the nature of these X rays. gases to conduct electricity. Albert Michelson thought they British physicists weighed in on might be vortices in the ether. the side that X rays were impulses in Thomas Edison and Oliver Lodge the ether rather than continuous suggested acoustical or gravitation- waves. Lucasian Professor of Math- al waves. But the rays ability to pho- ematics at Cambridge, Sir George tograph was decisive, and serious Gabriel Stokes, and his colleague and thinkers settled on three possibili- director of the Cavendish Laborato- ties, all of them of electromagnetic ry, J.J. Thomson, committed them- origin: the waves were very high fre- selves to the impulse hypothesis in quency light; they were longitudinal 1896. It was consistent with their waves (Roentgen’s initial suggestion); conception of cathode rays as parti- or they were transverse, discontin- cles (Thomson was to announce the uous impulses of the ether. discovery of the corpuscle or electron Quite early on the hypothesis that one year later.) The abrupt stop of a they were longitudinal waves was charged particle would result, after a discarded, despite the support of tiny delay, in the propagation out- Henri Poincaré and Lord Kelvin. The ward of an electromagnetic pulse. crux of the question was whether the With Thomson’s exact measurement waves were polarizable. If so they of the charge-to-mass ratio and H.A. could not be longitudinal waves. Lorentz’ successful theory of the Although the early experiments on electron, which explained many polarization were negative or unclear, intriguing phenomena, Continen- with the discovery of another ray, tal physicists began to accept, to Henri Bequerel’s uranium rays for Lenard’s dismay, cathode rays as which he claimed to have found po- material particles and X rays as First X ray made in public. Hand of the famed larization, those on the Continent impulses in the ether. anatomist, Albert von Kölliker, made during set up a convincing typology. It went Soon new results began to Roentgen's initial lecture before the Würzburg from lower to higher frequency come in. Two Dutch physicists, Physical Medical Society on January 23, 1896. BEAM LINE 15
  7. 7. Arnold Johannes Wilhelm Sommerfeld, 1868–1951. (Courtesy of the AIP Niels Bohr Library) director of the Cavendish Laboratory, had separated α rays, stoppable by metal foil or paper sheets, from the more penetrating β rays. In 1900, Rutherford had identified the βs as high-speed electrons: deflected in a magnetic field they showed the cor- rect charge-to-mass ratio. A third component of the uranium rays, undeviable and highly penetrating, was discovered by Paul Villard at the Hermann Haga and Cornelius Werd, Ecole Normale Superieur in Paris. announced that X rays could be dif- Rutherford named these γ rays. In WE WANT TO KNOW fracted, and a Privatdozent at Göt- her 1903 thesis Marie Curie made tingen named Arnold Sommerfeld these comparisons: γ rays to X rays; carried out a mathematical analy- β rays to cathode rays; and α rays to If the Roentgen rays, that are way ahead, Will show us in simple note, sis of diffraction to show that their canal rays. (Canal rays were streams How, when we ask our best girl to wed, results could be explained in terms of positively charged molecules.) That lump will look in our throat. of aperiodic impulses. In 1904, A few years later another story Charles Glover Barkla, a student of came out. The British scientist If the cathode rays, that we hear all about, both Stokes and Thomson at Cam- William Henry Bragg announced in When the burglar threatens to shoot, bridge, showed that X rays were 1907 that X rays and γ rays were not Will they show us the picture without any doubt, plane polarizable while experiment- in fact ether waves, but rather par- Of the heart that we feel in our boot. ing with secondary and tertiary ticles, a neutral pair at that: electron If the new x-rays, that the papers do laud, X rays. (These were produced by plus positively charged particle. When the ghosts do walk at night, directing X rays against solids.) Bragg’s serious research began at a Will show ’neath our hat to the world abroad As X rays began to show, more and late age, 41, after twenty pleasant How our hair stands on end in our fright. more, the properties of light, urani- years at the University of Adelaide, um rays provided new mysteries. Australia, where he played golf and If the wonderful, new, electric rays, Will do all the people have said, They themselves were composed of hobnobbed with government of- And show us quite plainly, before many days, three sorts of distinct rays: α, β, and ficials. He announced his new in- Those wheels that we have in our head. γ rays. What were these? Suddenly tellectual work in a Presidential physics, which had seemed to some Address to the Australian Associa- If the Roentgen, cathode, electric, x-light, to be coming to a conclusion, was tion for the Advancement of Science Invisible! Think of that! during which he made a critical faced with unexplainable, qualita- Can ever be turned on the Congressman bright And show him just where he is at. tive discoveries. They were not “in review of Rutherford’s work, ques- the sixth place of the decimals,” as tioning the law of exponential Oh, if these rays should strike you and me, Michelson had predicted. At the decrease for the absorption of α rays. Going through us without any pain, international congress on physics, For two and a half years he published Oh, what a fright they would give us to see staged in Paris in 1900 by the French a paper every few months, work that The mess which our stomachs contain! Physical Society, fully nine percent led him to make the radical state- of the papers delivered were on the ment that X rays were particles. His —Homer C. Bennett, American X-ray Journal, 1897 new ray physics. idea was based on two facts: (i) X rays In 1899 Ernest Rutherford, another excite fewer gas molecules in their student of Thomson’s and the man path than would be expected from who would become his successor as a wave-like disturbance, and (ii) the 16 SUMMER 1995
  8. 8. velocity of the electrons excited by X rays is greater than could be giv- en to them by a wave. By this time Bragg and his physicist son were back in England, and their theory caused great controversy even in the country where particles were in favor and where exotic modeling of physical phenomena was well tolerated. Their most vociferous opponent was wondered whether such a discipline Roentgen picture of a newborn rabbit made Charles Barkla, who argued that the distinct from chemistry existed!) by J. N. Eder and E. Valenta of Vienna, 1896. ionization of matter was a secondary When in 1899 Roentgen was of- (Burndy Library, Dibner Institute, Cambridge, effect not needing to be directly fered a position at Munich and Massachusetts.) attributable to the wave-like nature the chance to build up physics there, of X rays. We will return later to the he accepted. Five years later, in problem of the concentration of X- negotiations with the minister of ray energy, unexplainable in terms education over another possible of waves, as it bears on Louis de move, this time to the Reichsanstalt, Broglie’s insight into the wave nature Roentgen received, in return for a of matter. pledge to stay in Munich, a second institute, for theoretical physics, to complement his existing institute X RAYS AS A PROBE OF THE Radiographs of tropical fish made for experimental physics. When Emil by J.N. Eder and E. Valenta of Vienna, STRUCTURE OF MATTER Cohn and Emil Weichert succes- Jaunary 1896 and presented to Before we turn to our final act in sively declined the offer of a position, Roentgen. (Burndy Library, Dibner the almost thirty year drama to un- it was given to Privatdozent Som- Institute, Cambridge, Massachusetts.) derstand the nature of X rays, let us merfeld, who joined Roentgen in turn aside to follow another direc- Munich and shared his desire to build tion that the work on X rays took, up physics there to the quality of the a shift from the investigation of the institutes in Göttingen, Berlin, and nature of X rays to their use in prob- Leipzig. In the work on quantum ing the structure of crystals and of theory of the next two decades, atoms. That story will take us back Munich would join Copenhagen and to Roentgen and the center for phys- Göttingen as the main centers on the ics he built up at Munich. While Continent. at Würzberg, Roentgen had been Sommerfeld was initially unen- agitating for an extra position in thusiastic about assistant Max von physics. He wanted a position for Laue’s idea that regularly spaced theoretical physics, a newly emerg- atoms in a crystal might act as a dif- ing specialty of German origin that fraction grating for X rays, the fine followed by several decades the crys- distances between the atoms serving, tallization of physics itself in the as no hand- or machine-ruled grating mid-nineteenth century. (In 1871 could, to diffract ultra-high frequen- James Clerk Maxwell hesitated in cies. If, of course, that is what one giving his support to the creation of thought X rays were! Sommerfeld, a Physical Society in London. He pushing the impulse hypothesis, was BEAM LINE 17
  9. 9. White radiation Laue diffraction pattern from the protein trimethylene dehydrogenase (an enzyme that catalyzes the conversion of trimethylamine to dimethylamine and formaldehyde) recorded on SSRL beam 10–2 with a 5 msec X-ray exposure. The photograph was taken by Scott Matthews and Scott White of Washington University, St. Louis, and Mike Soltis, Henry Bellamy, and Paul Phizackerly of SSRL/SLAC. engaging in dis- Later others would suggest that the cussions with crystal itself imposed structure on Johannes Stark the incoming radiation. Laue pub- over the quan- lished a rather long article on his tum nature of X theory of diffraction in the Enzyk- rays. Stark was lopadie der Mathematische Wis- one of the few senschaften, and much later (1941) physicists who he went on to publish a 350-page in 1911 took se- review of the subject, Roentgen- riously Einstein’s strahlen-Interferenzen, in which he suggestion that light comes in quan- included the effects of electron ta of energy. Applying the notion to interference. X rays, Stark was able to assign them Perhaps as was fitting for an early a frequency and to explain the high proponent of relativity and a defender velocity of electrons that had been of Einstein throughout the Nazi pe- excited by X rays, one of the phe- riod, Laue made little of quantum nomena that so exercised Bragg and theory and remained skeptical of the Barkla. Copenhagen interpretation through- Laue persisted in asking that the out his life. He became director of experimentalists try out X rays on the Kaiser Wilhelm Institute in the crystals. A student of Max Planck’s years before World War II, resigning (in fact, his favorite), Laue had his position in 1943, at which time worked on a theory of the interfer- the Institute was directed towards ence of light in plane parallel plates. the building of an atomic bomb un- By 1912 his specialty had become the der the leadership of Werner Heisen- theory of relativity, but he was not berg. After the war Laue worked to averse to following Sommerfeld in rebuild German science. In the fall working on a theory of diffraction. of 1946 he helped create the German Laue’s guess was that it would be Physical Society in the British Zone, only the secondary X rays, not the and worked to revive the first of the chaotic Bremsstrahlung identified national bureaus of standards, the with the initial deceleration of Physikalische-Technische-Reich- electrons, that would interfere con- sanstalt. Towards the end of his life structively in the crystal. In April he assumed the directorship of the 1912 Walther Friedrich and Paul now one of several Kaiser Wilhelm Knipping shone secondary X rays on Institutes, this one devoted to elec- copper sulfate and zinc sulfate sur- trochemistry in Berlin-Dahlem. Laue faces and found that dark spots in died in an auto accident at the age of successive circles appeared on pho- eighty-one. tographic plates placed behind them. Laue was representative of the At this time both the nature of X rays German talent for institution build- and the structure of crystals was a ing in the support of science and the puzzle. Laue’s analysis of the situa- German fascination for fundamen- tion was to identify five distinct tal principles and theories. Those wavelengths of incoming X rays who would apply Laue’s idea and between 1.27 and 4.83 × 10 −9 cm. build on Friedrich and Knipping’s 18 SUMMER 1995
  10. 10. Top right: Sir William Henry Bragg, 1862–1942. Lower right: Sir William Lawrence Bragg, 1890–1971. (Courtesy of the AIP Niels Bohr Library) experimental demonstration were using a photographic plate or an the British, specifically the Braggs ionization chamber (depending on and Henry Moseley. In view of the the strength of the incoming rays) as German results the Braggs had come a detector—the Braggs proceeded to believe that X rays were of an elec- with the first measurements in X-ray tromagnetic nature, but they insist- spectroscopy. By 1913, just a year af- ed that the rays must have some sort ter they had pioneered the method, of dual existence as they were able crystal analysis with X rays had to concentrate their energy. But the become a standard technique. The continuing puzzle as to their nature results not only gave insight into the did not stop the Braggs from recog- structure of crystals but also into the nizing the practicability and impor- nature of the anti-cathode that tance of a new field of study, X-ray produced the rays. crystallography. The first person to notice that X The new field was pioneered by rays can be characteristic of the sub- the Braggs. They were inspired by the stance that emits them was Charles Cambridge theorists who argued that Barkla, the opponent of the Braggs in a diffraction grating imposes a struc- the matter of X rays as neutral par- ture on an inhomogeneous pulse of ticles and a professor at the Univer- white light, picking out, as if in a sity of Edinburgh who spent over Fourier transform, the wavelengths forty years examining the properties into which the beam can be decom- of secondary X rays. Between 1906 posed. William Henry Bragg and his and 1908 he had noticed that ele- son, William Lawrence Bragg, argued ments emit secondary X rays with by analogy that the crystal, by dint a penetrating power in aluminum of the distance between planes of that is distinct for each element. To atoms, imposes a similar structure distinguish between the hardness on an inhomogeneous pulse of X of the characteristic rays, he intro- rays. When the X rays are reflected duced the terminology K and L rays. off two successive planes of atoms in It was for this discovery that he was the crystal, they interfere construc- awarded the Nobel Prize in 1917. tively if the difference in the distance (His subsequent work earned Barkla traveled is equal to an integral num- the reputation as something of a sci- ber of wavelengths. Thus the famous entific crank.) What the Braggs no- Bragg condition ticed (see figure on next page) was that a pattern of multiple peaks with n λ = 2d sin θ, varying intensities was produced no matter what the crystal (shifted only where d is the distance between by the varying distances between planes and θ is the angle of reflection. planes of atoms) as long as the ele- Using an X-ray tube and a colli- ment of the anti-cathode remained mating slit to produce the incom- the same. In other words, the pattern ing rays; using various minerals, was analogous to spectral lines emit- quartz, rock salt, iron, pyrite, ted by gases in the optical frequen- zincblende, and calcite, as three- cies. The person to explore this anal- dimensional diffraction gratings; and ogy to its fullest was Henry Moseley, BEAM LINE 19
  11. 11. B1 II B2 B Ionization C1 C A C2 A2 A1 5 10 15 20 25 I B2 Degrees C2 A2 C3? B3 0 10 20 30 Degrees a young researcher working in gave on Friday on X rays. It was One of the earliest examples of X-ray Rutherford’s Manchester laborato- rather anxious work, as Bragg, the spectroscopy. The Braggs made a ry during the time when Niels Bohr chief authority on the subject (Pro- seredipitous discovery: while studying fessor of Leeds) was present, and was visiting regularly. the scattering of X rays off of crystals as I had to be cautious. However it Moseley’s two grandfathers had proved quite successful and I they noticed that a distinctive pattern of peaks appeared for each of the different been fellows of the Royal Society, and managed to completely disguise anti-cathodes being used to produce his father had founded a school of my nervousness. I was talking the rays. What had initially started out as zoology at Oxford. Mosely himself chiefly about the new German a study of the structure of crystals led to was perhaps the only important experiments of passing rays an investigation of the atomic structure atomic physicist to be educated at through crystals. The men who did of the anti-cathode elements. [Bragg Oxford. In the fall of 1910 he came the work entirely failed to under- and Bragg, PRS, 88A, 413 (1913).] stand what it meant, and gave an to work as a demonstrator under explanation which was obviously Rutherford, his salary being paid by wrong. After much hard work Dar- a Manchester industrialist. He was win and I found the real meaning assigned a research problem to which of the experiments.* everyone knew the answer: how many β particles are emitted in the For a time the Braggs, Moseley, radioactive disintegration of radium and Darwin continued on the same B (Pb214) to radium C (Bi214). On find- track, even though Rutherford pre- ing the answer everyone expected, sented difficulties which were final- one, he proved his competency as an ly overcome by Moseley’s persistent experimentalist. However, his next enthusiasm and by Bragg’s offer to experiments would not be so cut and Moseley of a visit to Leeds to teach dried, nor would they receive the him the techniques of X-ray spec- ready approval of Rutherford. Like troscopy. Some of the questions they the Braggs, and quite independent- pursued were the old ones about the ly of them, Moseley was stimulat- nature of X rays. How to reconcile ed by the photographs of Friedrich the corpuscular nature of the rays and Knipping, and felt that Laue had with their ability to interfere? Bragg misinterpreted them as evidence of had compared this conundrum in the five homogeneous X rays. He teamed electromagnetic theory of X rays to up with Charles G. Darwin, grand- the physical impossibility of a spread- son of the famous evolutionist, and ing circle of water waves, caused by turned to, as he said, the “real mean- the fall of a rock, to excite another ing” of the German experiments. rock to jump the same distance the The Laue dots connoted the struc- wave-producing rock had fallen. ture of the crystal, not the structure The new questions concerned the of the incoming rays. When pre- elements. In July of 1913 Bohr paid a senting his results to a Friday phys- visit to Manchester and discussed ics colloquium which father Bragg atomic structure with Moseley, attended, Moseley discovered the Darwin, and George Hevesey. The similarity in their understanding of discussion revolved around the sim- H. G. J. Moseley in Balliol-Trinity the phenomena, and afterwards he ilarity, and possible differences, Laboratory, Oxford, circa 1910. (Courtesy of University of Oxford, wrote to his mother: Museum of the History of Science and I have been lazy for a couple of *Nov. 4, 1912. Quoted in J.L. Heilbron, the AIP Niels Bohr Library) days recouping after the lecture I H.G.J. Moseley, p. 194. 20 SUMMER 1995
  12. 12. Charles G. Darwin, L. M. Thomas, and Gregory Breit. (Courtesy of the Goudsmit Collection) between the atomic weight of an element (A) and its nuclear charge (Z). Geiger’s and Marsden’s scatter- ing experiments and Rutherford’s theory had proposed that the newly discovered nucleus held a charge half that of the atomic weight. A Dutch lawyer and would-be interpreter of Mendeleev’s table, Van de Broek, had suggested that the nuclear charge of an element set its place in the table. Now the frequency of characteris- 3 tic K rays gave another quantity with νK = ν (Z − 1)2 . α 4 0 which to mark the elements. What Moseley interpreted his formula would the X-ray spectroscope have as a vindication of Bohr’s theory, to say about those places in the table which at the time was being pub- where the atomic weights did not fol- lished in three lengthy and famous low in increasing order the serial papers “On the Constitution of numbers: between nickel and cobalt; Atoms.” Moseley argued, not quite between argon and potassium; and convincingly, that his results could between iodine and tellurium? be used to support the quantization Would the hardness of the K rays or- of an electron’s angular momentum. der the elements by atomic weight Frederick A. Lindeman, a fellow Eng- or by nuclear charge? lishman working with Walther Moseley used an ingenious de- Nernst on the Continent but with vice of G. W. C. Kaye’s to exam- his eye on the same chair of phys- ine the K rays from copper, nickel, ics as Moseley, Clifton’s chair at Ox- cobalt, iron, manganese, chromi- ford, criticized both Bohr and Mose- um, and titanium. By putting the ley. He was working out of an already X-ACTLY SO! different elements which served as successful tradition which applied anticathodes on a magnetized truck the condition of quantized frequen- The Roentgen Rays, the Roentgen Rays, and rail inside the evacuated cham- cies to the motion of atoms to pre- What is this craze? ber, Moseley was able to change The town’s ablaze dict specific heats in a solid and to anti-cathodes with an external With the new phase the motions of molecules in a gas magnet without disrupting the in- Of X-ray’s ways. to predict the patterns of rotational tegrity of the chamber. After and vibrational infrared spectra. (A I’m full of daze, switching from detecting the K rays quantum theory of molecular spec- Shock and amaze; by ionization to detecting them by tra preceded one of atomic spectra!) For nowadays photography, his work went quick- More successful than Moseley’s I hear they’ll gaze ly, and in several weeks he showed Thro’ cloak and gown—and even stays, argument in favor of Bohr’s atomic that the ranking of elements by K These naughty, naughty Roentgen Rays. theory was his help to the chemists rays followed their ranking by nu- in sorting out the confusions of the clear charge, Z. The relation was —Wilhelma, Electrical Review, rare earths. In November of 1913, simple as well. The darker of the April 17, 1896 Moseley moved to Oxford where two primary K lines, K α , fit the he worked with equipment at the form BEAM LINE 21
  13. 13. Arthur Holly Compton, 1892–1962. (Courtesy of the AIP Niels Bohr Library) brought to a halt frequency of X rays emitted. By 1916, by the end of the the Duane-Hunt law was the best World War I. The way to determine h, Planck’s con- Braggs’ work stant, although neither Millikan nor continued after Duane then subscribed to the view the war. The el- that energy came in discrete quan- der Bragg revivi- tized units. fied the Royal Arthur Holly Compton also Institution, initially interpreted his results on where Sir Hum- X-ray scattering from electrons as a phry Davy and cut-off relation that was governed in Michael Faraday this case by Planck’s constant rather had made their than as proof of the quantum nature chemical and of radiation. Compton, who was later electrical dis- to run the Manhattan Project’s Met- coveries, by es- allurgical Laboratory at Chicago tablishing a re- during World War II, received his search school for Ph.D. from Princeton just before the the analysis of First World War for work on X-ray organic crystals. diffraction and scattering. After Electrical Laboratory but with no This work would become central to several years spent at Westinghouse salary. Moseley began a thorough in- the developing field of molecular bi- Manufacturing Company working vestigation of Mendeleev’s table us- ology. During the war the elder Bragg on fluorescent lamps, he spent a year ing X rays, and moving from calcium worked for the Navy board to eval- at Cambridge’s Cavendish Laborato- to zinc and then to the rare earths, uate inventions and to promote re- ry where he developed a friendship lanthanum to erbium. George Ur- search with military applications. with J.J. Thomson and carried out an bain, a Professor of Chemistry in Like many British and U.S. scientists investigation into the orderly change Paris at one of the Grands Ecoles had he eventually found himself work- of X-ray frequency with scattering been engaged for years in fraction- ing on problems of submarine de- angle as the X rays scattered from ating the elemental rare earths in tection. Moseley, with his Eton pa- electrons. As a new professor at competition with Carl Auer von triotism, practically forced himself Washington University in St. Louis, Welsback, who performed his frac- upon the Royal Engineers along with Compton published a mass of data tionations in an Austrian castle. his friend Henry Tizard. Tizard sur- on the relation between X-ray fre- Urbain recognized the power of vived the Great War and subse- quency and scattering angle taken Moseley’s technique and paid him quently led the minds of British sci- with a Bragg crystal spectrometer. In a visit with precious samples of the entists into World War II. Moseley, 1922, a year after he had taken the last four rare earths, thulium, ytter- however, died at Gallipoli in the bat- measurements, and along with Peter bium, lutecium, and celtium. He was tle of Sari Bari. Debye in Germany who had seen his astounded at how quickly Mose- As Europe engaged itself in the results in the Bulletin of the Na- ley’s X-ray spectrometer could de- Great War, interesting work on X tional Research Council, Compton termine that celtium was not his rays began to come out of the Unit- accepted Einstein’s light quantum sought after new element, but was ed States. Following Robert Mil- and by extension the X-ray quantum. only a combination of lutecium and likan’s work on the photoelectric The explanation of the Compton- ytterbium! effect, William Duane at Harvard effect then became a simple scatter- The Braggs’ work on crystals and gave an exact law that related the en- ing of two elastic particles. that of Moseley’s on elements was ergy of cathode ray electrons to the 22 SUMMER 1995
  14. 14. Niels Bohr, 1885–1962. (Courtesy of the AIP Niels Bohr Library Few physicists had taken Einstein seriously when he predicted the light quantum in 1905. Bohr had pooh-poohed the idea. But by 1921 evidence was mounting and so was Einstein’s fame. The de Broglie brothers, Maurice and Louis, were two others who had learned from the studies of X rays of the dual nature of radiation, and Louis was inspired to suggest that matter too might have vinced most physicists that they this dual nature. Maurice de Broglie’s were waves. The Braggs, not quite interest in the quantum had been giving up, insisted that they had the sparked by his secretaryship of the properties of both waves and parti- first Solvay Conference called in cles. By 1922 the startling explana- 1911 by chemist Walther Nernst to tion by Compton of his scattering introduce the quantum concept to experiments—X-ray energy was physical scientists, and he decided to concentrated into particle points— investigate the energies of electrons helped convince the science com- excited by K and L frequency X rays. munity to take Einstein’s notion of He found the old problem over which light quanta seriously. And finally Bragg and Barkla had argued: X rays the work on X rays by the de Broglies, can concentrate their entire energy and the younger brother’s desire to and pass it on to electrons. And like put on an equal footing light and Bragg he concluded that X rays act matter, gave Louis de Broglie the both as waves and as particles. His courage to suggest that even the good younger brother, Louis, in a spirit old electron (the cathode ray parti- of unification, longed to treat light cles!) partook of wave qualities. and matter as equals. Both could be understood as particles following ADDENDUM waves, he proposed. A “mobile” of light or X rays or of matter followed The early history of X rays follows along behind an “onde fictive.” another path that I have not covered So the discussion of X rays had here. As the physicists wondered come around full circle. They were about the nature of X rays and used discovered in Roentgen’s laborato- them to probe the structure of crys- ry as this newcomer to cathode rays tals and atoms, medical doctors used was trying to puzzle out his coun- them to probe the human body and tryman Lenard’s challenge to the to diagnose and treat disease. Roent- British. Lenard believed cathode rays gen by presenting an X-ray photo- to be ethereal. The British thought graph of his wife’s hand to the them particles. Soon X rays became Würzburg Physical and Medical the new mystery. Were they elec- Society in January of 1896 began the tromagnetic waves or were they practice of radiology. A month later neutral pairs of particles? By 1913 a German doctor used an X ray to the interference of X rays had con- diagnose sarcoma of the tibia in the BEAM LINE 23
  15. 15. Louis de Broglie, 1892–1987. (Courtesy of the AIP Niels Bohr Library) right leg of a young boy. The military Two of the three discoveries that first used X rays in Naples in May of helped shake physics out of its fin- 1896 to locate bullets in the forearms de-siècle malaise, X rays and of two soldiers who had been wound- radioactivity, would be taken up, ed in Italy’s Ethiopian campaign. almost immediately, by doctors in Radiology would be advanced their medical practice. And as by the strong tradition of medical physicists began to require substan- research in France. Antoine Béclère tial funds to continue their quest to set up the first X-ray machine in discover the smallest structures of which a patient was strapped and matter, the link between physics and moved around for complete X rays of medicine would be pushed. Ernest the chest. For those taking pictures Lawrence regularly raised money for he introduced safety equipment, lead his laboratory’s cyclotrons by vir- aprons and lead rubber gloves. He tually promising cures for cancer: “It pioneered the first use of radiography is almost unthinkable that the man- of the stomach in 1906. The patient ifold new radiations and radioactive had first a meal of bismuth. Through substances [produced by his cy- the work of Béclère and others the clotrons] should not greatly extend practice of medical diagnosis changed the successful range of application significantly. Soon to follow was the of radiation therapy.”* use of X rays to treat cancer. The rays An informal moment at an informal *Quoted in J.L. Heilbron and Robert of the chemists and physicists conference called by Paul Ewald Seidel, Lawrence and his Laboratory, in 1925. From left: Paul P. Ewald, seemed to inspire doctors: α, β, and γ (Berkeley, University of California Press, Charles G. Darwin, H. Ott, rays were also beamed at cancerous 1989) p. 215. William L. Bragg, and R. W. James. tumors. (The Isidor Fankuchen Collection) FOR FURTHER READING John Heilbron, H.G.J. Moseley: The Life and Letters of an Eng- lish Physicist (Berkeley, U.C. Press, 1974). Bruce Wheaton, The Tiger and the Shark: Empirical Roots of Wave-Particle Dualism (Cam- bridge, Cambridge University Press, 1983) One can also find good bibli- ographies in the numerous articles in the Dictionary of Scientific Biography under the individual scientists discussed above. 24 SUMMER 1995