Early History of X Raysys by ALEXI ASSMUS The The discovery ofX rays TEET吓阻 in 1895 was the THROUCH A BLOCK OF WOOD beginning of a “如 unl Ihe Goins wihin your PuIse revolutionary change ADMISSION 3d AL DAv in our understanding WONDERFUL NET X RAY PHOTOGRAPHS TAKEN of the physical world SEES THROUGH N THE WINTER of the year of his fiftieth birthday, and the year following his appointment to the leadership of the University of Wurzburg, Rector Wilhelm Conrad Roentgen noticed a barium platinocyanide screen fluorescing in his laboratory as he ARE 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 colleague 10 SUMMER 1995
Early History of X Rays by ALEXI ASSMUS 10 SUMMER 1995 The discovery of X rays in 1895 was the beginning of a revolutionary change in our understanding of the physical world. I N 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 laboratory, working alone, and sharing nothing with his colleagues
Wilhelm Conrad Roentgen (1845-1923, (Courtesy of AlP Emilio Segre Visual Three days before Christmas he that jolted the fin- brought his wife into his laborato- de-siecle 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 wurzburg 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 al ary. The next day the wiener Press physical phenomena in carried the news, and the day fol- mathematically precise lowing word of Roentgens 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 resented 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 theThe 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 th 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. "(an. 16, 1896, p 9) ly on, English The public was enthralled by this scientist new form of photography and curi- investigating the 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
BEAM LINE 11 Three days before Christmas he brought his wife into his laboratory, and they emerged with a photograph of the bones in her hand and of the ring on her finger. The Würzburg Physico-Medical Society was the first to hear of the new rays that could penetrate the body and photograph its bones. Roentgen delivered the news on the 28th of December 1895. Emil Warburg relayed it to the Berlin Physical Society on the 4th of January. The next day the Wiener Press carried the news, and the day following word of Roentgen’s discovery began to spread by telegraph around the world. On the 13th of January, Roentgen presented himself to the Kaiser and was awarded the Prussian Order of the Crown, Second Class. And on the 16th of January the The New-York Times announced the discovery as a new form of photography, which revealed hidden solids, penetrated wood, paper, and flesh, and exposed the bones of the human frame. “Men of science in this city are awaiting with the utmost impatience the arrival of English technical journals which will give them the full particulars of Professor Roentgen’s discovery of a method of photographing opaque bodies,” The New-York Times began, and it concluded by predicting the “transformation of modern surgery by enabling the surgeon to detect the presence of foreign bodies.” (Jan. 16, 1896, p. 9) The public was enthralled by this new form of photography and curious to know the nature of the new rays. Physicians put it to immediate use. Physicists sat up and took notice. The discovery of X rays was the first in a series of three discoveries that jolted the finde-siècle discipline out of its mood of finality, of closing down the books with ever more precise measurements, of losing itself in debates over statistical mechanics, or of trying to ground all physical phenomena in mathematically precise fluctuations of the ether. All three discoveries, X rays, uranium rays, and the electron, followed from one of the major experimental traditions in the second half of the nineteenth century, the study of the discharge of electricity in gases. All three contributed to a profound transformation of physics. In the 20th century, the discipline has been grounded in the study of elementary particles. As with the invention of incandescent light bulbs, the study of electrical discharge through gases was made possible by the development of improved vacuum technology in the 1850s. Early on, English scientists were investigating the patterns of light and dark that appeared in sealed lead-glass tubes. The patterns in Wilhelm Conrad Roentgen (1845–1923). (Courtesy of AIP Emilio Segré Visual Archives) Forms of tube used by Roentgen in 1895–1896 for the production of X rays. German Museum, Munich
Roentgens 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 enard, began to study them more carefully. Lenard designed a tube 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 Faradays: 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 Roentgens 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 Wurzburg, 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 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
12 SUMMER 1995 transparent to ultra-violet light. When Heinrich Hertz found that he could pass the rays through metal foil, a fellow German scientist, Philip 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 fluorescence. Defined in this way, the range of the cathode rays was six to eight centimeters. Lenard’s experiments inspired Roentgen to wonder if the rays in an attenuated form really traveled farther, and he planned experiments to see if a sensitive electroscope would measure a discharge at four times the distance Lenard had identified. This line of work was outside Roentgen’s usual research pursuits, which had by this time gained him great stature in German science. Son of a cloth manufacturer and merchant from the Rhine province, Roentgen was not a particularly diligent student in his youth. He eventually made his way to the Polytechnic in Zurich, where he obtained a diploma in mechanical engineering in 1868 and a doctorate one year later. In Zurich he became an assistant to August Kundt and moved along with him to the University of Würzburg, and then on to the Physical Institute at Strasbourg. His first move on his own was to the chair of physics at Giessen in Hesse in 1879, from which he received many offers to go elsewhere. The path upward in the German university system was to follow calls to universities of higher and higher stature, and finally to obtain an institute of one’s own. Roentgen these partially evacuated tubes were stimulated by a voltage drop between a cathode and an anode: typically there was a dark space, called Crookes’ dark space; then a glow, called negative light; then another dark space, this one called Faraday’s; and a final glow of positive light. If the air in the tube was exhausted until the first dark space expanded to fill the entire tube and all glows disappeared, then the rays emitted from the cathode could be investigated. The rays cast shadows, and were deflected by magnetic fields, but appeared to be immune to the effects of static electric forces. As was to be characteristic of the new ray physics to come—the physics of cathode rays, X rays, alpha rays, beta rays, gamma rays, and N rays— the nature of the cathode rays was in dispute, the British favoring a stream of particles, those on the Continent preferring to think of them as some sort of disturbance of the ether. (The British position, and the research program developed by J.J. Thomson at the Cavendish Laboratory to study ionization in gases, would result in the discovery of the electron. But our story does not take us that way). A strong reason for believing that the cathode rays were particles was the observation that they would not pass through matter that was Sir Joseph John Thomson, 1856–1940. (Courtesy of the AIP Niels Bohr Library) Roentgen’s apparatus for studying the ionization of air by X rays, 1906. German Museum, Munich
efused the calls until the universi- centimeters that Lenard had found y of Wurzburg 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 Wurzburg. 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- Philip Lenard, 1862-1947.(Courtesy of one's profession is demanded, but not ognize the significance of this effect. Ulstein Bilderdienst and the A/P 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 Thomson Roentgens 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 Wurzburg. These early interests fogged photographic plates. But anged 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 2b even Faradays measurement of the N rotation of polarized light in gas oentgen 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 cro jokes was operating. The screen sat much farther away than the six to eight Quoted in"Wilhelm Conrad Roentgen, Dictionary of Scientific Biography (New York: Scribners, 1975), p. 531 BEAM LINE 13
BEAM LINE 13 refused the calls until the University of Würzburg offered him the Directorship of their Physical Institute. In 1894 he was elected Rector at Würzburg. In his inaugural address, given the year before his discovery of X rays, Roentgen stated that the “university is a nursery of scientific research and mental education” and cautioned that “pride in one’s profession is demanded, but not professional conceit, snobbery, or academic arrogance, all of which grow from false egoism.”* Roentgen’s pride could rest in the over forty papers he had published from Strasbourg, Giessen, and Würzburg. These early interests ranged widely—crystals, pyroelectrical and piezoelectrical phenomena, and the effects of pressure on liquids and solids—but did not yet include electrical discharges in gases. He had taken his turn at measuring the specific heat ratios of gases using a sensitive thermometer of his own making. He was an exact experimenter who often made his own apparatus—a skill learned during his training as an engineer in Zurich— and he was able to measure extremely 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 cathode rays. In the course of repeating the experiments of Hertz and Lenard, he happened to notice a glowing fluorescent screen set off quite some distance from the Crookes’ tube he was operating. The screen sat much farther away than the six to eight centimeters that Lenard had found to be the maximum distance for which cathode rays maintain their power to induce fluorescence. Roentgen recognized the effect as worthy of his undivided attention and devoted the next six weeks to its uninterrupted study. Historians have speculated about why Roentgen was the first to recognize the significance of this effect. The equipment, a cathode ray tube and a fluorescing screen, had been in use for decades. In 1894 J.J. Thomson had seen fluorescence in Germanglass tubing several feet from the discharge tube. Others had noted fogged photographic plates. But before Lenard’s work, the object of study was always the effects inside the tube itself, and stray ultra-ultraviolet light could be used to explain the fogging of photographic plates. Lenard’s great interest was in proving, in contradiction to the British, the ethereal nature of cathode rays, and he was the first to study the Demonstration by Crookes that cathode rays travel in straight lines: a) cathode; b) aluminum cross and anode; d) dark shadow; c) fluorescent image. Phillip Lenard, 1862–1947. (Courtesy of Ullstein Bilderdienst and the AIP Niels Bohr Library) *Quoted in “Wilhelm Conrad Roentgen,” Dictionary of Scientific Biography (New York: Scribner’s, 1975), p. 531
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 rvant 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 Segre Visual Archives) to refract them with water and car- photography. Since the rays expose 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 Rontgen, then the news is true could pass freely through thick lay- ferent from visible, infra-red and And not a trick fide 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 We do not want like Dr: Swift. because of refraction and reflection, tudinal vibrations of the ether rather To take our fesh offand is hardly passed at all. He concluded than transverse ones ones or that X rays were not susceptible to Now that their existence was And joint for you to poke e your nose n egular 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 Each others usual full-dress photo originate from the bright fluores- Roentgen himself published only Your worse than altogether"state cence on the tube where the cathode three papers on the subject, but oth- Ofportraiture 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 Would probably be voted tame work/ themselves are insensitive to the He boasted to reporters that any magnet Roentgen concluded that one could make photographs of No, keep them for your epita 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 Orgo 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
14 SUMMER 1995 effects of the rays in air or in a second glass tube into which he directed them. Roentgen, a meticulous and observant experimenter, made the obvious tests on the new X rays: Were they propagated in straight lines? Were they refracted? Were they reflected? Were they distinct from cathode rays? What were they? Like the cathode rays, they moved in straight lines. Roentgen was unable to refract them with water and carbon bisulphide in mica prisms. Nor could he concentrate the rays with ebonite or glass lenses. With ebonite and aluminum prisms he noted the possibility of refracted rays on a photographic plate but could not observe this effect on a fluorescent screen. Testing further, he found that X rays could pass freely through thick layers of finely powdered rock salt, electrolytic salt powder, and zinc dust, unlike visible light which, because of refraction and reflection, is hardly passed at all. He concluded that X rays were not susceptible to regular refraction or reflection. Roentgen found that the X rays originate from the bright fluorescence on the tube where the cathode rays strike the glass and spread out. The point of origin of the X rays moves as the cathode rays are moved by a magnetic field, but the X rays themselves are insensitive to the magnet. Roentgen concluded that they are distinct from cathode rays, since Lenard’s work had shown that cathode rays passing through the tube maintained their direction but were susceptible to magnetic deflection. Roentgen justified calling the new phenomena rays because of the O, Röntgen, then the news is true, And not a trick of idle rumour, That bids us each beware of you, And of your grim and graveyard humour. We do not want, like Dr. Swift, To take our flesh off and to pose in Our bones, or show each little rift And joint for you to poke your nose in. We only crave to contemplate Each other’s usual full-dress photo; Your worse than “altogether” state Of portraiture we bar in toto! The fondest swain would scarcely prize A picture of his lady’s framework; To gaze on this with yearning eyes Would probably be voted tame work! No, keep them for your epitaph, these tombstone-souvenirs unpleasant; Or go away and photograph Mahatmas, spooks, and Mrs. B-s-nt! —Punch, January 25, 1896 shadowy pictures they produce: bones in a hand, a wire wrapped around a bobbin, weights in a box, a compass card and needle hidden away in a metal case, the inhomogeneity of a metal. The ability of the new rays to produce photographs gave them great popular appeal and brought Roentgen fame. Many articles appeared in photography journals, and The New-York Times indexed the new discovery under photography. Since the rays exposed photographic plate, the public assumed they were some form of light. The physicist Roentgen concurred. Accepting Lenard’s claim that cathode rays were vibrations of the ether, Roentgen compared the new rays to them and forwarded the opinion that the two were ethereal, although different from visible, infra-red and ultra-violet light in that they did not reflect or refract. He suggested that cathode rays and X rays were longitudinal vibrations of the ether rather than transverse ones. Now that their existence was established, it was easy enough to experiment with the new X rays. Roentgen himself published only three papers on the subject, but others jumped quickly into the field. And not just physicists. Thomas Edison used modified incandescent light bulbs to produce the new rays. He boasted to reporters that anyone could make photographs of skeleton hands; that was mere child’s play. Within a month of Roentgen’s announcement doctors were using the X rays to locate bullets in human flesh and photograph broken bones. Dr. Henry W. Cattell, Demonstrator of Morbid Anatomy at the University of Pennsylvania, confirmed their Heinrich Rudolf Hertz, 1857–1894. (Courtesy of Deutsches Museum and AIP Emilio Segrè Visual Archives)
Henri Poincare, 1854-1912.(Courtesy of AlP Emilio Segre 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 everity 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(Roentgens 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 Poincare and Lord Kelvin. The ward of an electromagnetic pulse crux of the question was whether the With Thomsons exact measurement waves were polarizable. If so they of the charge-to-mass ratio and HA 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 Kolliker made durin set up a convincing typology. It went Soon new results began to Roentgen's initial lecture before the Wurzburg from lower to higher frequency come in. Two Dutch physicists, Physical Medical Society on January 23,1896 BEAM LINE 15
BEAM LINE 15 importance for the diagnosis of kidney stones and cirrhotic livers and commented that “The surgical imagination can pleasurably lose itself in devising endless applications of this wonderful process.” (The New-York Times, Feb. 15, 1896, p. 9). In the first six months after their discovery Viennese mummies were undressed, doctors claimed to have photographed their own brains, and the human heart was uncovered. By 1897 the rays’ dangerous side began to be reported: examples included loss of hair and skin burns of varying severity. Electricians and physicists speculated on the nature of these X rays. Albert Michelson thought they might be vortices in the ether. Thomas Edison and Oliver Lodge suggested acoustical or gravitational waves. But the rays ability to photograph was decisive, and serious thinkers settled on three possibilities, all of them of electromagnetic origin: the waves were very high frequency light; they were longitudinal waves (Roentgen’s initial suggestion); or they were transverse, discontinuous impulses of the ether. Quite early on the hypothesis that they were longitudinal waves was discarded, despite the support of Henri Poincaré and Lord Kelvin. The crux of the question was whether the waves were polarizable. If so they could not be longitudinal waves. Although the early experiments on polarization were negative or unclear, with the discovery of another ray, Henri Bequerel’s uranium rays for which he claimed to have found polarization, those on the Continent set up a convincing typology. It went from lower to higher frequency transverse ethereal vibrations: light, uranium rays, X rays. Uranium rays were given off by certain minerals, and they needed no apparatus to produce them, but they shared certain properties with X rays. They exposed photographic plates and they caused gases to conduct electricity. British physicists weighed in on the side that X rays were impulses in the ether rather than continuous waves. Lucasian Professor of Mathematics at Cambridge, Sir George Gabriel Stokes, and his colleague and director of the Cavendish Laboratory, J.J. Thomson, committed themselves to the impulse hypothesis in 1896. It was consistent with their conception of cathode rays as particles (Thomson was to announce the discovery of the corpuscle or electron one year later.) The abrupt stop of a charged particle would result, after a tiny delay, in the propagation outward of an electromagnetic pulse. With Thomson’s exact measurement of the charge-to-mass ratio and H.A. Lorentz’ successful theory of the electron, which explained many intriguing phenomena, Continental physicists began to accept, to Lenard’s dismay, cathode rays as material particles and X rays as impulses in the ether. Soon new results began to come in. Two Dutch physicists, First X ray made in public. Hand of the famed anatomist, Albert von Kölliker, made during Roentgen's initial lecture before the Würzburg Physical Medical Society on January 23, 1896. Henri Poincaré, 1854–1912. (Courtesy of AIP Emilio Segrè Visual Archives)
Arnold Johannes wilhelm sommerfeld 1868-1951.(Courtesy of the AIP Niels Bohr Library) had separated a rays, stoppable by metal foil or paper sheets, from the In1900 Rutherford had identified the Bs as high-speed electrons: deflected in a magnetic field they showed the cor 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 y rays. In WE WANT TOKNOW fracted. and a privatdozent at got her 1903 thesis Marie Curie made tingen named Arnold Sommerfeld these comparisons: y rays to X rays If thhe Roentgen rays, that are way ahead, carried out a mathematical analy- Brays to cathode rays, and a rays to 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 gir/ 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, vill they show us the picture without any doubt bridge, showed that X rays were 1907 that X rays and y rays were not Of the heart that we feel in our boot. plane polarizable while experiment- in fact ether waves, but rather par- 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. Braggs 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 If the wonderful new, electric rays, um rays provided new mysteries. Australia, where he played golf and 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: a, B, and ficials. He announced his new in- Those wheels that we have in our head 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/ faced with unexplainable, qualita- during which he made a critical Can ever be turned on the Congressman brightI tive discoveries. They were not"in review of Rutherfords work, ques- And show him just where he is at the sixth place of the decimals, as tioning the law of exponential Oh, ifthese rays should strike you and me Michelson had predicted. At the decrease for the absorption of arays Going through us without any pai 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- -Homer C. Bennett. of the papers delivered were on the ment that X rays were particles. His X-ray Journal, 1897 new ray physics idea was based on two facts: (i)Xrays In 1899 Ernest Rutherford, another excite fewer gas molecules in their student of Thomsons 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
16 SUMMER 1995 Hermann Haga and Cornelius Werd, announced that X rays could be diffracted, and a Privatdozent at Göttingen named Arnold Sommerfeld carried out a mathematical analysis of diffraction to show that their results could be explained in terms of aperiodic impulses. In 1904, Charles Glover Barkla, a student of both Stokes and Thomson at Cambridge, showed that X rays were plane polarizable while experimenting with secondary and tertiary X rays. (These were produced by directing X rays against solids.) As X rays began to show, more and more, the properties of light, uranium rays provided new mysteries. They themselves were composed of three sorts of distinct rays: α, β, and γ rays. What were these? Suddenly physics, which had seemed to some to be coming to a conclusion, was faced with unexplainable, qualitative discoveries. They were not “in the sixth place of the decimals,” as Michelson had predicted. At the international congress on physics, staged in Paris in 1900 by the French Physical Society, fully nine percent of the papers delivered were on the new ray physics. In 1899 Ernest Rutherford, another student of Thomson’s and the man who would become his successor as WE WANT TO KNOW If the Roentgen rays, that are way ahead, Will show us in simple note, How, when we ask our best girl to wed, That lump will look in our throat. If the cathode rays, that we hear all about, When the burglar threatens to shoot, Will they show us the picture without any doubt, Of the heart that we feel in our boot. If the new x-rays, that the papers do laud, When the ghosts do walk at night, Will show ’neath our hat to the world abroad How our hair stands on end in our fright. If the wonderful, new, electric rays, Will do all the people have said, And show us quite plainly, before many days, Those wheels that we have in our head. If the Roentgen, cathode, electric, x-light, Invisible! Think of that! Can ever be turned on the Congressman bright And show him just where he is at. Oh, if these rays should strike you and me, Going through us without any pain, Oh, what a fright they would give us to see The mess which our stomachs contain! —Homer C. Bennett, American X-ray Journal, 1897 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 correct charge-to-mass ratio. A third component of the uranium rays, undeviable and highly penetrating, was discovered by Paul Villard at the Ecole Normale Superieur in Paris. Rutherford named these γ rays. In her 1903 thesis Marie Curie made these comparisons: γ rays to X rays; β rays to cathode rays; and α rays to canal rays. (Canal rays were streams of positively charged molecules.) A few years later another story came out. The British scientist William Henry Bragg announced in 1907 that X rays and γ rays were not in fact ether waves, but rather particles, a neutral pair at that: electron plus positively charged particle. Bragg’s serious research began at a late age, 41, after twenty pleasant years at the University of Adelaide, Australia, where he played golf and hobnobbed with government officials. He announced his new intellectual work in a Presidential Address to the Australian Association for the Advancement of Science during which he made a critical review of Rutherford’s work, questioning the law of exponential decrease for the absorption of α rays. For two and a half years he published a paper every few months, work that led him to make the radical statement that X rays were particles. His idea was based on two facts: (i) X rays excite fewer gas molecules in their path than would be expected from a wave-like disturbance, and (ii) the Arnold Johannes Wilhelm Sommerfeld, 1868–1951. (Courtesy of the AIP Niels Bohr Library)
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 reat 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 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 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 X RAYS AS A PROBE OF THE omplement his existing institute STRUCTURE OF MATTER for experimental physics. When Emil by J.N. Eder and E Valenta of Vienna 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 Gottingen, 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- Gottingen as the main centers on the ics he built up at Munich. While Continent. at Wurzberg, 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 followedyof physics itself in the as no hand-or machine-ruled grating several decades the crys- distances between the atoms serving tallizati 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 hypothe BEAM LINE 17
BEAM LINE 17 velocity of the electrons excited by X rays is greater than could be given 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 Charles Barkla, who argued that the ionization of matter was a secondary effect not needing to be directly attributable to the wave-like nature of X rays. We will return later to the problem of the concentration of Xray energy, unexplainable in terms of waves, as it bears on Louis de Broglie’s insight into the wave nature of matter. X RAYS AS A PROBE OF THE STRUCTURE OF MATTER Before we turn to our final act in the almost thirty year drama to understand the nature of X rays, let us turn aside to follow another direction that the work on X rays took, a shift from the investigation of the nature of X rays to their use in probing the structure of crystals and of atoms. That story will take us back to Roentgen and the center for physics he built up at Munich. While at Würzberg, Roentgen had been agitating for an extra position in physics. He wanted a position for theoretical physics, a newly emerging specialty of German origin that followed by several decades the crystallization of physics itself in the mid-nineteenth century. (In 1871 James Clerk Maxwell hesitated in giving his support to the creation of a Physical Society in London. He wondered whether such a discipline distinct from chemistry existed!) When in 1899 Roentgen was offered a position at Munich and the chance to build up physics there, he accepted. Five years later, in negotiations with the minister of education over another possible move, this time to the Reichsanstalt, Roentgen received, in return for a pledge to stay in Munich, a second institute, for theoretical physics, to complement his existing institute for experimental physics. When Emil Cohn and Emil Weichert successively declined the offer of a position, it was given to Privatdozent Sommerfeld, who joined Roentgen in Munich and shared his desire to build up physics there to the quality of the institutes in Göttingen, Berlin, and Leipzig. In the work on quantum theory of the next two decades, Munich would join Copenhagen and Göttingen as the main centers on the Continent. Sommerfeld was initially unenthusiastic about assistant Max von Laue’s idea that regularly spaced atoms in a crystal might act as a diffraction grating for X rays, the fine distances between the atoms serving, as no hand- or machine-ruled grating could, to diffract ultra-high frequencies. If, of course, that is what one thought X rays were! Sommerfeld, pushing the impulse hypothesis, was Radiographs of tropical fish made by J.N. Eder and E. Valenta of Vienna, Jaunary 1896 and presented to Roentgen. (Burndy Library, Dibner Institute, Cambridge, Massachusetts.) Roentgen picture of a newborn rabbit made by J. N. Eder and E. Valenta of Vienna, 1896. (Burndy Library, Dibner Institute, Cambridge, Massachusetts.)
White radiation Laue diffraction pattern from the protein trimethylene dehydrogenase (an enzyme that catalyzes the conversion of trimethylamine to dimethylamine and :,..,...,., X-ray exposure. The photograph was taken by Scott Matthews formaldehyde) recorded on ssRL beam 10-2 with a 5 msec 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) "t.:-:.... in 1911 took se. review of the subject, Roentgen riously Einsteins strahlen-Interferenzen, in which he suggestion that light comes in quan included the effects of electron ta of energy. Applying the notion to interference Xrays, 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 Il, 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- BEFORE LEA HE EXHIBITION By 1912 his specialty had be come e der the leadership of Werner Heisen- theory of relativity, but he was not berg. After the war Laue worked te averse to following Sommerfeld in rebuild German science. In the fall THE WONDROUS working on a theory of diffraction. of 1946 he helped create the german X RAYS 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 Th chaotic Bremsstrahlung identified national bureaus of standards, the Greatest Scientific Discovery 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 enabled to see 1912 Walther Friedrich and Paul now one of several Kaiser wilhelm THROUGH A SHEET OF mMETAL" Knipping shone secondary xrays on Institutes this one devoted to elec THROUCH A BLOCK OF wooD" faces and found that dark spots in died in an auto accident at the age of Count the Coins witin your Purse. tographic plates placed behinds oho. eighty-one successive circles appeared on em. Laue was representative of the At this time both the nature of X rays german talent for institution build- ADMISSION.3d. and the structure of crystals was a ing in the support of science and the OPEN A puzzle. Laue's analysis of the situa- German fascination for fundamen tion was to identify five distinct tal principles and theories. Those X RAY PHOTOCRAPHS TAKEN. wavelengths of incoming X rays who would apply Laue's idea and between 1.27 and 4.83 x 10- cm. build on Friedrich and Knipping's 18 SUMMER 1995
18 SUMMER 1995 engaging in discussions with Johannes Stark over the quantum nature of X rays. Stark was one of the few physicists who in 1911 took seriously Einstein’s suggestion that light comes in quanta of energy. Applying the notion to X rays, Stark was able to assign them a frequency and to explain the high velocity of electrons that had been excited by X rays, one of the phenomena that so exercised Bragg and Barkla. Laue persisted in asking that the experimentalists try out X rays on crystals. A student of Max Planck’s (in fact, his favorite), Laue had worked on a theory of the interference of light in plane parallel plates. By 1912 his specialty had become the theory of relativity, but he was not averse to following Sommerfeld in working on a theory of diffraction. Laue’s guess was that it would be only the secondary X rays, not the chaotic Bremsstrahlung identified with the initial deceleration of electrons, that would interfere constructively in the crystal. In April 1912 Walther Friedrich and Paul Knipping shone secondary X rays on copper sulfate and zinc sulfate surfaces and found that dark spots in successive circles appeared on photographic plates placed behind them. At this time both the nature of X rays and the structure of crystals was a puzzle. Laue’s analysis of the situation was to identify five distinct wavelengths of incoming X rays between 1.27 and 4.83 × 10−9 cm. 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. Later others would suggest that the crystal itself imposed structure on the incoming radiation. Laue published a rather long article on his theory of diffraction in the Enzyklopadie der Mathematische Wissenschaften, and much later (1941) he went on to publish a 350-page review of the subject, Roentgenstrahlen-Interferenzen, in which he included the effects of electron interference. Perhaps as was fitting for an early proponent of relativity and a defender of Einstein throughout the Nazi period, Laue made little of quantum theory and remained skeptical of the Copenhagen interpretation throughout his life. He became director of the Kaiser Wilhelm Institute in the years before World War II, resigning his position in 1943, at which time the Institute was directed towards the building of an atomic bomb under the leadership of Werner Heisenberg. After the war Laue worked to rebuild German science. In the fall of 1946 he helped create the German Physical Society in the British Zone, and worked to revive the first of the national bureaus of standards, the Physikalische-Technische-Reichsanstalt. Towards the end of his life he assumed the directorship of the now one of several Kaiser Wilhelm Institutes, this one devoted to electrochemistry in Berlin-Dahlem. Laue died in an auto accident at the age of eighty-one. Laue was representative of the German talent for institution building in the support of science and the German fascination for fundamental principles and theories. Those who would apply Laue’s idea and build on Friedrich and Knipping’s
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 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 tance of a new field of study, X-ray produced the ray ti-cathode that nizing the practicability and impor- nature of the an 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 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 nλ=2 d sin e, arying intensities was produced no matter what the crystal(shifted only where d is the distance between by the varying distances between planes and Ais 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
experimental demonstration were the British, specifically the Braggs and Henry Moseley. In view of the German results the Braggs had come to believe that X rays were of an electromagnetic nature, but they insisted that the rays must have some sort of dual existence as they were able to concentrate their energy. But the continuing puzzle as to their nature did not stop the Braggs from recognizing the practicability and importance of a new field of study, X-ray crystallography. The new field was pioneered by the Braggs. They were inspired by the Cambridge theorists who argued that a diffraction grating imposes a structure on an inhomogeneous pulse of white light, picking out, as if in a Fourier transform, the wavelengths into which the beam can be decomposed. William Henry Bragg and his son, William Lawrence Bragg, argued by analogy that the crystal, by dint of the distance between planes of atoms, imposes a similar structure on an inhomogeneous pulse of X rays. When the X rays are reflected off two successive planes of atoms in the crystal, they interfere constructively if the difference in the distance traveled is equal to an integral number of wavelengths. Thus the famous Bragg condition n λ = 2d sin θ, where d is the distance between planes and θ is the angle of reflection. Using an X-ray tube and a collimating slit to produce the incoming rays; using various minerals, quartz, rock salt, iron, pyrite, zincblende, and calcite, as threedimensional diffraction gratings; and BEAM LINE 19 using a photographic plate or an ionization chamber (depending on the strength of the incoming rays) as a detector—the Braggs proceeded with the first measurements in X-ray spectroscopy. By 1913, just a year after they had pioneered the method, crystal analysis with X rays had become a standard technique. The results not only gave insight into the structure of crystals but also into the nature of the anti-cathode that produced the rays. The first person to notice that X rays can be characteristic of the substance that emits them was Charles Barkla, the opponent of the Braggs in the matter of X rays as neutral particles and a professor at the University of Edinburgh who spent over forty years examining the properties of secondary X rays. Between 1906 and 1908 he had noticed that elements emit secondary X rays with a penetrating power in aluminum that is distinct for each element. To distinguish between the hardness of the characteristic rays, he introduced the terminology K and L rays. It was for this discovery that he was awarded the Nobel Prize in 1917. (His subsequent work earned Barkla the reputation as something of a scientific crank.) What the Braggs noticed (see figure on next page) was that a pattern of multiple peaks with varying intensities was produced no matter what the crystal (shifted only by the varying distances between planes of atoms) as long as the element of the anti-cathode remained the same. In other words, the pattern was analogous to spectral lines emitted by gases in the optical frequencies. The person to explore this analogy to its fullest was Henry Moseley, Top right: Sir William Henry Bragg, 1862–1942. Lower right: Sir William Lawrence Bragg, 1890–1971. (Courtesy of the AIP Niels Bohr Library)
