Early history of x rays

Early History of X Raysby 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.

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.

zycnzj.com/ www.zycnzj.com

zycnzj.com/http://www.zycnzj.com/

BEAM LINE 11

Three days before Christmas hebrought his wife into his laborato-ry, and they emerged with a photo-graph of the bones in her hand and ofthe ring on her finger. The WürzburgPhysico-Medical Society was the firstto hear of the new rays that couldpenetrate the body and photographits bones. Roentgen delivered thenews on the 28th of December 1895.Emil Warburg relayed it to the BerlinPhysical Society on the 4th of Janu-ary. The next day the Wiener Presscarried the news, and the day fol-lowing word of Roentgen’s discoverybegan to spread by telegraph aroundthe world.

On the 13th of January, Roentgenpresented himself to the Kaiser andwas awarded the Prussian Order ofthe Crown, Second Class. And on the16th of January the The New-YorkTimes announced the discovery asa new form of photography, whichrevealed hidden solids, penetratedwood, paper, and flesh, and exposedthe bones of the human frame. “Menof science in this city are awaitingwith the utmost impatience thearrival of English technical journalswhich will give them the full par-ticulars of Professor Roentgen’s dis-covery of a method of photographingopaque bodies,” The New-YorkTimes began, and it concluded by pre-dicting the “transformation of mod-ern surgery by enabling the surgeonto detect the presence of foreignbodies.” (Jan. 16, 1896, p. 9)

The public was enthralled by thisnew form of photography and curi-ous to know the nature of the newrays. Physicians put it to immediateuse. Physicists sat up and took no-tice. The discovery of X rays was thefirst in a series of three discoveries

that jolted the fin-de-siècle disci-pline out of itsmood of finality,of closing downthe books withever more precisemeasurements, oflosing itself in de-bates over statisticalmechanics, or of try-ing to ground allphysical phenomena inmathematically precisefluctuations of the ether.All three discoveries, X rays,uranium rays, and the elec-tron, followed from one of themajor experimental traditions in thesecond half of the nineteenthcentury, the study of the dischargeof electricity in gases. All threecontributed to a profound transfor-mation of physics. In the 20th cen-tury, the discipline has been ground-ed in the study of elementaryparticles.

As with the invention of in-candescent lightbulbs, the studyof electrical dis-charge throughgases was madepossible by thedevelopment ofimproved vacu-um technologyin the 1850s. Ear-ly on, Englishscientists wereinvestigating thepatterns of lightand dark that ap-peared in sealedlead-glass tubes.The patterns in

Wilhelm Conrad Roentgen (1845–1923).(Courtesy of AIP Emilio Segré VisualArchives)

Forms of tube used by Roentgen in 1895–1896 for the production of X rays.

Ger

man

Mus

eum

, Mun

ich



12 SUMMER 1995

transparent to ultra-violet light.When Heinrich Hertz found that hecould pass the rays through metalfoil, a fellow German scientist, PhilipLenard, began to study them morecarefully. Lenard designed a tubewith a thin aluminum windowthrough which the rays couldemerge, and he measured how farthey could travel and still inducefluorescence. Defined in this way,the range of the cathode rays was sixto eight centimeters. Lenard’s ex-periments inspired Roentgen to won-der if the rays in an attenuated formreally traveled farther, and heplanned experiments to see if asensitive electroscope would mea-sure a discharge at four times thedistance Lenard had identified.

This line of work was outsideRoentgen’s usual research pursuits,which had by this time gained himgreat stature in German science. Sonof a cloth manufacturer and mer-chant from the Rhine province,Roentgen was not a particularlydiligent student in his youth. Heeventually made his way to thePolytechnic in Zurich, where heobtained a diploma in mechanicalengineering in 1868 and a doctor-ate one year later. In Zurich hebecame an assistant to August Kundtand moved along with him to theUniversity of Würzburg, and then onto the Physical Institute at Stras-bourg. His first move on his own wasto the chair of physics at Giessenin Hesse in 1879, from which hereceived many offers to go elsewhere.The path upward in the Germanuniversity system was to follow callsto universities of higher and higherstature, and finally to obtain aninstitute of one’s own. Roentgen

these partially evacuated tubes werestimulated by a voltage drop betweena cathode and an anode: typicallythere was a dark space, calledCrookes’ dark space; then a glow,called negative light; then anotherdark space, this one called Faraday’s;and a final glow of positive light. Ifthe air in the tube was exhausted un-til the first dark space expanded tofill the entire tube and all glows dis-appeared, then the rays emitted fromthe cathode could be investigated.The rays cast shadows, and weredeflected by magnetic fields, butappeared to be immune to the ef-fects of static electric forces.

As was to be characteristic of thenew ray physics to come—the phys-ics of cathode rays, X rays, alpha rays,beta rays, gamma rays, and N rays—the nature of the cathode rays was indispute, the British favoring a streamof particles, those on the Continentpreferring to think of them as somesort of disturbance of the ether. (TheBritish position, and the research pro-gram developed by J.J. Thomson atthe Cavendish Laboratory to studyionization in gases, would result inthe discovery of the electron. But ourstory does not take us that way).

A strong reason for believing thatthe cathode rays were particles wasthe observation that they wouldnot pass through matter that was

Sir Joseph John Thomson, 1856–1940.(Courtesy of the AIP Niels Bohr Library)

Roentgen’s apparatus for studying theionization of air by X rays, 1906.

Ger

man

Mus

eum

, Mun

ich



BEAM LINE 13

refused the calls until the Universi-ty of Würzburg offered him theDirectorship of their Physical Insti-tute. In 1894 he was elected Rectorat Würzburg. In his inaugural ad-dress, given the year before his dis-covery of X rays, Roentgen statedthat the “university is a nursery ofscientific research and mental edu-cation” and cautioned that “pride inone’s profession is demanded, but notprofessional conceit, snobbery, oracademic arrogance, all of whichgrow from false egoism.”*

Roentgen’s pride could rest in theover forty papers he had publishedfrom Strasbourg, Giessen, andWürzburg. These early interestsranged widely—crystals, pyroelec-trical and piezoelectrical phenomena,and the effects of pressure on liquidsand solids—but did not yet includeelectrical discharges in gases. He hadtaken his turn at measuring thespecific heat ratios of gases using asensitive thermometer of his ownmaking. He was an exact experi-menter who often made his ownapparatus—a skill learned during histraining as an engineer in Zurich—and he was able to measure ex-tremely small effects, surpassingeven Faraday’s measurement of therotation of polarized light in gases.

Roentgen turned to a new interestin October of 1895: the study of cath-ode rays. In the course of repeatingthe experiments of Hertz and Lenard,he happened to notice a glowing flu-orescent screen set off quite somedistance from the Crookes’ tube hewas operating. The screen sat muchfarther away than the six to eight

centimeters that Lenard had foundto be the maximum distance forwhich cathode rays maintain theirpower to induce fluorescence. Roent-gen recognized the effect as wor-thy of his undivided attention anddevoted the next six weeks to itsuninterrupted study.

Historians have speculated aboutwhy Roentgen was the first to rec-ognize the significance of this effect.The equipment, a cathode ray tubeand a fluorescing screen, had been inuse for decades. In 1894 J.J. Thomsonhad seen fluorescence in German-glass tubing several feet from thedischarge tube. Others had notedfogged photographic plates. Butbefore Lenard’s work, the object ofstudy was always the effects insidethe tube itself, and stray ultra-ultra-violet light could be used to explainthe fogging of photographic plates.Lenard’s great interest was in prov-ing, in contradiction to the British,the ethereal nature of cathode rays,and he was the first to study the

Demonstration by Crookes that cathoderays travel in straight lines: a) cathode;b) aluminum cross and anode; d) darkshadow; c) fluorescent image.

Phillip Lenard, 1862–1947. (Courtesy ofUllstein Bilderdienst and the AIP NielsBohr Library)

*Quoted in “Wilhelm Conrad Roentgen,”Dictionary of Scientific Biography(New York: Scribner’s, 1975), p. 531.



14 SUMMER 1995

effects of the rays in air or in a sec-ond glass tube into which he directedthem.

Roentgen, a meticulous and ob-servant experimenter, made theobvious tests on the new X rays:Were they propagated in straightlines? Were they refracted? Were theyreflected? Were they distinct fromcathode rays? What were they? Likethe cathode rays, they moved instraight lines. Roentgen was unableto refract them with water and car-bon bisulphide in mica prisms. Norcould he concentrate the rays withebonite or glass lenses. With eboniteand aluminum prisms he noted thepossibility of refracted rays on a pho-tographic plate but could not observethis effect on a fluorescent screen.Testing further, he found that X rayscould pass freely through thick lay-ers of finely powdered rock salt,electrolytic salt powder, and zincdust, unlike visible light which,because of refraction and reflection,is hardly passed at all. He concludedthat X rays were not susceptible toregular refraction or reflection.

Roentgen found that the X raysoriginate from the bright fluores-cence on the tube where the cathoderays strike the glass and spread out.The point of origin of the X raysmoves as the cathode rays are movedby a magnetic field, but the X raysthemselves are insensitive to themagnet. Roentgen concluded thatthey are distinct from cathode rays,since Lenard’s work had shown thatcathode rays passing through thetube maintained their directionbut were susceptible to magneticdeflection.

Roentgen justified calling the newphenomena 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 inOur bones, or show each little rift

And joint for you to poke your nose in.

We only crave to contemplateEach other’s usual full-dress photo;Your worse than “altogether” state

Of portraiture we bar in toto!

The fondest swain would scarcely prizeA 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 photographMahatmas, spooks, and Mrs. B-s-nt!

—Punch, January 25, 1896

shadowy pictures they produce:bones in a hand, a wire wrappedaround a bobbin, weights in a box,a compass card and needle hiddenaway in a metal case, the inhomo-geneity of a metal. The ability of thenew rays to produce photographsgave them great popular appeal andbrought Roentgen fame. Many arti-cles appeared in photography jour-nals, and The New-York Times in-dexed the new discovery underphotography. Since the rays exposedphotographic plate, the public as-sumed they were some form of light.The physicist Roentgen concurred.Accepting Lenard’s claim that cath-ode rays were vibrations of the ether,Roentgen compared the new rays tothem and forwarded the opinion thatthe two were ethereal, although dif-ferent from visible, infra-red andultra-violet light in that they did notreflect or refract. He suggested thatcathode rays and X rays were longi-tudinal vibrations of the ether ratherthan transverse ones.

Now that their existence wasestablished, it was easy enough toexperiment with the new X rays.Roentgen himself published onlythree papers on the subject, but oth-ers jumped quickly into the field.And not just physicists. ThomasEdison used modified incandescentlight bulbs to produce the new rays.He boasted to reporters that any-one could make photographs ofskeleton hands; that was mere child’splay. Within a month of Roentgen’sannouncement doctors were usingthe X rays to locate bullets in humanflesh and photograph broken bones.Dr. Henry W. Cattell, Demonstratorof Morbid Anatomy at the Univer-sity of Pennsylvania, confirmed their

Heinrich Rudolf Hertz, 1857–1894.(Courtesy of Deutsches Museum andAIP Emilio Segrè Visual Archives)



BEAM LINE 15

importance for the diagnosis ofkidney stones and cirrhotic livers andcommented that “The surgicalimagination can pleasurably lose it-self in devising endless applicationsof this wonderful process.” (TheNew-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 thehuman heart was uncovered. By 1897the rays’ dangerous side began tobe reported: examples included lossof hair and skin burns of varyingseverity.

Electricians and physicists specu-lated on the nature of these X rays.Albert Michelson thought theymight be vortices in the ether.Thomas Edison and Oliver Lodgesuggested acoustical or gravitation-al waves. But the rays ability to pho-tograph was decisive, and seriousthinkers settled on three possibili-ties, all of them of electromagneticorigin: the waves were very high fre-quency light; they were longitudinalwaves (Roentgen’s initial suggestion);or they were transverse, discontin-uous impulses of the ether.

Quite early on the hypothesis thatthey were longitudinal waves wasdiscarded, despite the support ofHenri Poincaré and Lord Kelvin. Thecrux of the question was whether thewaves were polarizable. If so theycould not be longitudinal waves.Although the early experiments onpolarization were negative or unclear,with the discovery of another ray,Henri Bequerel’s uranium rays forwhich he claimed to have found po-larization, those on the Continentset up a convincing typology. It wentfrom lower to higher frequency

transverse ethereal vibrations: light,uranium rays, X rays. Uranium rayswere given off by certain minerals,and they needed no apparatus to pro-duce them, but they shared certainproperties with X rays. They exposedphotographic plates and they causedgases to conduct electricity.

British physicists weighed in onthe side that X rays were impulses inthe ether rather than continuouswaves. Lucasian Professor of Math-ematics at Cambridge, Sir GeorgeGabriel Stokes, and his colleague anddirector of the Cavendish Laborato-ry, J.J. Thomson, committed them-selves to the impulse hypothesis in1896. It was consistent with theirconception of cathode rays as parti-cles (Thomson was to announce thediscovery of the corpuscle or electronone year later.) The abrupt stop of acharged particle would result, after atiny delay, in the propagation out-ward of an electromagnetic pulse.With Thomson’s exact measurementof the charge-to-mass ratio and H.A.Lorentz’ successful theory of theelectron, which explained manyintriguing phenomena, Continen-tal physicists began to accept, toLenard’s dismay, cathode rays asmaterial particles and X rays asimpulses in the ether.

Soon new results began tocome in. Two Dutch physicists,

First X ray made in public. Hand of the famedanatomist, Albert von Kölliker, made duringRoentgen's initial lecture before the WürzburgPhysical Medical Society on January 23, 1896.

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



16 SUMMER 1995

Hermann Haga and Cornelius Werd,announced that X rays could be dif-fracted, and a Privatdozent at Göt-tingen named Arnold Sommerfeldcarried out a mathematical analy-sis of diffraction to show that theirresults could be explained in termsof aperiodic impulses. In 1904,Charles Glover Barkla, a student ofboth Stokes and Thomson at Cam-bridge, showed that X rays wereplane polarizable while experiment-ing with secondary and tertiaryX rays. (These were produced bydirecting X rays against solids.)

As X rays began to show, more andmore, the properties of light, urani-um rays provided new mysteries.They themselves were composed ofthree sorts of distinct rays: α, β, andγ rays. What were these? Suddenlyphysics, which had seemed to someto be coming to a conclusion, wasfaced with unexplainable, qualita-tive discoveries. They were not “inthe sixth place of the decimals,” asMichelson had predicted. At theinternational congress on physics,staged in Paris in 1900 by the FrenchPhysical Society, fully nine percentof the papers delivered were on thenew ray physics.

In 1899 Ernest Rutherford, anotherstudent of Thomson’s and the manwho 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 abroadHow 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 brightAnd 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 seeThe mess which our stomachs contain!

—Homer C. Bennett, American X-ray Journal, 1897

director of the Cavendish Laboratory,had separated α rays, stoppable bymetal foil or paper sheets, from themore penetrating β rays. In 1900,Rutherford had identified the βs ashigh-speed electrons: deflected in amagnetic field they showed the cor-rect charge-to-mass ratio. A thirdcomponent of the uranium rays,undeviable and highly penetrating,was discovered by Paul Villard at theEcole Normale Superieur in Paris.Rutherford named these γ rays. Inher 1903 thesis Marie Curie madethese comparisons: γ rays to X rays;β rays to cathode rays; and α rays tocanal rays. (Canal rays were streamsof positively charged molecules.)

A few years later another storycame out. The British scientistWilliam Henry Bragg announced in1907 that X rays and γ rays were notin fact ether waves, but rather par-ticles, a neutral pair at that: electronplus positively charged particle.Bragg’s serious research began at alate age, 41, after twenty pleasantyears at the University of Adelaide,Australia, where he played golf andhobnobbed with government of-ficials. He announced his new in-tellectual work in a PresidentialAddress to the Australian Associa-tion for the Advancement of Scienceduring which he made a criticalreview of Rutherford’s work, ques-tioning the law of exponentialdecrease for the absorption of α rays.For two and a half years he publisheda paper every few months, work thatled him to make the radical state-ment that X rays were particles. Hisidea was based on two facts: (i) X raysexcite fewer gas molecules in theirpath than would be expected froma wave-like disturbance, and (ii) the

Arnold Johannes Wilhelm Sommerfeld,1868–1951. (Courtesy of the AIP NielsBohr Library)



BEAM LINE 17

velocity of the electrons excited byX rays is greater than could be giv-en to them by a wave. By this timeBragg and his physicist son were backin England, and their theory causedgreat controversy even in the countrywhere particles were in favor andwhere exotic modeling of physicalphenomena was well tolerated. Theirmost vociferous opponent wasCharles Barkla, who argued that theionization of matter was a secondaryeffect not needing to be directlyattributable to the wave-like natureof X rays. We will return later to theproblem of the concentration of X-ray energy, unexplainable in termsof waves, as it bears on Louis deBroglie’s insight into the wave natureof matter.

X RAYS AS A PROBE OF THESTRUCTURE OF MATTER

Before we turn to our final act inthe almost thirty year drama to un-derstand the nature of X rays, let usturn aside to follow another direc-tion that the work on X rays took,a shift from the investigation of thenature of X rays to their use in prob-ing the structure of crystals and ofatoms. That story will take us backto Roentgen and the center for phys-ics he built up at Munich. Whileat Würzberg, Roentgen had beenagitating for an extra position inphysics. He wanted a position fortheoretical physics, a newly emerg-ing specialty of German origin thatfollowed by several decades the crys-tallization of physics itself in themid-nineteenth century. (In 1871James Clerk Maxwell hesitated ingiving his support to the creation ofa Physical Society in London. He

wondered whether such a disciplinedistinct from chemistry existed!)

When in 1899 Roentgen was of-fered a position at Munich andthe chance to build up physics there,he accepted. Five years later, innegotiations with the minister ofeducation over another possiblemove, this time to the Reichsanstalt,Roentgen received, in return for apledge to stay in Munich, a secondinstitute, for theoretical physics, tocomplement his existing institutefor experimental physics. When EmilCohn and Emil Weichert succes-sively declined the offer of a position,it was given to Privatdozent Som-merfeld, who joined Roentgen inMunich and shared his desire to buildup physics there to the quality of theinstitutes in Göttingen, Berlin, andLeipzig. In the work on quantumtheory of the next two decades,Munich would join Copenhagen andGöttingen as the main centers on theContinent.

Sommerfeld was initially unen-thusiastic about assistant Max vonLaue’s idea that regularly spacedatoms in a crystal might act as a dif-fraction grating for X rays, the finedistances between the atoms serving,as no hand- or machine-ruled gratingcould, to diffract ultra-high frequen-cies. If, of course, that is what onethought 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 toRoentgen. (Burndy Library, DibnerInstitute, 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.)



18 SUMMER 1995

engaging in dis-cussions withJohannes Starkover the quan-tum nature of Xrays. Stark wasone of the fewphysicists whoin 1911 took se-riously Einstein’s

suggestion that light comes in quan-ta of energy. Applying the notion toX rays, Stark was able to assign thema frequency and to explain the highvelocity of electrons that had beenexcited by X rays, one of the phe-nomena that so exercised Bragg andBarkla.

Laue persisted in asking that theexperimentalists try out X rays oncrystals. A student of Max Planck’s(in fact, his favorite), Laue hadworked on a theory of the interfer-ence of light in plane parallel plates.By 1912 his specialty had become thetheory of relativity, but he was notaverse to following Sommerfeld inworking on a theory of diffraction.Laue’s guess was that it would beonly the secondary X rays, not thechaotic Bremsstrahlung identifiedwith the initial deceleration ofelectrons, that would interfere con-structively in the crystal. In April1912 Walther Friedrich and PaulKnipping shone secondary X rays oncopper sulfate and zinc sulfate sur-faces and found that dark spots insuccessive circles appeared on pho-tographic plates placed behind them.At this time both the nature of X raysand the structure of crystals was apuzzle. Laue’s analysis of the situa-tion was to identify five distinctwavelengths of incoming X raysbetween 1.27 and 4.83 × 10−9 cm.

White radiation Laue diffraction pattern from the proteintrimethylene dehydrogenase (an enzyme that catalyzes the

conversion of trimethylamine to dimethylamine andformaldehyde) recorded on SSRL beam 10–2 with a 5 msec

X-ray exposure. The photograph was taken by Scott Matthewsand Scott White of Washington University, St. Louis, and Mike

Soltis, Henry Bellamy, and Paul Phizackerly of SSRL/SLAC.

Later others would suggest that thecrystal itself imposed structure onthe incoming radiation. Laue pub-lished a rather long article on histheory of diffraction in the Enzyk-lopadie der Mathematische Wis-senschaften, and much later (1941)he went on to publish a 350-pagereview of the subject, Roentgen-strahlen-Interferenzen, in which heincluded the effects of electroninterference.

Perhaps as was fitting for an earlyproponent of relativity and a defenderof Einstein throughout the Nazi pe-riod, Laue made little of quantumtheory and remained skeptical of theCopenhagen interpretation through-out his life. He became director ofthe Kaiser Wilhelm Institute in theyears before World War II, resigninghis position in 1943, at which timethe Institute was directed towardsthe building of an atomic bomb un-der the leadership of Werner Heisen-berg. After the war Laue worked torebuild German science. In the fallof 1946 he helped create the GermanPhysical Society in the British Zone,and worked to revive the first of thenational bureaus of standards, thePhysikalische-Technische-Reich-sanstalt. Towards the end of his lifehe assumed the directorship of thenow one of several Kaiser WilhelmInstitutes, this one devoted to elec-trochemistry in Berlin-Dahlem. Lauedied in an auto accident at the age ofeighty-one.

Laue was representative of theGerman talent for institution build-ing in the support of science and theGerman fascination for fundamen-tal principles and theories. Thosewho would apply Laue’s idea andbuild on Friedrich and Knipping’s



experimental demonstration werethe British, specifically the Braggsand Henry Moseley. In view of theGerman results the Braggs had cometo believe that X rays were of an elec-tromagnetic nature, but they insist-ed that the rays must have some sortof dual existence as they were ableto concentrate their energy. But thecontinuing puzzle as to their naturedid not stop the Braggs from recog-nizing the practicability and impor-tance of a new field of study, X-raycrystallography.

The new field was pioneered bythe Braggs. They were inspired by theCambridge theorists who argued thata diffraction grating imposes a struc-ture on an inhomogeneous pulse ofwhite light, picking out, as if in aFourier transform, the wavelengthsinto which the beam can be decom-posed. William Henry Bragg and hisson, William Lawrence Bragg, arguedby analogy that the crystal, by dintof the distance between planes ofatoms, imposes a similar structureon an inhomogeneous pulse of Xrays. When the X rays are reflectedoff two successive planes of atoms inthe crystal, they interfere construc-tively if the difference in the distancetraveled is equal to an integral num-ber of wavelengths. Thus the famousBragg condition

n λ = 2d sin θ,

where d is the distance betweenplanes and θ is the angle of reflection.

Using an X-ray tube and a colli-mating slit to produce the incom-ing rays; using various minerals,quartz, rock salt, iron, pyrite,zincblende, and calcite, as three-dimensional diffraction gratings; and

BEAM LINE 19

using a photographic plate or anionization chamber (depending onthe strength of the incoming rays) asa detector—the Braggs proceededwith the first measurements in X-rayspectroscopy. By 1913, just a year af-ter they had pioneered the method,crystal analysis with X rays hadbecome a standard technique. Theresults not only gave insight into thestructure of crystals but also into thenature of the anti-cathode thatproduced the rays.

The first person to notice that Xrays can be characteristic of the sub-stance that emits them was CharlesBarkla, the opponent of the Braggs inthe matter of X rays as neutral par-ticles and a professor at the Univer-sity of Edinburgh who spent overforty years examining the propertiesof secondary X rays. Between 1906and 1908 he had noticed that ele-ments emit secondary X rays witha penetrating power in aluminumthat is distinct for each element. Todistinguish between the hardnessof the characteristic rays, he intro-duced the terminology K and L rays.It was for this discovery that he wasawarded the Nobel Prize in 1917.(His subsequent work earned Barklathe reputation as something of a sci-entific crank.) What the Braggs no-ticed (see figure on next page) wasthat a pattern of multiple peaks withvarying intensities was produced nomatter what the crystal (shifted onlyby the varying distances betweenplanes of atoms) as long as the ele-ment of the anti-cathode remainedthe same. In other words, the patternwas analogous to spectral lines emit-ted by gases in the optical frequen-cies. The person to explore this anal-ogy to its fullest was Henry Moseley,

Top right: Sir William Henry Bragg, 1862–1942. Lower right: Sir William LawrenceBragg, 1890–1971. (Courtesy of the AIP Niels Bohr Library)



20 SUMMER 1995

a young researcher working inRutherford’s Manchester laborato-ry during the time when Niels Bohrwas visiting regularly.

Moseley’s two grandfathers hadbeen fellows of the Royal Society, andhis father had founded a school ofzoology at Oxford. Mosely himselfwas perhaps the only importantatomic physicist to be educated atOxford. In the fall of 1910 he cameto work as a demonstrator underRutherford, his salary being paid bya Manchester industrialist. He wasassigned a research problem to whicheveryone knew the answer: howmany β particles are emitted in theradioactive disintegration of radiumB (Pb214) to radium C (Bi214). On find-ing the answer everyone expected,one, he proved his competency as anexperimentalist. However, his nextexperiments would not be so cut anddried, nor would they receive theready approval of Rutherford. Likethe Braggs, and quite independent-ly of them, Moseley was stimulat-ed by the photographs of Friedrichand Knipping, and felt that Laue hadmisinterpreted them as evidence offive homogeneous X rays. He teamedup with Charles G. Darwin, grand-son of the famous evolutionist, andturned to, as he said, the “real mean-ing” of the German experiments.The Laue dots connoted the struc-ture of the crystal, not the structureof the incoming rays. When pre-senting his results to a Friday phys-ics colloquium which father Braggattended, Moseley discovered thesimilarity in their understanding ofthe phenomena, and afterwards hewrote to his mother:

I have been lazy for a couple ofdays recouping after the lecture I

*Nov. 4, 1912. Quoted in J.L. Heilbron,H.G.J. Moseley, p. 194.

gave on Friday on X rays. It wasrather anxious work, as Bragg, thechief authority on the subject (Pro-fessor of Leeds) was present, andas I had to be cautious. However itproved quite successful and Imanaged to completely disguisemy nervousness. I was talkingchiefly about the new Germanexperiments of passing raysthrough crystals. The men who didthe work entirely failed to under-stand what it meant, and gave anexplanation which was obviouslywrong. After much hard work Dar-win and I found the real meaningof the experiments.*

For a time the Braggs, Moseley,and Darwin continued on the sametrack, even though Rutherford pre-sented difficulties which were final-ly overcome by Moseley’s persistententhusiasm and by Bragg’s offer toMoseley of a visit to Leeds to teachhim the techniques of X-ray spec-troscopy. Some of the questions theypursued were the old ones about thenature of X rays. How to reconcilethe corpuscular nature of the rayswith their ability to interfere? Bragghad compared this conundrum in theelectromagnetic theory of X rays tothe physical impossibility of a spread-ing circle of water waves, caused bythe fall of a rock, to excite anotherrock to jump the same distance thewave-producing rock had fallen.

The new questions concerned theelements. In July of 1913 Bohr paid avisit to Manchester and discussedatomic structure with Moseley,Darwin, and George Hevesey. Thediscussion revolved around the sim-ilarity, and possible differences,H. G. J. Moseley in Balliol-Trinity

Laboratory, Oxford, circa 1910.(Courtesy of University of Oxford,

Museum of the History of Science andthe AIP Niels Bohr Library)

0 10

5 10 15 20 25

20 30Degrees

DegreesIon

izat

ion C1

BC A

B1

B2A2

C3? B3

A1

C2

C2

B2A2

I

II

One of the earliest examples of X-rayspectroscopy. The Braggs made aseredipitous discovery: while studyingthe scattering of X rays off of crystalsthey noticed that a distinctive pattern ofpeaks appeared for each of the differentanti-cathodes being used to producethe rays. What had initially started out asa study of the structure of crystals led toan investigation of the atomic structureof the anti-cathode elements. [Braggand Bragg, PRS, 88A, 413 (1913).]



BEAM LINE 21

between the atomic weight of anelement (A) and its nuclear charge(Z). Geiger’s and Marsden’s scatter-ing experiments and Rutherford’stheory had proposed that the newlydiscovered nucleus held a charge halfthat of the atomic weight. A Dutchlawyer and would-be interpreter ofMendeleev’s table, Van de Broek, hadsuggested that the nuclear charge ofan element set its place in the table.Now the frequency of characteris-tic K rays gave another quantity withwhich to mark the elements. Whatwould the X-ray spectroscope haveto say about those places in the tablewhere the atomic weights did not fol-low in increasing order the serialnumbers: between nickel and cobalt;between argon and potassium; andbetween iodine and tellurium?Would the hardness of the K rays or-der the elements by atomic weightor by nuclear charge?

Moseley used an ingenious de-vice of G. W. C. Kaye’s to exam-ine the K rays from copper, nickel,cobalt, iron, manganese, chromi-um, and titanium. By putting thedifferent elements which served asanticathodes on a magnetized truckand rail inside the evacuated cham-ber, Moseley was able to changeanti-cathodes with an externalmagnet without disrupting the in-tegrity of the chamber. Afterswitching from detecting the K raysby ionization to detecting them byphotography, his work went quick-ly, and in several weeks he showedthat the ranking of elements by Krays followed their ranking by nu-clear charge, Z. The relation wassimple as well. The darker of thetwo primary K lines, Kα , fit theform

Moseley interpreted his formulaas a vindication of Bohr’s theory,which at the time was being pub-lished in three lengthy and famouspapers “On the Constitution ofAtoms.” Moseley argued, not quiteconvincingly, that his results couldbe used to support the quantizationof an electron’s angular momentum.Frederick A. Lindeman, a fellow Eng-lishman working with WaltherNernst on the Continent but withhis eye on the same chair of phys-ics as Moseley, Clifton’s chair at Ox-ford, criticized both Bohr and Mose-ley. He was working out of an alreadysuccessful tradition which appliedthe condition of quantized frequen-cies to the motion of atoms to pre-dict specific heats in a solid and tothe motions of molecules in a gasto predict the patterns of rotationaland vibrational infrared spectra. (Aquantum theory of molecular spec-tra preceded one of atomic spectra!)

More successful than Moseley’sargument in favor of Bohr’s atomictheory was his help to the chemistsin sorting out the confusions of therare earths. In November of 1913,Moseley moved to Oxford wherehe worked with equipment at the

να

νK Z= −34 0 12( ) .

X-ACTLY SO!

The Roentgen Rays, the Roentgen Rays,What is this craze?The town’s ablaze

With the new phaseOf X-ray’s ways.

I’m full of daze,Shock and amaze;

For nowadaysI hear they’ll gaze

Thro’ cloak and gown—and even stays,These naughty, naughty Roentgen Rays.

—Wilhelma, Electrical Review, April 17, 1896

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

Goudsmit Collection)



22 SUMMER 1995

frequency of X rays emitted. By 1916,the Duane-Hunt law was the bestway to determine h, Planck’s con-stant, although neither Millikan norDuane then subscribed to the viewthat energy came in discrete quan-tized units.

Arthur Holly Compton alsoinitially interpreted his results onX-ray scattering from electrons as acut-off relation that was governed inthis case by Planck’s constant ratherthan as proof of the quantum natureof radiation. Compton, who was laterto run the Manhattan Project’s Met-allurgical Laboratory at Chicagoduring World War II, received hisPh.D. from Princeton just before theFirst World War for work on X-raydiffraction and scattering. Afterseveral years spent at WestinghouseManufacturing Company workingon fluorescent lamps, he spent a yearat Cambridge’s Cavendish Laborato-ry where he developed a friendshipwith J.J. Thomson and carried out aninvestigation into the orderly changeof X-ray frequency with scatteringangle as the X rays scattered fromelectrons. As a new professor atWashington University in St. Louis,Compton published a mass of dataon the relation between X-ray fre-quency and scattering angle takenwith a Bragg crystal spectrometer. In1922, a year after he had taken themeasurements, and along with PeterDebye in Germany who had seen hisresults in the Bulletin of the Na-tional Research Council, Comptonaccepted Einstein’s light quantumand by extension the X-ray quantum.The explanation of the Compton-effect then became a simple scatter-ing of two elastic particles.

brought to a haltby the end of theWorld War I. TheB r a g g s ’ w o r kcontinued afterthe war. The el-der Bragg revivi-fied the RoyalI n s t i t u t i o n ,where Sir Hum-phry Davy andMichael Faradayhad made theirchemical andelectrical dis-coveries, by es-tablishing a re-search school forthe analysis oforganic crystals.

This work would become central tothe developing field of molecular bi-ology. During the war the elder Braggworked for the Navy board to eval-uate inventions and to promote re-search with military applications.Like many British and U.S. scientistshe eventually found himself work-ing on problems of submarine de-tection. Moseley, with his Eton pa-triotism, practically forced himselfupon the Royal Engineers along withhis friend Henry Tizard. Tizard sur-vived the Great War and subse-quently led the minds of British sci-entists into World War II. Moseley,however, died at Gallipoli in the bat-tle of Sari Bari.

As Europe engaged itself in theGreat War, interesting work on Xrays began to come out of the Unit-ed States. Following Robert Mil-likan’s work on the photoelectriceffect, William Duane at Harvardgave an exact law that related the en-ergy of cathode ray electrons to the

Arthur Holly Compton, 1892–1962.(Courtesy of the AIP Niels Bohr Library)

Electrical Laboratory but with nosalary. Moseley began a thorough in-vestigation of Mendeleev’s table us-ing X rays, and moving from calciumto zinc and then to the rare earths,lanthanum to erbium. George Ur-bain, a Professor of Chemistry inParis at one of the Grands Ecoles hadbeen engaged for years in fraction-ating the elemental rare earths incompetition with Carl Auer vonWelsback, who performed his frac-tionations in an Austrian castle.Urbain recognized the power ofMoseley’s technique and paid hima visit with precious samples of thelast four rare earths, thulium, ytter-bium, lutecium, and celtium. He wasastounded at how quickly Mose-ley’s X-ray spectrometer could de-termine that celtium was not hissought after new element, but wasonly a combination of lutecium andytterbium!

The Braggs’ work on crystals andthat of Moseley’s on elements was



BEAM LINE 23

Few physicists had taken Einsteinseriously when he predicted thelight quantum in 1905. Bohr hadpooh-poohed the idea. But by 1921evidence was mounting and so wasEinstein’s fame. The de Brogliebrothers, Maurice and Louis, weretwo others who had learned from thestudies of X rays of the dual natureof radiation, and Louis was inspiredto suggest that matter too might havethis dual nature. Maurice de Broglie’sinterest in the quantum had beensparked by his secretaryship of thefirst Solvay Conference called in1911 by chemist Walther Nernst tointroduce the quantum concept tophysical scientists, and he decided toinvestigate the energies of electronsexcited by K and L frequency X rays.He found the old problem over whichBragg and Barkla had argued: X rayscan concentrate their entire energyand pass it on to electrons. And likeBragg he concluded that X rays actboth as waves and as particles. Hisyounger brother, Louis, in a spiritof unification, longed to treat lightand matter as equals. Both could beunderstood as particles followingwaves, he proposed. A “mobile” oflight or X rays or of matter followedalong behind an “onde fictive.”

So the discussion of X rays hadcome around full circle. They werediscovered in Roentgen’s laborato-ry as this newcomer to cathode rayswas trying to puzzle out his coun-tryman Lenard’s challenge to theBritish. Lenard believed cathode raysto be ethereal. The British thoughtthem particles. Soon X rays becamethe new mystery. Were they elec-tromagnetic waves or were theyneutral pairs of particles? By 1913the interference of X rays had con-

vinced most physicists that theywere waves. The Braggs, not quitegiving up, insisted that they had theproperties of both waves and parti-cles. By 1922 the startling explana-tion by Compton of his scatteringexperiments—X-ray energy wasconcentrated into particle points—helped convince the science com-munity to take Einstein’s notion oflight quanta seriously. And finallythe work on X rays by the de Broglies,and the younger brother’s desire toput on an equal footing light andmatter, gave Louis de Broglie thecourage to suggest that even the goodold electron (the cathode ray parti-cles!) partook of wave qualities.

ADDENDUM

The early history of X rays followsanother path that I have not coveredhere. As the physicists wonderedabout the nature of X rays and usedthem to probe the structure of crys-tals and atoms, medical doctors usedthem to probe the human body andto diagnose and treat disease. Roent-gen by presenting an X-ray photo-graph of his wife’s hand to theWürzburg Physical and MedicalSociety in January of 1896 began thepractice of radiology. A month latera German doctor used an X ray todiagnose sarcoma of the tibia in the

Niels Bohr, 1885–1962. (Courtesy of theAIP Niels Bohr Library



24 SUMMER 1995

Two of the three discoveries thathelped shake physics out of its fin-de-siècle malaise, X rays andradioactivity, would be taken up,almost immediately, by doctors intheir medical practice. And asphysicists began to require substan-tial funds to continue their quest todiscover the smallest structures ofmatter, the link between physics andmedicine would be pushed. ErnestLawrence regularly raised money forhis laboratory’s cyclotrons by vir-tually promising cures for cancer: “Itis almost unthinkable that the man-ifold new radiations and radioactivesubstances [produced by his cy-clotrons] should not greatly extendthe successful range of applicationof radiation therapy.”*

*Quoted in J.L. Heilbron and RobertSeidel, Lawrence and his Laboratory,(Berkeley, University of California Press,1989) p. 215.

right leg of a young boy. The militaryfirst used X rays in Naples in May of1896 to locate bullets in the forearmsof two soldiers who had been wound-ed in Italy’s Ethiopian campaign.

Radiology would be advancedby the strong tradition of medicalresearch in France. Antoine Béclèreset up the first X-ray machine inwhich a patient was strapped andmoved around for complete X rays ofthe chest. For those taking pictureshe introduced safety equipment, leadaprons and lead rubber gloves. Hepioneered the first use of radiographyof the stomach in 1906. The patienthad first a meal of bismuth. Throughthe work of Béclère and others thepractice of medical diagnosis changedsignificantly. Soon to follow was theuse of X rays to treat cancer. The raysof the chemists and physicistsseemed to inspire doctors: α, β, and γrays were also beamed at canceroustumors.

Louis de Broglie, 1892–1987. (Courtesy of the AIP Niels Bohr Library)

An informal moment at an informalconference called by Paul Ewald in 1925. From left: Paul P. Ewald,Charles G. Darwin, H. Ott, William L. Bragg, and R. W. James. (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 andthe Shark: Empirical Roots ofWave-Particle Dualism (Cam-bridge, Cambridge UniversityPress, 1983)

One can also find good bibli-ographies in the numerous articlesin the Dictionary of ScientificBiography under the individualscientists discussed above.



Early history of x rays

Documents

Transcript of Early history of x rays