Early History of X Rays

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     Early History of X Rays
     by ALEXI ASSMUS



     The discovery of X rays
     in 1895 was the
     beginning of a
     revolutionary change
     in our understanding
     of the physical world.




                   IN THE WINTER of the year of his fiftieth birthday, and the year

                       following his appointment to the leadership of the University

                   of Würzburg, Rector Wilhelm Conrad Roentgen noticed a barium

                            platinocyanide screen fluorescing in his laboratory as he

                       generated cathode rays in a Crookes tube some distance away.

                       Leaving aside for a time his duties to the university and to his

                   students, Rector Roentgen spent the next six weeks in his labora-

                       tory, working alone, and sharing nothing with his colleagues.

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




   Three days before Christmas he        that jolted the fin-
brought his wife into his laborato-      de-siècle disci-
ry, and they emerged with a photo-       pline out of its
graph of the bones in her hand and of    mood of finality,
the ring on her finger. The Würzburg      of closing down
Physico-Medical Society was the first     the books with
to hear of the new rays that could       ever more precise
penetrate the body and photograph        measurements, of
its bones. Roentgen delivered the        losing itself in de-
news on the 28th of December 1895.       bates over statistical
Emil Warburg relayed it to the Berlin    mechanics, or of try-
Physical Society on the 4th of Janu-     ing to ground all
ary. The next day the Wiener Press       physical phenomena in
carried the news, and the day fol-       mathematically precise
lowing word of Roentgen’s discovery      fluctuations of the ether.
began to spread by telegraph around      All three discoveries, X rays,
the world.                               uranium rays, and the elec-
   On the 13th of January, Roentgen      tron, followed from one of the
presented himself to the Kaiser and      major experimental traditions in the
was awarded the Prussian Order of        second half of the nineteenth
the Crown, Second Class. And on the      century, the study of the discharge
16th of January the The New-York         of electricity in gases. All three
Times announced the discovery as         contributed to a profound transfor-
a new form of photography, which         mation of physics. In the 20th cen-
revealed hidden solids, penetrated       tury, the discipline has been ground-
wood, paper, and flesh, and exposed       ed in the study of elementary             Forms of tube used by Roentgen
the bones of the human frame. “Men       particles.                                in 1895–1896 for the production
of science in this city are awaiting        As with the invention of in-           of X rays.
with the utmost impatience the           candescent light
arrival of English technical journals    bulbs, the study
which will give them the full par-       of electrical dis-
ticulars of Professor Roentgen’s dis-    charge through
covery of a method of photographing      gases was made
opaque bodies,” The New-York             possible by the
Times began, and it concluded by pre-    development of
dicting the “transformation of mod-      improved vacu-
ern surgery by enabling the surgeon      um technology
to detect the presence of foreign        in the 1850s. Ear-
bodies.” (Jan. 16, 1896, p. 9)           ly on, English
   The public was enthralled by this     scientists were
new form of photography and curi-        investigating the
                                                           German Museum, Munich




ous to know the nature of the new        patterns of light
rays. Physicians put it to immediate     and dark that ap-
use. Physicists sat up and took no-      peared in sealed
tice. The discovery of X rays was the    lead-glass tubes.
first in a series of three discoveries   The patterns in


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




                                                                                                                   transparent to ultra-violet light.
                                                                                                                   When Heinrich Hertz found that he
                                                                                                                   could pass the rays through metal
                                                                                                                   foil, a fellow German scientist, Philip




                                                                                           German Museum, Munich
                                                                                                                   Lenard, began to study them more
                                                                                                                   carefully. Lenard designed a tube
                                                                                                                   with a thin aluminum window
                                                                                                                   through which the rays could
                                                                                                                   emerge, and he measured how far
                                                                                                                   they could travel and still induce
                                                these partially evacuated tubes were                               fluorescence. Defined in this way,
                                                stimulated by a voltage drop between                               the range of the cathode rays was six
                                                a cathode and an anode: typically                                  to eight centimeters. Lenard’s ex-
                                                there was a dark space, called                                     periments inspired Roentgen to won-
                                                Crookes’ dark space; then a glow,                                  der if the rays in an attenuated form
                                                called negative light; then another                                really traveled farther, and he
                                                dark space, this one called Faraday’s;                             planned experiments to see if a
                                                and a final glow of positive light. If                             sensitive electroscope would mea-
                                                the air in the tube was exhausted un-                              sure a discharge at four times the
                                                til the first dark space expanded to                               distance Lenard had identified.
                                                fill the entire tube and all glows dis-                                This line of work was outside
                                                appeared, then the rays emitted from                               Roentgen’s usual research pursuits,
                                                the cathode could be investigated.                                 which had by this time gained him
                                                The rays cast shadows, and were                                    great stature in German science. Son
                                                deflected by magnetic fields, but                                  of a cloth manufacturer and mer-
                                                appeared to be immune to the ef-                                   chant from the Rhine province,
                                                fects of static electric forces.                                   Roentgen was not a particularly
                                                   As was to be characteristic of the                              diligent student in his youth. He
                                                new ray physics to come—the phys-                                  eventually made his way to the
                                                ics of cathode rays, X rays, alpha rays,                           Polytechnic in Zurich, where he
                                                beta rays, gamma rays, and N rays—                                 obtained a diploma in mechanical
                                                the nature of the cathode rays was in                              engineering in 1868 and a doctor-
                                                dispute, the British favoring a stream                             ate one year later. In Zurich he
                                                of particles, those on the Continent                               became an assistant to August Kundt
                                                preferring to think of them as some                                and moved along with him to the
                                                sort of disturbance of the ether. (The                             University of Würzburg, and then on
                                                British position, and the research pro-                            to the Physical Institute at Stras-
                                                gram developed by J.J. Thomson at                                  bourg. His first move on his own was
                                                the Cavendish Laboratory to study                                  to the chair of physics at Giessen
                                                ionization in gases, would result in                               in Hesse in 1879, from which he
                                                the discovery of the electron. But our                             received many offers to go elsewhere.
                                                story does not take us that way).                                  The path upward in the German
                                                   A strong reason for believing that                              university system was to follow calls
                                                the cathode rays were particles was                                to universities of higher and higher
     Sir Joseph John Thomson, 1856–1940.        the observation that they would                                    stature, and finally to obtain an
     (Courtesy of the AIP Niels Bohr Library)   not pass through matter that was                                   institute of one’s own. Roentgen


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refused the calls until the Universi-    centimeters that Lenard had found
ty of Würzburg offered him the           to be the maximum distance for
Directorship of their Physical Insti-    which cathode rays maintain their
tute. In 1894 he was elected Rector      power to induce fluorescence. Roent-
at Würzburg. In his inaugural ad-        gen recognized the effect as wor-
dress, given the year before his dis-    thy of his undivided attention and
covery of X rays, Roentgen stated        devoted the next six weeks to its
that the “university is a nursery of     uninterrupted study.
scientific research and mental edu-         Historians have speculated about
cation” and cautioned that “pride in     why Roentgen was the first to rec-        Phillip Lenard, 1862–1947. (Courtesy of
one’s profession is demanded, but not    ognize the significance of this effect.    Ullstein Bilderdienst and the AIP Niels
professional conceit, snobbery, or       The equipment, a cathode ray tube         Bohr Library)
academic arrogance, all of which         and a fluorescing screen, had been in
grow from false egoism.”*                use for decades. In 1894 J.J. Thomson
   Roentgen’s pride could rest in the    had seen fluorescence in German-
over forty papers he had published       glass tubing several feet from the
from Strasbourg, Giessen, and            discharge tube. Others had noted
Würzburg. These early interests          fogged photographic plates. But
ranged widely—crystals, pyroelec-        before Lenard’s work, the object of
trical and piezoelectrical phenomena,    study was always the effects inside
and the effects of pressure on liquids   the tube itself, and stray ultra-ultra-
and solids—but did not yet include       violet light could be used to explain
electrical discharges in gases. He had   the fogging of photographic plates.
taken his turn at measuring the          Lenard’s great interest was in prov-
                                                                                   Demonstration by Crookes that cathode
specific heat ratios of gases using a    ing, in contradiction to the British,     rays travel in straight lines: a) cathode;
sensitive thermometer of his own         the ethereal nature of cathode rays,      b) aluminum cross and anode; d) dark
making. He was an exact experi-          and he was the first to study the         shadow; c) fluorescent image.
menter who often made his own
apparatus—a skill learned during his
training as an engineer in Zurich—
and he was able to measure ex-
tremely small effects, surpassing
even Faraday’s measurement of the
rotation of polarized light in gases.
   Roentgen turned to a new interest
in October of 1895: the study of cath-
ode rays. In the course of repeating
the experiments of Hertz and Lenard,
he happened to notice a glowing flu-
orescent screen set off quite some
distance from the Crookes’ tube he
was operating. The screen sat much
farther away than the six to eight
*Quoted in “Wilhelm Conrad Roentgen,”
 Dictionary of Scientific Biography
 (New York: Scribner’s, 1975), p. 531.



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                                                    effects of the rays in air or in a sec-   shadowy pictures they produce:
                                                    ond glass tube into which he directed     bones in a hand, a wire wrapped
                                                    them.                                     around a bobbin, weights in a box,
                                                       Roentgen, a meticulous and ob-         a compass card and needle hidden
                                                    servant experimenter, made the            away in a metal case, the inhomo-
                                                    obvious tests on the new X rays:          geneity of a metal. The ability of the
                                                    Were they propagated in straight          new rays to produce photographs
                                                    lines? Were they refracted? Were they     gave them great popular appeal and
                                                    reflected? Were they distinct from        brought Roentgen fame. Many arti-
                                                    cathode rays? What were they? Like        cles appeared in photography jour-
        Heinrich Rudolf Hertz, 1857–1894.           the cathode rays, they moved in           nals, and The New-York Times in-
        (Courtesy of Deutsches Museum and           straight lines. Roentgen was unable       dexed the new discovery under
        AIP Emilio Segrè Visual Archives)           to refract them with water and car-       photography. Since the rays exposed
                                                    bon bisulphide in mica prisms. Nor        photographic plate, the public as-
                                                    could he concentrate the rays with        sumed they were some form of light.
                                                    ebonite or glass lenses. With ebonite     The physicist Roentgen concurred.
                                                    and aluminum prisms he noted the          Accepting Lenard’s claim that cath-
                                                    possibility of refracted rays on a pho-   ode rays were vibrations of the ether,
                                                    tographic plate but could not observe     Roentgen compared the new rays to
                                                    this effect on a fluorescent screen.      them and forwarded the opinion that
                                                    Testing further, he found that X rays     the two were ethereal, although dif-
        O, Röntgen, then the news is true,          could pass freely through thick lay-      ferent from visible, infra-red and
          And not a trick of idle rumour,           ers of finely powdered rock salt,         ultra-violet light in that they did not
         That bids us each beware of you,           electrolytic salt powder, and zinc        reflect or refract. He suggested that
     And of your grim and graveyard humour.         dust, unlike visible light which,         cathode rays and X rays were longi-
                                                    because of refraction and reflection,      tudinal vibrations of the ether rather
         We do not want, like Dr. Swift,
       To take our flesh off and to pose in          is hardly passed at all. He concluded     than transverse ones.
        Our bones, or show each little rift         that X rays were not susceptible to          Now that their existence was
      And joint for you to poke your nose in.       regular refraction or reflection.          established, it was easy enough to
                                                       Roentgen found that the X rays         experiment with the new X rays.
          We only crave to contemplate              originate from the bright fluores-        Roentgen himself published only
        Each other’s usual full-dress photo;
                                                    cence on the tube where the cathode       three papers on the subject, but oth-
        Your worse than “altogether” state
          Of portraiture we bar in toto!            rays strike the glass and spread out.     ers jumped quickly into the field.
                                                    The point of origin of the X rays         And not just physicists. Thomas
      The fondest swain would scarcely prize        moves as the cathode rays are moved       Edison used modified incandescent
        A picture of his lady’s framework;          by a magnetic field, but the X rays       light bulbs to produce the new rays.
       To gaze on this with yearning eyes           themselves are insensitive to the         He boasted to reporters that any-
       Would probably be voted tame work!           magnet. Roentgen concluded that           one could make photographs of
         No, keep them for your epitaph,
                                                    they are distinct from cathode rays,      skeleton hands; that was mere child’s
      these tombstone-souvenirs unpleasant;         since Lenard’s work had shown that        play. Within a month of Roentgen’s
           Or go away and photograph                cathode rays passing through the          announcement doctors were using
       Mahatmas, spooks, and Mrs. B-s-nt!           tube maintained their direction           the X rays to locate bullets in human
                                                    but were susceptible to magnetic          flesh and photograph broken bones.
                   —Punch, January 25, 1896         deflection.                                Dr. Henry W. Cattell, Demonstrator
                                                       Roentgen justified calling the new      of Morbid Anatomy at the Univer-
                                                    phenomena rays because of the             sity of Pennsylvania, confirmed their


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




importance for the diagnosis of
kidney stones and cirrhotic livers and
commented that “The surgical
imagination can pleasurably lose it-
self in devising endless applications
of this wonderful process.” (The
New-York Times, Feb. 15, 1896, p. 9).
In the first six months after their dis-
covery Viennese mummies were un-
dressed, doctors claimed to have pho-
tographed their own brains, and the        transverse ethereal vibrations: light,
human heart was uncovered. By 1897         uranium rays, X rays. Uranium rays
the rays’ dangerous side began to          were given off by certain minerals,
be reported: examples included loss        and they needed no apparatus to pro-
of hair and skin burns of varying          duce them, but they shared certain
severity.                                  properties with X rays. They exposed
   Electricians and physicists specu-      photographic plates and they caused
lated on the nature of these X rays.       gases to conduct electricity.
Albert Michelson thought they                 British physicists weighed in on
might be vortices in the ether.            the side that X rays were impulses in
Thomas Edison and Oliver Lodge             the ether rather than continuous
suggested acoustical or gravitation-       waves. Lucasian Professor of Math-
al waves. But the rays ability to pho-     ematics at Cambridge, Sir George
tograph was decisive, and serious          Gabriel Stokes, and his colleague and
thinkers settled on three possibili-       director of the Cavendish Laborato-
ties, all of them of electromagnetic       ry, J.J. Thomson, committed them-
origin: the waves were very high fre-      selves to the impulse hypothesis in
quency light; they were longitudinal       1896. It was consistent with their
waves (Roentgen’s initial suggestion);     conception of cathode rays as parti-
or they were transverse, discontin-        cles (Thomson was to announce the
uous impulses of the ether.                discovery of the corpuscle or electron
   Quite early on the hypothesis that      one year later.) The abrupt stop of a
they were longitudinal waves was           charged particle would result, after a
discarded, despite the support of          tiny delay, in the propagation out-
Henri Poincaré and Lord Kelvin. The        ward of an electromagnetic pulse.
crux of the question was whether the       With Thomson’s exact measurement
waves were polarizable. If so they         of the charge-to-mass ratio and H.A.
could not be longitudinal waves.           Lorentz’ successful theory of the
Although the early experiments on          electron, which explained many
polarization were negative or unclear,     intriguing phenomena, Continen-
with the discovery of another ray,         tal physicists began to accept, to
Henri Bequerel’s uranium rays for          Lenard’s dismay, cathode rays as
which he claimed to have found po-         material particles and X rays as
                                                                                    First X ray made in public. Hand of the famed
larization, those on the Continent         impulses in the ether.                   anatomist, Albert von Kölliker, made during
set up a convincing typology. It went         Soon new results began to             Roentgen's initial lecture before the Würzburg
from lower to higher frequency             come in. Two Dutch physicists,           Physical Medical Society on January 23, 1896.


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




                                                                                                   director of the Cavendish Laboratory,
                                                                                                   had separated α rays, stoppable by
                                                                                                   metal foil or paper sheets, from the
                                                                                                   more penetrating β rays. In 1900,
                                                                                                   Rutherford had identified the βs as
                                                                                                   high-speed electrons: deflected in a
                                                                                                   magnetic field they showed the cor-
                                                                                                   rect charge-to-mass ratio. A third
                                                                                                   component of the uranium rays,
                                                                                                   undeviable and highly penetrating,
                                                                                                   was discovered by Paul Villard at the
                                                         Hermann Haga and Cornelius Werd,          Ecole Normale Superieur in Paris.
                                                         announced that X rays could be dif-       Rutherford named these γ rays. In
              WE WANT TO KNOW                            fracted, and a Privatdozent at Göt-       her 1903 thesis Marie Curie made
                                                         tingen named Arnold Sommerfeld            these comparisons: γ rays to X rays;
                                                         carried out a mathematical analy-         β rays to cathode rays; and α rays to
      If the Roentgen rays, that are way ahead,
              Will show us in simple note,               sis of diffraction to show that their     canal rays. (Canal rays were streams
        How, when we ask our best girl to wed,           results could be explained in terms       of positively charged molecules.)
           That lump will look in our throat.            of aperiodic impulses. In 1904,              A few years later another story
                                                         Charles Glover Barkla, a student of       came out. The British scientist
  If the cathode rays, that we hear all about,           both Stokes and Thomson at Cam-           William Henry Bragg announced in
      When the burglar threatens to shoot,               bridge, showed that X rays were           1907 that X rays and γ rays were not
 Will they show us the picture without any doubt,
                                                         plane polarizable while experiment-       in fact ether waves, but rather par-
       Of the heart that we feel in our boot.
                                                         ing with secondary and tertiary           ticles, a neutral pair at that: electron
      If the new x-rays, that the papers do laud,        X rays. (These were produced by           plus positively charged particle.
           When the ghosts do walk at night,             directing X rays against solids.)         Bragg’s serious research began at a
      Will show ’neath our hat to the world abroad          As X rays began to show, more and      late age, 41, after twenty pleasant
      How our hair stands on end in our fright.          more, the properties of light, urani-     years at the University of Adelaide,
                                                         um rays provided new mysteries.           Australia, where he played golf and
         If the wonderful, new, electric rays,
           Will do all the people have said,             They themselves were composed of          hobnobbed with government of-
     And show us quite plainly, before many days,        three sorts of distinct rays: α, β, and   ficials. He announced his new in-
      Those wheels that we have in our head.             γ rays. What were these? Suddenly         tellectual work in a Presidential
                                                         physics, which had seemed to some         Address to the Australian Associa-
       If the Roentgen, cathode, electric, x-light,      to be coming to a conclusion, was         tion for the Advancement of Science
                Invisible! Think of that!                                                          during which he made a critical
                                                         faced with unexplainable, qualita-
     Can ever be turned on the Congressman bright
           And show him just where he is at.             tive discoveries. They were not “in       review of Rutherford’s work, ques-
                                                         the sixth place of the decimals,” as      tioning the law of exponential
     Oh, if these rays should strike you and me,         Michelson had predicted. At the           decrease for the absorption of α rays.
       Going through us without any pain,                international congress on physics,        For two and a half years he published
     Oh, what a fright they would give us to see         staged in Paris in 1900 by the French     a paper every few months, work that
      The mess which our stomachs contain!               Physical Society, fully nine percent      led him to make the radical state-
                                                         of the papers delivered were on the       ment that X rays were particles. His
                            —Homer C. Bennett,
                   American X-ray Journal, 1897          new ray physics.                          idea was based on two facts: (i) X rays
                                                            In 1899 Ernest Rutherford, another     excite fewer gas molecules in their
                                                         student of Thomson’s and the man          path than would be expected from
                                                         who would become his successor as         a wave-like disturbance, and (ii) the


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velocity of the electrons excited by
X rays is greater than could be giv-
en to them by a wave. By this time
Bragg and his physicist son were back
in England, and their theory caused
great controversy even in the country
where particles were in favor and
where exotic modeling of physical
phenomena was well tolerated. Their
most vociferous opponent was             wondered whether such a discipline         Roentgen picture of a newborn rabbit made
Charles Barkla, who argued that the      distinct from chemistry existed!)          by J. N. Eder and E. Valenta of Vienna, 1896.
ionization of matter was a secondary        When in 1899 Roentgen was of-           (Burndy Library, Dibner Institute, Cambridge,
effect not needing to be directly        fered a position at Munich and             Massachusetts.)
attributable to the wave-like nature     the chance to build up physics there,
of X rays. We will return later to the   he accepted. Five years later, in
problem of the concentration of X-       negotiations with the minister of
ray energy, unexplainable in terms       education over another possible
of waves, as it bears on Louis de        move, this time to the Reichsanstalt,
Broglie’s insight into the wave nature   Roentgen received, in return for a
of matter.                               pledge to stay in Munich, a second
                                         institute, for theoretical physics, to
                                         complement his existing institute
X RAYS AS A PROBE OF THE                                                            Radiographs of tropical fish made
                                         for experimental physics. When Emil        by J.N. Eder and E. Valenta of Vienna,
STRUCTURE OF MATTER
                                         Cohn and Emil Weichert succes-             Jaunary 1896 and presented to
   Before we turn to our final act in     sively declined the offer of a position,   Roentgen. (Burndy Library, Dibner
the almost thirty year drama to un-      it was given to Privatdozent Som-          Institute, Cambridge, Massachusetts.)
derstand the nature of X rays, let us    merfeld, who joined Roentgen in
turn aside to follow another direc-      Munich and shared his desire to build
tion that the work on X rays took,       up physics there to the quality of the
a shift from the investigation of the    institutes in Göttingen, Berlin, and
nature of X rays to their use in prob-   Leipzig. In the work on quantum
ing the structure of crystals and of     theory of the next two decades,
atoms. That story will take us back      Munich would join Copenhagen and
to Roentgen and the center for phys-     Göttingen as the main centers on the
ics he built up at Munich. While         Continent.
at Würzberg, Roentgen had been              Sommerfeld was initially unen-
agitating for an extra position in       thusiastic about assistant Max von
physics. He wanted a position for        Laue’s idea that regularly spaced
theoretical physics, a newly emerg-      atoms in a crystal might act as a dif-
ing specialty of German origin that      fraction grating for X rays, the fine
followed by several decades the crys-    distances between the atoms serving,
tallization of physics itself in the     as no hand- or machine-ruled grating
mid-nineteenth century. (In 1871         could, to diffract ultra-high frequen-
James Clerk Maxwell hesitated in         cies. If, of course, that is what one
giving his support to the creation of    thought X rays were! Sommerfeld,
a Physical Society in London. He         pushing the impulse hypothesis, was


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                                                     White radiation Laue diffraction pattern from the protein
                                                 trimethylene dehydrogenase (an enzyme that catalyzes the
                                                           conversion of trimethylamine to dimethylamine and
                                                 formaldehyde) recorded on SSRL beam 10–2 with a 5 msec
                                              X-ray exposure. The photograph was taken by Scott Matthews
                                               and Scott White of Washington University, St. Louis, and Mike
                                                   Soltis, Henry Bellamy, and Paul Phizackerly of SSRL/SLAC.




                                              engaging in dis-     Later others would suggest that the
                                              cussions with        crystal itself imposed structure on
                                              Johannes Stark       the incoming radiation. Laue pub-
                                              over the quan-       lished a rather long article on his
                                              tum nature of X      theory of diffraction in the Enzyk-
                                              rays. Stark was      lopadie der Mathematische Wis-
                                              one of the few       senschaften, and much later (1941)
                                              physicists who       he went on to publish a 350-page
                                              in 1911 took se-     review of the subject, Roentgen-
                                              riously Einstein’s   strahlen-Interferenzen, in which he
                         suggestion that light comes in quan-      included the effects of electron
                         ta of energy. Applying the notion to      interference.
                         X rays, Stark was able to assign them        Perhaps as was fitting for an early
                         a frequency and to explain the high       proponent of relativity and a defender
                         velocity of electrons that had been       of Einstein throughout the Nazi pe-
                         excited by X rays, one of the phe-        riod, Laue made little of quantum
                         nomena that so exercised Bragg and        theory and remained skeptical of the
                         Barkla.                                   Copenhagen interpretation through-
                            Laue persisted in asking that the      out his life. He became director of
                         experimentalists try out X rays on        the Kaiser Wilhelm Institute in the
                         crystals. A student of Max Planck’s       years before World War II, resigning
                         (in fact, his favorite), Laue had         his position in 1943, at which time
                         worked on a theory of the interfer-       the Institute was directed towards
                         ence of light in plane parallel plates.   the building of an atomic bomb un-
                         By 1912 his specialty had become the      der the leadership of Werner Heisen-
                         theory of relativity, but he was not      berg. After the war Laue worked to
                         averse to following Sommerfeld in         rebuild German science. In the fall
                         working on a theory of diffraction.       of 1946 he helped create the German
                         Laue’s guess was that it would be         Physical Society in the British Zone,
                         only the secondary X rays, not the        and worked to revive the first of the
                         chaotic Bremsstrahlung identified         national bureaus of standards, the
                         with the initial deceleration of          Physikalische-Technische-Reich-
                         electrons, that would interfere con-      sanstalt. Towards the end of his life
                         structively in the crystal. In April      he assumed the directorship of the
                         1912 Walther Friedrich and Paul           now one of several Kaiser Wilhelm
                         Knipping shone secondary X rays on        Institutes, this one devoted to elec-
                         copper sulfate and zinc sulfate sur-      trochemistry in Berlin-Dahlem. Laue
                         faces and found that dark spots in        died in an auto accident at the age of
                         successive circles appeared on pho-       eighty-one.
                         tographic plates placed behind them.         Laue was representative of the
                         At this time both the nature of X rays    German talent for institution build-
                         and the structure of crystals was a       ing in the support of science and the
                         puzzle. Laue’s analysis of the situa-     German fascination for fundamen-
                         tion was to identify five distinct        tal principles and theories. Those
                         wavelengths of incoming X rays            who would apply Laue’s idea and
                         between 1.27 and 4.83 × 10 −9 cm.         build on Friedrich and Knipping’s


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




experimental demonstration were            using a photographic plate or an
the British, specifically the Braggs       ionization chamber (depending on
and Henry Moseley. In view of the          the strength of the incoming rays) as
German results the Braggs had come         a detector—the Braggs proceeded
to believe that X rays were of an elec-    with the first measurements in X-ray
tromagnetic nature, but they insist-       spectroscopy. By 1913, just a year af-
ed that the rays must have some sort       ter they had pioneered the method,
of dual existence as they were able        crystal analysis with X rays had
to concentrate their energy. But the       become a standard technique. The
continuing puzzle as to their nature       results not only gave insight into the
did not stop the Braggs from recog-        structure of crystals but also into the
nizing the practicability and impor-       nature of the anti-cathode that
tance of a new field of study, X-ray       produced the rays.
crystallography.                              The first person to notice that X
   The new field was pioneered by          rays can be characteristic of the sub-
the Braggs. They were inspired by the      stance that emits them was Charles
Cambridge theorists who argued that        Barkla, the opponent of the Braggs in
a diffraction grating imposes a struc-     the matter of X rays as neutral par-
ture on an inhomogeneous pulse of          ticles and a professor at the Univer-
white light, picking out, as if in a       sity of Edinburgh who spent over
Fourier transform, the wavelengths         forty years examining the properties
into which the beam can be decom-          of secondary X rays. Between 1906
posed. William Henry Bragg and his         and 1908 he had noticed that ele-
son, William Lawrence Bragg, argued        ments emit secondary X rays with
by analogy that the crystal, by dint       a penetrating power in aluminum
of the distance between planes of          that is distinct for each element. To
atoms, imposes a similar structure         distinguish between the hardness
on an inhomogeneous pulse of X             of the characteristic rays, he intro-
rays. When the X rays are reflected        duced the terminology K and L rays.
off two successive planes of atoms in      It was for this discovery that he was
the crystal, they interfere construc-      awarded the Nobel Prize in 1917.
tively if the difference in the distance   (His subsequent work earned Barkla
traveled is equal to an integral num-      the reputation as something of a sci-
ber of wavelengths. Thus the famous        entific crank.) What the Braggs no-
Bragg condition                            ticed (see figure on next page) was
                                           that a pattern of multiple peaks with
             n λ = 2d sin θ,               varying intensities was produced no
                                           matter what the crystal (shifted only
where d is the distance between            by the varying distances between
planes and θ is the angle of reflection.    planes of atoms) as long as the ele-
   Using an X-ray tube and a colli-        ment of the anti-cathode remained
mating slit to produce the incom-          the same. In other words, the pattern
ing rays; using various minerals,          was analogous to spectral lines emit-
quartz, rock salt, iron, pyrite,           ted by gases in the optical frequen-
zincblende, and calcite, as three-         cies. The person to explore this anal-
dimensional diffraction gratings; and      ogy to its fullest was Henry Moseley,


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                                 B1
                                                            II        B2
                                                   B
 Ionization




                            C1                 C        A        C2        A2
                                      A1

                                           5       10         15    20      25
                        I                                   B2   Degrees
                                                   C2            A2
                                                                     C3?        B3

              0             10                 20                     30
                                           Degrees
                                                                                     a young researcher working in               gave on Friday on X rays. It was
                  One of the earliest examples of X-ray                              Rutherford’s Manchester laborato-           rather anxious work, as Bragg, the
                  spectroscopy. The Braggs made a                                    ry during the time when Niels Bohr          chief authority on the subject (Pro-
                  seredipitous discovery: while studying                                                                         fessor of Leeds) was present, and
                                                                                     was visiting regularly.
                  the scattering of X rays off of crystals                                                                       as I had to be cautious. However it
                                                                                        Moseley’s two grandfathers had           proved quite successful and I
                  they noticed that a distinctive pattern of
                  peaks appeared for each of the different
                                                                                     been fellows of the Royal Society, and      managed to completely disguise
                  anti-cathodes being used to produce                                his father had founded a school of          my nervousness. I was talking
                  the rays. What had initially started out as                        zoology at Oxford. Mosely himself           chiefly about the new German
                  a study of the structure of crystals led to                        was perhaps the only important              experiments of passing rays
                  an investigation of the atomic structure                           atomic physicist to be educated at          through crystals. The men who did
                  of the anti-cathode elements. [Bragg                               Oxford. In the fall of 1910 he came         the work entirely failed to under-
                  and Bragg, PRS, 88A, 413 (1913).]                                                                              stand what it meant, and gave an
                                                                                     to work as a demonstrator under             explanation which was obviously
                                                                                     Rutherford, his salary being paid by        wrong. After much hard work Dar-
                                                                                     a Manchester industrialist. He was          win and I found the real meaning
                                                                                     assigned a research problem to which        of the experiments.*
                                                                                     everyone knew the answer: how
                                                                                     many β particles are emitted in the         For a time the Braggs, Moseley,
                                                                                     radioactive disintegration of radium     and Darwin continued on the same
                                                                                     B (Pb214) to radium C (Bi214). On find-   track, even though Rutherford pre-
                                                                                     ing the answer everyone expected,        sented difficulties which were final-
                                                                                     one, he proved his competency as an      ly overcome by Moseley’s persistent
                                                                                     experimentalist. However, his next       enthusiasm and by Bragg’s offer to
                                                                                     experiments would not be so cut and      Moseley of a visit to Leeds to teach
                                                                                     dried, nor would they receive the        him the techniques of X-ray spec-
                                                                                     ready approval of Rutherford. Like       troscopy. Some of the questions they
                                                                                     the Braggs, and quite independent-       pursued were the old ones about the
                                                                                     ly of them, Moseley was stimulat-        nature of X rays. How to reconcile
                                                                                     ed by the photographs of Friedrich       the corpuscular nature of the rays
                                                                                     and Knipping, and felt that Laue had     with their ability to interfere? Bragg
                                                                                     misinterpreted them as evidence of       had compared this conundrum in the
                                                                                     five homogeneous X rays. He teamed        electromagnetic theory of X rays to
                                                                                     up with Charles G. Darwin, grand-        the physical impossibility of a spread-
                                                                                     son of the famous evolutionist, and      ing circle of water waves, caused by
                                                                                     turned to, as he said, the “real mean-   the fall of a rock, to excite another
                                                                                     ing” of the German experiments.          rock to jump the same distance the
                                                                                     The Laue dots connoted the struc-        wave-producing rock had fallen.
                                                                                     ture of the crystal, not the structure      The new questions concerned the
                                                                                     of the incoming rays. When pre-          elements. In July of 1913 Bohr paid a
                                                                                     senting his results to a Friday phys-    visit to Manchester and discussed
                                                                                     ics colloquium which father Bragg        atomic structure with Moseley,
                                                                                     attended, Moseley discovered the         Darwin, and George Hevesey. The
                                                                                     similarity in their understanding of     discussion revolved around the sim-
                         H. G. J. Moseley in Balliol-Trinity
                                                                                     the phenomena, and afterwards he         ilarity, and possible differences,
                          Laboratory, Oxford, circa 1910.
                        (Courtesy of University of Oxford,                           wrote to his mother:
                    Museum of the History of Science and                                 I have been lazy for a couple of     *Nov. 4, 1912. Quoted in J.L. Heilbron,
                               the AIP Niels Bohr Library)                             days recouping after the lecture I      H.G.J. Moseley, p. 194.



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




between the atomic weight of an
element (A) and its nuclear charge
(Z). Geiger’s and Marsden’s scatter-
ing experiments and Rutherford’s
theory had proposed that the newly
discovered nucleus held a charge half
that of the atomic weight. A Dutch
lawyer and would-be interpreter of
Mendeleev’s table, Van de Broek, had
suggested that the nuclear charge of
an element set its place in the table.
Now the frequency of characteris-                                  3
tic K rays gave another quantity with                 νK       =     ν (Z − 1)2 .
                                                           α       4 0
which to mark the elements. What
                                              Moseley interpreted his formula
would the X-ray spectroscope have
                                           as a vindication of Bohr’s theory,
to say about those places in the table
                                           which at the time was being pub-
where the atomic weights did not fol-
                                           lished in three lengthy and famous
low in increasing order the serial
                                           papers “On the Constitution of
numbers: between nickel and cobalt;
                                           Atoms.” Moseley argued, not quite
between argon and potassium; and
                                           convincingly, that his results could
between iodine and tellurium?
                                           be used to support the quantization
Would the hardness of the K rays or-
                                           of an electron’s angular momentum.
der the elements by atomic weight
                                           Frederick A. Lindeman, a fellow Eng-
or by nuclear charge?
                                           lishman working with Walther
   Moseley used an ingenious de-
                                           Nernst on the Continent but with
vice of G. W. C. Kaye’s to exam-
                                           his eye on the same chair of phys-
ine the K rays from copper, nickel,
                                           ics as Moseley, Clifton’s chair at Ox-
cobalt, iron, manganese, chromi-
                                           ford, criticized both Bohr and Mose-
um, and titanium. By putting the
                                           ley. He was working out of an already               X-ACTLY SO!
different elements which served as
                                           successful tradition which applied
anticathodes on a magnetized truck
                                           the condition of quantized frequen-      The Roentgen Rays, the Roentgen Rays,
and rail inside the evacuated cham-
                                           cies to the motion of atoms to pre-               What is this craze?
ber, Moseley was able to change                                                               The town’s ablaze
                                           dict specific heats in a solid and to
anti-cathodes with an external                                                               With the new phase
                                           the motions of molecules in a gas
magnet without disrupting the in-                                                             Of X-ray’s ways.
                                           to predict the patterns of rotational
tegrity of the chamber. After
                                           and vibrational infrared spectra. (A                  I’m full of daze,
switching from detecting the K rays
                                           quantum theory of molecular spec-                   Shock and amaze;
by ionization to detecting them by
                                           tra preceded one of atomic spectra!)                  For nowadays
photography, his work went quick-
                                              More successful than Moseley’s                   I hear they’ll gaze
ly, and in several weeks he showed                                                   Thro’ cloak and gown—and even stays,
                                           argument in favor of Bohr’s atomic
that the ranking of elements by K                                                   These naughty, naughty Roentgen Rays.
                                           theory was his help to the chemists
rays followed their ranking by nu-
                                           in sorting out the confusions of the
clear charge, Z. The relation was                                                          —Wilhelma, Electrical Review,
                                           rare earths. In November of 1913,
simple as well. The darker of the                                                                          April 17, 1896
                                           Moseley moved to Oxford where
two primary K lines, K α , fit the
                                           he worked with equipment at the
form


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




                                                                  brought to a halt     frequency of X rays emitted. By 1916,
                                                                  by the end of the     the Duane-Hunt law was the best
                                                                  World War I. The      way to determine h, Planck’s con-
                                                                  Braggs’ work          stant, although neither Millikan nor
                                                                  continued after       Duane then subscribed to the view
                                                                  the war. The el-      that energy came in discrete quan-
                                                                  der Bragg revivi-     tized units.
                                                                  fied the Royal           Arthur Holly Compton also
                                                                  Institution,          initially interpreted his results on
                                                                  where Sir Hum-        X-ray scattering from electrons as a
                                                                  phry Davy and         cut-off relation that was governed in
                                                                  Michael Faraday       this case by Planck’s constant rather
                                                                  had made their        than as proof of the quantum nature
                                                                  chemical and          of radiation. Compton, who was later
                                                                  electrical dis-       to run the Manhattan Project’s Met-
                                                                  coveries, by es-      allurgical Laboratory at Chicago
                                                                  tablishing a re-      during World War II, received his
                                                                  search school for     Ph.D. from Princeton just before the
                                                                  the analysis of       First World War for work on X-ray
                                                                  organic crystals.     diffraction and scattering. After
     Electrical Laboratory but with no        This work would become central to         several years spent at Westinghouse
     salary. Moseley began a thorough in-     the developing field of molecular bi-      Manufacturing Company working
     vestigation of Mendeleev’s table us-     ology. During the war the elder Bragg     on fluorescent lamps, he spent a year
     ing X rays, and moving from calcium      worked for the Navy board to eval-        at Cambridge’s Cavendish Laborato-
     to zinc and then to the rare earths,     uate inventions and to promote re-        ry where he developed a friendship
     lanthanum to erbium. George Ur-          search with military applications.        with J.J. Thomson and carried out an
     bain, a Professor of Chemistry in        Like many British and U.S. scientists     investigation into the orderly change
     Paris at one of the Grands Ecoles had    he eventually found himself work-         of X-ray frequency with scattering
     been engaged for years in fraction-      ing on problems of submarine de-          angle as the X rays scattered from
     ating the elemental rare earths in       tection. Moseley, with his Eton pa-       electrons. As a new professor at
     competition with Carl Auer von           triotism, practically forced himself      Washington University in St. Louis,
     Welsback, who performed his frac-        upon the Royal Engineers along with       Compton published a mass of data
     tionations in an Austrian castle.        his friend Henry Tizard. Tizard sur-      on the relation between X-ray fre-
     Urbain recognized the power of           vived the Great War and subse-            quency and scattering angle taken
     Moseley’s technique and paid him         quently led the minds of British sci-     with a Bragg crystal spectrometer. In
     a visit with precious samples of the     entists into World War II. Moseley,       1922, a year after he had taken the
     last four rare earths, thulium, ytter-   however, died at Gallipoli in the bat-    measurements, and along with Peter
     bium, lutecium, and celtium. He was      tle of Sari Bari.                         Debye in Germany who had seen his
     astounded at how quickly Mose-              As Europe engaged itself in the        results in the Bulletin of the Na-
     ley’s X-ray spectrometer could de-       Great War, interesting work on X          tional Research Council, Compton
     termine that celtium was not his         rays began to come out of the Unit-       accepted Einstein’s light quantum
     sought after new element, but was        ed States. Following Robert Mil-          and by extension the X-ray quantum.
     only a combination of lutecium and       likan’s work on the photoelectric         The explanation of the Compton-
     ytterbium!                               effect, William Duane at Harvard          effect then became a simple scatter-
        The Braggs’ work on crystals and      gave an exact law that related the en-    ing of two elastic particles.
     that of Moseley’s on elements was        ergy of cathode ray electrons to the


22   SUMMER 1995                        zycnzj.com/http://www.zycnzj.com/
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 Niels Bohr, 1885–1962. (Courtesy of the
                  AIP Niels Bohr Library




   Few physicists had taken Einstein
seriously when he predicted the
light quantum in 1905. Bohr had
pooh-poohed the idea. But by 1921
evidence was mounting and so was
Einstein’s fame. The de Broglie
brothers, Maurice and Louis, were
two others who had learned from the
studies of X rays of the dual nature
of radiation, and Louis was inspired
to suggest that matter too might have      vinced most physicists that they
this dual nature. Maurice de Broglie’s     were waves. The Braggs, not quite
interest in the quantum had been           giving up, insisted that they had the
sparked by his secretaryship of the        properties of both waves and parti-
first Solvay Conference called in          cles. By 1922 the startling explana-
1911 by chemist Walther Nernst to          tion by Compton of his scattering
introduce the quantum concept to           experiments—X-ray energy was
physical scientists, and he decided to     concentrated into particle points—
investigate the energies of electrons      helped convince the science com-
excited by K and L frequency X rays.       munity to take Einstein’s notion of
He found the old problem over which        light quanta seriously. And finally
Bragg and Barkla had argued: X rays        the work on X rays by the de Broglies,
can concentrate their entire energy        and the younger brother’s desire to
and pass it on to electrons. And like      put on an equal footing light and
Bragg he concluded that X rays act         matter, gave Louis de Broglie the
both as waves and as particles. His        courage to suggest that even the good
younger brother, Louis, in a spirit        old electron (the cathode ray parti-
of unification, longed to treat light      cles!) partook of wave qualities.
and matter as equals. Both could be
understood as particles following
                                           ADDENDUM
waves, he proposed. A “mobile” of
light or X rays or of matter followed         The early history of X rays follows
along behind an “onde fictive.”             another path that I have not covered
   So the discussion of X rays had         here. As the physicists wondered
come around full circle. They were         about the nature of X rays and used
discovered in Roentgen’s laborato-         them to probe the structure of crys-
ry as this newcomer to cathode rays        tals and atoms, medical doctors used
was trying to puzzle out his coun-         them to probe the human body and
tryman Lenard’s challenge to the           to diagnose and treat disease. Roent-
British. Lenard believed cathode rays      gen by presenting an X-ray photo-
to be ethereal. The British thought        graph of his wife’s hand to the
them particles. Soon X rays became         Würzburg Physical and Medical
the new mystery. Were they elec-           Society in January of 1896 began the
tromagnetic waves or were they             practice of radiology. A month later
neutral pairs of particles? By 1913        a German doctor used an X ray to
the interference of X rays had con-        diagnose sarcoma of the tibia in the


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                                    zycnzj.com/ www.zycnzj.com


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




                                              right leg of a young boy. The military        Two of the three discoveries that
                                              first used X rays in Naples in May of       helped shake physics out of its fin-
                                              1896 to locate bullets in the forearms     de-siècle malaise, X rays and
                                              of two soldiers who had been wound-        radioactivity, would be taken up,
                                              ed in Italy’s Ethiopian campaign.          almost immediately, by doctors in
                                                 Radiology would be advanced             their medical practice. And as
                                              by the strong tradition of medical         physicists began to require substan-
                                              research in France. Antoine Béclère        tial funds to continue their quest to
                                              set up the first X-ray machine in          discover the smallest structures of
                                              which a patient was strapped and           matter, the link between physics and
                                              moved around for complete X rays of        medicine would be pushed. Ernest
                                              the chest. For those taking pictures       Lawrence regularly raised money for
                                              he introduced safety equipment, lead       his laboratory’s cyclotrons by vir-
                                              aprons and lead rubber gloves. He          tually promising cures for cancer: “It
                                              pioneered the first use of radiography      is almost unthinkable that the man-
                                              of the stomach in 1906. The patient        ifold new radiations and radioactive
                                              had first a meal of bismuth. Through        substances [produced by his cy-
                                              the work of Béclère and others the         clotrons] should not greatly extend
                                              practice of medical diagnosis changed      the successful range of application
                                              significantly. Soon to follow was the       of radiation therapy.”*
                                              use of X rays to treat cancer. The rays
     An informal moment at an informal                                                   *Quoted in J.L. Heilbron and Robert
                                              of the chemists and physicists
     conference called by Paul Ewald                                                      Seidel, Lawrence and his Laboratory,
     in 1925. From left: Paul P. Ewald,       seemed to inspire doctors: α, β, and γ      (Berkeley, University of California Press,
     Charles G. Darwin, H. Ott,               rays were also beamed at cancerous          1989) p. 215.
     William L. Bragg, and R. W. James.       tumors.
     (The Isidor Fankuchen Collection)




                                                                                              FOR FURTHER READING

                                                                                                John Heilbron, H.G.J. Moseley:
                                                                                            The Life and Letters of an Eng-
                                                                                            lish Physicist (Berkeley, U.C.
                                                                                            Press, 1974).
                                                                                            Bruce Wheaton, The Tiger and
                                                                                            the Shark: Empirical Roots of
                                                                                            Wave-Particle Dualism (Cam-
                                                                                            bridge, Cambridge University
                                                                                            Press, 1983)
                                                                                                One can also find good bibli-
                                                                                            ographies in the numerous articles
                                                                                            in the Dictionary of Scientific
                                                                                            Biography under the individual
                                                                                            scientists discussed above.



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