Marie Curie's Doctoral Thesis: Prelude to a Nobel Prize Robert L. Wolke University of Pittsburgh, Pittsburgh. PA 15260 Marie Sklodowska Curie's gender, coupled with the bizarre new materials that she gave to the world, made her a kind of public novelty in the early years of the 20th century. As the first woman in history to reach the heights of scientific accomplishment, she became more of a public figure than many of her niale colleagues. And "her" radium, a substance that glowed in the dark, remained perpetually charged, burnedhuman flesh without the senaatlonof heat, and shot mysterious rays through solid matter, was a mar\,elous new toy that everyone could discuss a t breakfast along with the latest news. Americans became personally familiar with Marie Curie not only through radio, newspapers, newsreels, and books both factual and sentimental, hut also through.two national public subscriptions that raised the money to buy two grams of radium for her continuing research. These gifts, largely from the women of America, were graciously accepted by the "radium lady" during two visits to the United States in 1921 and 1929. Nor did the newsmedia fail to note with irony that F,g.re 1 Pnotograph 01 2 7 gRaBr,, taken oy 81sown ghl on 15 Oclooer 1922 the discoverer of radium herself was unable some two de' , the Tnc nscrlpl on aeclarer lhts sampleto oe the premerraosdmonten. 1 cades later to purchase "her own" element, which was then 00en lac'or! in Be g0.m (Co.nesy of A r m ves P w r e el Marme C u r e I selling for between 70 and 100 thousand dollars a gram (Fig. 1). But the transformation of a historical figure into a folk The Thesis heroine can lead to her being remembered for reasons that Marie Curie's doctoral thesis contains the entire kernel of are only partially correct. Who, for example, has not heard her most important work, embodying in a single document Marie Curie identified simply as "the discoverer of raan area of research that had otherwise been described only dium"? piecemeal, if a t all, in journal articles. This thesis is the rare It is true, of course, that the discovery of new elements has result of a central scientific figure's taking time out from her always been an easily identifiable landmark in chemistry; research to create a snapshot of a rapidly changing field at a there have been only about 20 since Mendeleev's time, and crucial moment in history. the number is quite unlikely to increase, at least in more t ! m t h m t t ~ a ~ & ~ t o% f Pkke~~t~.W~~~i 'sl no e r , k r e In May of 1903, when the thesis was published, virtually the only major finding that was claimed with certainty by doubt that Marie Curie brought a great deal of ingenuity to Curie was that a new element "with veryccrriocrsproperties" bear upon the identification and isolation of the new erehad been discovered. It was upon this peg that she hung her ment radium in pure form. She herself was so impressed with academic hood,so to speak. But in her introduction to the this unequivocal result that she lavished upon it the only the&, she l a d c>a~malso t o "a n m m t ! 6 t h i 9 . A italics in her doctoral dissertation's summary of conclusions: T h e U I O T ~has proued tnat raaikrn LS a new crternctac'ec& rescdrcd . . . dmx'm &L ~ - & & & & X X ~ - & ment." an atomic property . . . ." This we now know as the radioNevertheless, to identify her discovery of even two elechemical method, which served so well in later years to ments-radium and polonium-as the salient outcome of elucidate the process of nuclear fission and which provided her scientific career is to deny the importance of her contrithe basis of all radioactive tracer techniques. butions to understanding the phenomenon of radioactivity, The thesis (Fig. 4 ) , entitled "Radioactive Substances", whichat the turn of the century was a mystery, theenormity was first examined and approved by Paul Appell, Dean of of which we cannot beein to imaeine todav unless we should the Facult6 des Sciences de Paris, on 11 May 1903; it was approved for publication by Louis Liard, Vice-Rector of the suddenly find evidence that atoms are aiive. It was for her work on radioactivity, not for her discovery of radium and AcadBmie de Paris, on the same date. (One is tempted to picture the candidate, walking it around from office to office polonium, that she shared the 1903 Nobel Prize in physics (Fig. 2). Only later, in 1911 (Fig. 3), did asecond Nobel Prize, to obtain the requisite signatures.) The formal defense took this time in chemistry, honor the discovery of these eleplace on 12 June 1903 before an examining committee ments. But even then it was not so much for the simple act of chaired by Gabriel Lippman, Professeur Honoraire (emeridiscovery as for surmounting the incredible difficulty of the tus) of physics (and Nobel Prizewinner in 1908) and with quest. physics Professeur Honoraire Bouty and chemistry profesThe present paper is a critical account of the most signifisor Henri Moissan (Nobel Prize in Chemistry, 1906) as excant body of Marie Sklodowska Curie's scientific work, preaminers. sented in the hope that she he remembered for more of the The text used in the present study was a copy of the "right" reasons. I t also offers some new insights into her original thesis in French, obtained from the Bibliotheque de methods and results. L'Universit6, Marseille, France. An English translation Volume 65 Number 7 July 1988
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Figure 2. One of the three 1903 Nobei Prize cenificates in physics. The monogram " P C on the right showsthat this was Pierre Curie's copy. (Comtesy of Archives Pierre et Marie Curie.)
A
LA FACULTB DBS SCIENCES DE PARIS
... N'"' SKLODOWBKA CURIE.
PARIS.
Figure 3. Marie Curie in 1912, following the awarding of her Nobel Prize in Chemistry. (Courtesy of Archives Pierre et Marie Curie.)
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Journal of Chemical Education
Figure 4. Tnle page of Marie Curie's doctoral thesis. (Courtesy of Archives Pierre et Marie Curie.)
(translator unknown) without the footnotes was ~ u b l i s h e d in 1903 in ~ h e m i c a l ~ e wLondon, s, Vol. 88, pp 85 e t seq., and was reprinted several times in later years. The original Chemical News edition is still available in some American libraries as a separately bound reprint.' All quotations in the present paper, however, are my own translations from the French oriainal, and may differ in some respects from the Chemical News version and its offprints. The thesis consists of an "Introduction", "Historical", four chanters of research descrintion.. a brief eniloeue . u on the nature and cause of radioactivity, and a summary of conclusions. The nresent naner deals onlv with the "Introduction". the "Histo;ical", and-the first two research chapters, which embody the chemical, as broadly distinguished from the physical, parts of the work. The (translated) subheadings of these chapters are as follow^:^
.
CHAPTER I. RADIOACTIVITY OF URANIUM AND THORIUM. RADIOACTIVE MATERIALS. Radioactivity of Uranium and Thorium; Radioactive Materials; Becquerel Rays; Measurement of the Intensity of the Radiation; Radioactivity of the Compounds of Uranium and Thorium; Is Atomic Radioactivity a General Phenomenon?; Radioactive Minerals. CHAPTER 11. METHOD OF RESEARCH. Polonium, Radium, Actinium; Spectrum of Radium; Extraction of the New Radioactive Substances; Preparation of the pure Chloride of Radium; Determination of the Atomic Weight of Radium; Characteristics of Radium Salts; Fractionation of Ordinary Barium Chloride.
Scientific Background The years 1898-1902, during which Marie Curie carried out the research described in her thesis, were a time of rapid change and of more than a little confusion in the worlds of chemistry and physics (1-3). Wilhelm RontPen had snarked a furor both within and outside the scienzfic world when he announced in M'ijrzburi: in Ilerernher of 1656 that he had discovered some mvsterious rays that could penetrate solid metal and produce photographs of the bones in a living human hand. Because Rontgen's rays came from fluorescent spots in his glass Crookes tubes (these spots were beina excited, we now know, hv electron homb&dment), HeuFi Poincar6 suggested to thk French Academy of Sciences on 20 January 1896 that other luminous materials might emit similar rays. Among many others, Henri Becquerel began to search for the emission of penetrating rays from a variety of materials that had been made luminescent by exposure to sunlight. Instead, however, he found that a penetrating radiation was emitted by potassium uranyl sulfate and other uranium salts. whether or not thev bad been e x ~ o s e dto light. Fortu"Y" r; " Z ; they nately, he did not christen these became known as "Becquerel rays" or "uranium rays", and the way was left open for Rutherford to subdivide them later accordina to their penetrating.abilities into "alpha", "beta", and "ga&na". Surprisingly little work was done on the uranium rays between 1896 and 1898 (2). During this period, reports of new rays, waves, and radiations, both real and illusory, were and Becquerel's observation appears to have gotten lost in the shuffle. Rays of the penetrating variety, i.e., those that could pass through an opaque barrier and expose a photographic plate, were being reported as emanat-
neo- rays
' It may be prudent lo check any such copies for rao oactivlty, on me possoiliry that they were borrowed and taken oy scientists into contam nated laoorarories before the hazards 01 radioactivity were fully understood. In the ChemicalNewstranslation of the thesis and In some of the early literature in both Enqiish and French, includinq some of Marie curie's own papers, her coined words. "radioacfivif?' and "radioae Bf" were written with hyphens: "radio-activity", etc. In the original French thesis, however, she did not use the hyphens.
ing from many luminescent materials and from sources as diverse as fireflies and freshly cleaned metal surfaces. An important part of the search for an understanding of the uranium rays was a search for the source of that energy which, people believed, must have been absorbed in order to he re-emitted. Otherwise, where could the energy be coming from? The law of conservation of energy had been a cornerstone of physical science for more than 50 years and was not about to he abandoned. And when Becquerel concluded that the rays were coming from the uranium-atoms themselves, it only reinforced the idea of phosphorescence, for if atoms were as unchaneeable as evervone had believed for almost a century, then the energetic radiations could not he coming from changes in the atoms themselves. The enerev had to have beenudeposited previously in the atoms frxm some external source. When Becquerel discovered that exposure to sunlight was an unnecessary prelude to uranium's emission of radiation, he still helievedthat the phenomenon must be an especially long-lived kind of -phosphorescence, the uranium previously h a & ~ been "exoosed" in some unknown wav. The Curies and oibers, however, held to a fluorescence hypothesis: that there was some unidentified but contemnorarv. . .. continuous source of exciting energy. Nevertheless, Marie Curie begins her thesis with areference to "the . ohos~horescence of urani. um, discovered by M. Becquerel". It is ironic that Becquerel, the discoverer of radioactivity and proponent of a soon-to-he-discredited phosphorescence hypothesis, was ultimately in a sense correct. Today we know that the energy that is being given off by long-lived radioactive elements such as uranium was indeed stored in them a t an earlier time: some 20 billion years ago, when their atoms were created in the uucleogenetic processes that followed the "bie bane". The enerev was stored. not in the form of absorbed excit& radiation, hut in the' form of mass, which is todav beina re-emitted as electromaanetic and Darticulate radiation. ~ a d i o ~ c t i vthen, i t ~ , may well be said tb be the ultimate phosphorescence: it is returning to us some of the primordial energy of the universe. Becquerel's notion was not quite so wrong, after all.
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Chronology The thesis work done by Marie Curie and her associates, which began on 16 December 1897, took place in the following sequence. (The months given are those of the first reports, either as published or as presented to a scientific academy.) 1895-(December) W. Riintgen discovers X-rays. 1896-(February) H. Becquerel discovers penetrating radiations from luminescent uranium salts. -(March) H. Becquerel reports that exposure of salts to sunlight is not necessary to produce uranium rays. ( M a y ) H. Becquerel reports that the rays come from the clement uranium,~rrgardl&of cmnpwnd. 1897 (L)ecemlwr, M. Curie hegins w r k on Becqurrel radiation*. 1698- t'rhruawr .. C.Schmidt dircuvers the radioactid! oi thorium. -(April) M. Curie independently discovers the radioactivityof thorium. -(July) M. and P. Curie discover polonium and coin the term "radioactivity". -(December) M. and P. Curie and G. BCmont announce the discovery of radium. 1899-A. Debierne (and F. Giesel, independently in 1901) discover actinium. 1902-(March) M. Curie isolates 120 mg of radium chloride, spectroscopically free of barium. -(July) M. Curie reports the atomic weight of radium. -(July) E. RutherfordandF. Soddy report the hypothesis that "radioactivity is a manifestation of sub-atomic chemical change" and that radioactivity involves the spontaneous transformation of atoms of one element into those of another. 1903-(June) M. Curie awarded doctorate in science. -(December) H. Becquerel, M. Curie, and P. Curie share the Volume 65
Number 7 July 1988
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third annual Nobel Prize in . ~hvsics . far their work on radioactivity. 1911-M. Curie is awarded the Nobel Prize in chemistry for her work on radium and polonium. In the following discussion quotations from the thesis are accompanied by their page numbers in the French original. Thesis "lntroductlon and Hlsiorlcal" Marie Curie began her work in order to study the phenomenon in uranium that she was soon to name "radioactivitv". When she found evidence that other radioactive substances exist. her objectives became "the extraction of new radioactive substances and to continue studying them" ( p 1).In this quest she had the collaboration of her physicist husband, Pierre; who "put aside his work in progress" (p 1) to join her (Fig. 5). It was a time in which "the results of the various French and foreign researches [were] necessarily confused, as is the case with all new studies in the course of their development. The aspect of the question [of radioactivity] [was changing], so to speak, from day to day" (p 2). Many new kinds of mysterious penetrating radiations, or rays, were heing reported. Their emission, or supposed emission, was almost invariably heing associated with excited luminescence, the only phenomenon of radiation (i.e., light) emission that was a t the time even partially understood. I t could readily be accepted that primary energy-the sun'swas somehow being absorbed by the radioactive materials and being re-emitted as another kind of radiation. The sun, after all, was the ultimate source of all energy on Earth, whether transient as heat and light or stored in the form of fossil fuels. But in Becquerel's phenomenon of radioactivity, human beines were unknowinelv and for the first time experiencing a no&olar source of energy: the instability of the atomiE nucleus. Thev faced the dilemma of having to explain this phenomenon as heing independent of solar stimuli~tiun,vet without abandoning their notim of theconservati~migfencrgy. The ability of t h e Curies and their contemporaries to formulate a workable explanation of radioactivity almost a decade before the atomic nucleus was discovered constituted some of the most creative scientific experimentation and reasonine that has ever been accomdished. "The Erst tubes for producing ~ d n t ~ rays e n were without . . . metallic lanodesl. The source of the Rontgen ravs was found to be the glasswalls that were struck h i t h e cathode rays; the wall was a t the same time actively [luminescent]" ( p 3). Thus did Marie Curie express the relationship that had been noted between luminescence and the emission of penetrating radiation, i.e., radiation that, like X-rays and Becuuerel ravs, could penetrate black paper and still expose a phbtographic plate. In her thesis, Curie mentions reports of such radiations emanating from zinc sulfide and calcium sulfide, but inasmuch as those observations "have not been reproduced despite a number of attempts to that end" (p 4) she rejects them as unproven. Many chemicals (and even luminous insects) were a t the time heing placed on paper-covered photographic plates and left there for long exposures; it was a temptingly simple experiment to do, and several false positive results had been reported. More than likely, such factors as sample moisture, pressure, or vapors such as sulfides or ammoniain the laboratory atmospheres had sensitized some of the plates to produce foggy images. Penetrating a sheet of Daoer. black or not., is., after all. not much of a trick. Having put aside as unverified the sundry reports of new ~ e n e t r a t i n eradiations. Curie then describes Becauerel's widely c o k r m e d observations that uranium, whether elemental or compounded and whether luminescent or not, does in fact emit penetrating radiations and that elemental uranium appears to be more active in this respect than any of ~~~~~~
. . .
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Fl~ure5. Pierre and Marie Curie in their laboratory in 1898.
its compounds. On the other hand,she points out that uthcrs hud failed to confirm Recquerel's rtpurt that the urdnium frays could be reflected, refracted, and polarized. His supposed observation of these effects was indeed in error. Following the presentation of this background, Marie Curie begins in the thesis to portray her own work. Thesis "Chapter I. Radioactivity of Uranium and Thorium. Radioactive Minerals." The Measurement of Radioactivity
Because Rutherford and others had found that Becquerel's uranium ravs caused gases to become feeble conductors of electricity, curie deci&d to measure the intensity of thc radiation by its'elfect . . . on the conductivity of air" tp 61. Induing soshe replaced with an arcurste electrical measurement the crude methods that were thrn in use for deterring penetrating radiations: the timt: required t o discharge an electrosrupe or tu expose a photographir plate. 'I'hrourh the use ot euuioment desiened nrimarilv hv" her physicis~collaborator~ l e r i ehers , were to de the first quantitativelv re~roduciblemeasurements of radioactive intensity. withbutthis major innovation it would have been impossible to accomplish either of the two salient advances of the period: (1)tracing the profusion of newly discovered radioactive species (radioisotopes) through various chemical changes and thereby determining thei; elemental identities and (2) characterizing the radiations themselves. These accomplishments and their outcome, the understanding of atomic transformation through radioactivity, far overshadow in importance the discovery of the new elements actinium, polonium, and radium. The Curies' radiation detector consisted of a pair of parallel, horizontal, circular metal plates with the powdered radioactive s a m d e spread uniformlv over the surface of the lower one. when ;potential diff&ence was applied across the plates and the circuit was completed, a small electrical current would flow through the radiation-ionized air between them. The current was opposed, or "bucked" by voltage from a stressed piezoelectric quartz crystal, connected into the circuit in the opposite sense from the ionization current. (Pierre Curie himself had discovered the piezoelectric effect in 1883 and was therefore well primed to think of its application in this way.) The measurement was accomplished by stressing the crystal with added weights until zero current in the circuit was indicated by a quadrant electromA
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Figure 11. Marie Curie'sdesk at the institut du Radium, of which she became the first director in 1913.
Figure 10. The Radium Institute, a part of the Curie Institute, as it is today an rue Pierre et Marie Curie in Paris.
In the thesis, Curie provides a table of the ionization currents observed from a variety of uranium compounds measured (or counted, to use today's terminology) under the same conditions. The thickness of the sample on the plate was found not to matter; she concluded correctly that the major portion of the radiation being detected (the alpha particles, as was later determined) were not very penetrating and were strongly absorbed in the samples themselves, so that only the top layers were being observed in any case. "It may he concluded . . . that the absorption of uranium rays by the material that emits them is very great, since the rays coming from deep layers do not produce a significant effect" (P 14). Curie knew that the measured activities3 should be proportional to the uranium contents of the samples, inasmuch i s Becquerel had shown that radioactivity is an atomic property of uranium itself. It is therefore curious that she did not comment on the presence or absence of such a proportionality in her table of data, being content to make a later generalization, without quoting evidence, that "chemical combinations and mixtures cont%uiug uranium or thorium are active in ~roportionto the amount of the metal they contain, all ina&e material acting . . . as an inert absorber of the radiation" (p 17). Today, the tabulated data cannot be interpreted very accurately because the names of some of the compounds given in the thesis are ambiguous: e.g., "hydrated uranic acid, copper uranyl phosphate, etc." (p 14) and their purities are uncertain. But because uranium is so heavy compared with the utht:r elrmrmts in the compuunds, appruvimate uranium contents can he estimated fairly accuriltcly without knwing 566
Journal of Chemical Education
the exact formulas, and an approximate ratio of activity to uranium content can thus be calculated. I have made such a calculation and it does reveal a rough constancy, with an average deviation of 10-15%, among six compounds that could be identified with reasonable certainty and uranium metal itself. More than likely, the proportionality was not any better than 10-15% because of the effects of alpha-particle absorption and scattering in samples of varying composition. One sample, however, the name and formula of which Curie gives as "black oxide of uranium, U205" (p 14),yields a ratio of activity to uranium content that is more than 40% higher than the average of the others, and higher indeed than uranium itself by 10-15%. Oddly, Curie does not comment on this point either, in spite of the fact that her major clue to the existence of radium a little later on was to be the fact that certain uranium-containing materials were more active than pure uranium. Excessive activity in a supposedly pure compound of uranium was certainly unexpected; why did she not mention the anomalous activity of this "UpOs" sample? I t is difficult to believe that she neglected to perform the cnlrulation of proportlonnlity h e t w r n activity anti urmium content or to comment uvon the L ' O result in her lnhoratory notebook.' But perhaps she deemed it not far enough outside of the experimental error to warrant explicit comment at this ~ o i n in t the thesis. I believe it is more ~ ~likelv. -- ~ however, that'sbe preferred to save her bombsbell-the existence of a new radioactive element-for a little later on in the thesis, where she was able to base her inference upon the more spectacularly excessive activity of certain natural ura~
A.
The word "activity" was used by Marie Curie in two senses: (1) any quantitative measure of the intensity of radiation being emitted from a sample and (2) the qualitative property of emifling radiation. i.e.. a short way of saying "radioactivity". This dual usage, unfortunately, persists today. When Rutherford and Soddy identified the emission of radiation with thetransformation of atoms, the quantity in the first definition was taken as being proportional to the number of atomic transformations taking place in a sample per unit of time: -dN/dt, where Nis the number of unstable atoms in the sample and t is time. The word acfivityis being used above by the present author in this auantitative sense. It is mobable that Curie either knew or strond~vsumected at the time df writino her thesis that the "activitv" of a &&(in the quantitative sense) Gas proportional to the ratebf atomic transformations,because Rutherfordand Soddy's transformation ideas were emerging in the literature at about the same time. 'This point could be checked by direct referenceto the notebooks. which are preserved at the Laboratoire Curie in Paris but which are quite contaminated ~
~
~~~
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nium minerals (as opposed to laboratory compounds), which she found to be as much as three or four times as active as elemental uranium. (See below.) In any event, we may today offer a likely explanation for the excessive activity of Curie's U205sample: that it was not apure laborawry reagent but imperfectly purified uraninite, U02UOs. We now know that radium and other unstable daughters of the uranium disintegration series are in radioacti;e equilibrium with the uranium in any mineral until such time as they are removed by a chemical purification nrocess. Therefore. if the UqOx "comoound" was an imoeriectly purified natural minerai, it would indeed have Eontained some radium and would have exhibited more activitv than would be expected from its uranium content alone. Among the many naturally occurring ores and minerals that Marie Curie then tested for radioactivity, four uranium minerals5 were found to exhibit higher activities than that of pure uranium: autunite, Ca(U02)2(P04)2.10-12HzO;chalcolite, C U ( U O ~ ) ~ ( P O ~ ) + ~ ~more H ~ Ocommonly , known as torhernite; carnotite, K~(UO~)Z(VO&~HZ; and three samples of pitchblende, UOzU03, from different sources. These minerals exceeded the activity of elemental uranium by factors of 1.2,2.3,2.7,2.8,3.0, and 3.6, respectively. These data were her now-historic clue to the existence in the minerals of some unknown natural substance or substances that were more intensely radioactive than uranium or thorium. (It may be noted that the "U205" sample discussed above was auite within the ranee of these minerals: about 1.4 times the activity of uranium.- hat it was toward the low end of that ranae - further indicates that it had heeo partiallv. .purified.) Curie writes (p 20) "A11 the minerals that showed radioactivity contained-uranium or thorium; their activity is therefore not surprising, but the intensity of the phenomenon in certain cases is unexpected. . . . [Previous] conclusions [indicate that] no mineral should he as active as [elemental] thorium or uranium" (D 20). With only thecoyly stated objective"toclarify this point" ( D 201. Curie Drenared a svnthetic s a m ~ l e of chalcolite from
