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HALF-LIFE O F
A LEGEND
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nce prominent scientists become part of the “lore”of a field, their reputations sometimes undergo a strange transformation. Albert Einstein’s equation, E = m?, appears on T-shirts, and Steven Hawking‘s book on cosmic theory, A Brief History of Time, has become a popular coffee table book. T-shirt slogans and glossy book covers celebrate the scientists as heroes but do not really make them or their work more accessible to the general public. When the hero is Marie Curie, the task of presenting her work and her contributions to physics and chemistry is clouded further by the details of Curie’s life, which are remarkable enough to steal the reader‘s attention from the details of her experimental methods and conclusions. Nobel prize winner, discoverer of new elements, historical figure-and of course, woman scientist-these legend-making phrases distance even a professional readership from any realistic picture of Curie as a working chemist. Her efforts to elucidate the nature of radioactivity are usually summarized, even in college chemistry texts, with a phrase reserved for feats that are great but generally incomprehensible: “She won the Nobel prize for her discovery of radium and polonium.” Her experimental methods and conclusions are almost always left out, as though discovery of an element were either self-explanatory or obscure. However, her doctoral thesis, entitled Radioactive Substances, which is only about 100 pages long in paperback, makes interesting reading today, particularly when compared with current knowledge about radioactivity (I). Written in a more personal style than is usual in today’s research paANALYTICAL CHEMISTRY, VOL. 65, NO. 4, FEBRUARY 15,1993
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pers and review articles, her thesis demystifies her discoveries by giving modern readers a feel for the day-today process of investigation and cooperation among peers in the fledgling field of radioactivity studies. The thesis Curie presented to the Faculty of Sciences a t the University of Paris not only reveals the straightforward nature of her experiments and the equipment she and her husband designed for them, but also reflects the level of chemical knowledge among their peers and the conditions of laboratory research a t that time. Originally published in Chemical News, London, in 1903, the English translation was republished in paperback in 1961, 1967, and 1971 in the United States. Part of Curie's research is sketched out in the 1937 biography written by her younger daughter Eve (21, as well as in more recent, less romanticized professional biographies by authors such as Rosalymd Pflaum (3). Ninety years after the publication of this thesis, Curie's career remains as much a curiosity of science lore as it was when she was alive. The first female recipient of the Nobel Prize, Curie was also the fwst woman in Europe to gain a full science professorship at a university, and the first to acquire and direct a large laboratory. She worked with her husband, Pierre, until his death in 1906, and although she was frequently a s sumed to be a mere rider on her husband's coattails rather than a re-
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Figure 1. Curie's piezoelectrometer.
Radioactive substances were powdered and placed on plate 8 of a condenser, generating electricity that was measured by electrometer E A quam electric M a n m 0 was anached at E with tension added at H to compensate the charge on plate A. This measured the current intensity in absolute units. (Adapted fmm Reference 1.)
did he give her full credit for her work and her reasoning, but so did most of their small circle of peers in radioactive studies: Rutherford, Becquerel, Perrin, Langevin, Meyer, and Giesel. The Nobel committee awarded both Curies, along with Becquerel, t h e 1903 prize i n physics a n d awarded Marie alone the chemistry prize in 1911. That second prize came five years after Pierre was killed by a wagon while crossing the street and after she had assumed his post as an assistant professor a t the FaculL.-CQ-: -_""- - P A L - TTniver-:'
Advertisement accompanying the original English publication of Radioactive Substances in Chemical News, London, Aug. 28,1903. 216 A
of Paris. (She was not immediately given the full professorship Pierre had held, because the academy was still uneasy over awarding such a position to a woman, but after she received the second Nobel, she was promoted.) Marie Sklodowska entered t h e University of Paris (Sorbonne) in 1891 a t the age of 24, after having worked for several years in Poland as a governess to fund her older sister's medical education in Paris. She began work in the laboratory of Gabriel Lippman, and after three years had gained a scholarship and an industrial commissioned project to measure the magnetism of steel samples. A professor introduced her to Pierre Curie, who had earned himself a name for his research on magnetism, and in 1895 they married, living in a self-imposed spartan style. Pierre gained a chair in physica a t the Ecole de Physique et de Chimie Industrielles de Paris (EPCI), and although the position added nothing to his laboratory space-which was simply part of an open hallway, with students passing by-the director allowed Marie to transfer her experiments to Pierre's lab from Lippman's overcrowded one. At that time, many universities considered teaching as fundamentally important but treated experimental research as a mere side interest of the professors and therefore not worth a large investment in facilities; hallways and exposed, u n heated sheds were considered sufficient. Years later, Curie blamed the lack of decent shelter for the inevitable contamination of her recrystallized radium chloride fractions and for the humidity that interfered with the electrometer and the precision balance. Work that had taken five years t o perform might only have taken two, she said, if she and Pierre had been able to work in a clean, enclosed, and heated indoor laboratory. After having suffered such setbacks in her work for so long, she led the way in changing the standard academic perception of laboratory research. With the Nobel prizes came popular fame, which not only helped her to persuade the Sorbonne to build a large new laboratory facility dedicated to radiation research, but also attracted multimillion-franc science scholarship funds in her name and Pierre's memory, donated by the likes of Andrew Carnegie and Edmond de Rothschild. The work that earned her that fame began with her doctoral research. Shortly after marrying Pierre,
ANALYTICAL CHEMISTRY,VOL. 65, NO. 4, FEBRUARY 15,1993
Marie read Becquerel’s paper on the invisible emanations from uranium salte, which were capable of exposing photographic plates through a n opaque layer, and which might be X-rays such as those Roentgen had recently reported generating with the Crookes discharge tube. Marie decided to make the study of ‘73ecquerel rays” the topic of her doctoral thesis. She began to measure the intensity of electric current they produced in air, using a quartz piezqelectrometer (depicted in Figure 1 ) that Pierre and his brother had invented while engaged in crystallographic research. She measured the electrical current produced in air by known compounds of uranium and thorium in a variety of physical forms and chemical formulas. When the current produced by radiation was compensated with added tension on a quartz electric balance attached to the electrometer, the electrometer needle would remain steady and would allow measurement of the intensity of current produced by the radioactive specimens in absolute units. This was an advantage, because at the limiting value, uranium and thorium produced a current only on the order of A for a plate distance of 3 cm. Within a few weeks, Curie found that the intensity was proportional to the amount of elemental uranium or thorium present in each sample, regardless of physical or chemical form. She concluded that radioactivity, as she named it, was an atomic property and, unlike fluorescence and phosphorescence, not just the result of external influences such as light exposure or chemical combination. At the time, that differentiation had not been settled conclusively. Curie demonstrated that phosphorus, which also induced current in the air between the plates of the electrometer, was active only as the phosphorescent white form, not as the red oxide, so that phosphorescence and radioactivity had to be separate phenomena. She then began testing as many other materials as she could to see whether other known elements were also radioactive. She found none, but noted that several natural uranium ores were more radioactive per gram than purified metallic uranium. Pitchblende, rich in uranium oxides, was four times as active, and chalcolite, a copper-uranium biphosphate, had twice the radioactivity of uranium metal. Curie synthesized chalcolite by mixing uranium nitrate solution with copper phosphate in
phosphoric acid. The synthetic crystals had the expected reduction in intensity of two and a half times less than that of pure uranium. Curie reasoned that because there must be less uranium in the ores than in metallic uranium, the excess radioactivity might be caused by some other minor element with higher radioactivity than that of uranium, one that had so far escaped discovery only because it was present in the ore in much smaller amounts than uranium, and because tools to measure the radiation intensity quantitatively had not been available. She and her husband, who abandoned his crystallographic studies to join her in her research, bought a ton of pitchblende ore from Joachimsthal in Bohemia that had already been extracted with soda and sulfuric acid in a refmery to remove the valuable u r a n i u m a n d was inexpensive enough for them to afford. EPCI allotted the Curies an open shed in which to store and process the ore in the hope of extracting a new radioactive element. They thought the mystery element might be present as 1-2% of the residue, which was four and a half times as radioactive as uranium, but from a ton of pitchblende slag, all that they eventually recovered was “a few deeigrams” of pure radium. According to some modern estimates, they achieved a 25% yield of the total radium present by theoretical calculations-a feat in itself, considering their laboratory conditions.
They broke down the used ore 20kg at a time and isolated first polonium, then radium, by fractionation and recrystallization. Polonium waa hard to separate from bismuth and lost its original level of radioactivity within months. However, radium chloride retained its activity and could be separated from barium chloride in dilute ethanol or hydrochloric acid, so they chose it for further purification and study. The ore contained the sulfates of most metals, including lead, aluminum, iron, bismuth, and barium. It was boiled in concentrated soda, and most of the sulfates dissolved, leaving behind some radioactive residue. The.residue was mostly dissolved by HC1-polonium coprecipitated with bismuth from the dissolved fraction. Barium and radium remained in the residue and were converted to carbonates by boiling with soda, then treating with dilute HCl, filtering, and precipitating with sulfuric acid. This process yielded 10-20 kg of crude active sulfates t h a t were 30-60 times as active as uranium per ton of used ore. The sulfates were converted to chlorides by treating them with hydrogen disulfide (bismuth and polonium sulfides precipitated), fdtering, oxidizing with chlorine, and precipitating with ammonia (actinium precipitated out). This filtrate was precipitated with sodium bicarbonate, and the precipitate was washed with concentrated HC1 to get rid of calcium chloride. From the ton of slag, 8 kg of crude
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barium chloride remained, 60 times as radioactive as metallic uranium. The crude chlorides were dissolved in distilled water, boiled, and allowed to recrystallize. The supernatant was evaporated to dryness and the resulting chlorides were about five times less active than the fnst fraction. Both fractions were repeatedly redissolved and recrystallized. Fractions with the same radioactivity were combined, and inactive fractions were discarded. The least solu, ble fraction was the most active, and by adding HC1 to the solutions, Marie Curie was able to enhance the separation of radium from barium. Andr6 Dibierne, who assisted the Curies, came up with a scheme to adapt the laboratory method for industrial production, and a government factory in Paris was equipped to process the ore more quickly. This upscaling, coupled with the discovery that radium induced X-rays and could destroy skin cancers on contact, started the enormously prosperous radium industry in Europe and eventually in the United States. The Curies identified the fractions they wanted by the intensity of radioactivity in them. “Our method of procedure could only be based on radioactivity, as we know of no other property of the hypothetical substance,” wrote Curie in her thesis. Highly active fractions were subjected to “photographic spark” emission spectroscopy in a neighbor‘s laboratory to confirm the hypothesis of the new radioactive element and to see how pure the active fractions were. The spectra of these samples revealed a faint new line a t 381.47 MI, in the ultraviolet, indicating the presence of a new element against the backdrop of the barium spectrum. As Curie purified the radium further, the line intensified, and two other major lines appeared at 468.30 and 434.06 nm. Achieving a high level of purity became a new technical challenge when she tried to determine the atomic weight of radium, using the silver chloride method and assuming radium to be bivalent, as barium was. The atomic weight of radium could not be differentiated from that of barium, even in samples with radioactivity up to 600 times that of uranium. Only when she had purified the radium to 3500 times the radioactivity of uranium did she begin to see a difference-the atomic weight rose from 137 or 138 to 140. “By using more and more active producta, and obtaining spectra of radium of increasing intensity, I found that the 218 A
figures obtained rose in proportion.” She found a maximum atomic weight a t 225, close to today’s figure of 226.03. The radioactivity of that sample was on the order of 10‘ times the activity of uranium. After receiving her second Nobel prize, she would isolate pure radium metal by electrolysis with a mercury cathode, and would revise the atomic weight upward. Less well remembered today are the experiments the Curies performed to elucidate the nature of radioactivity itself. Alpha, beta, and gamma rays were named a t about this time by Rutherford, to whom the Curies had given some radium samples, but the rays had not been characterized completely. The Curies used their electrometer to measure the behavior of the rays in magnetic fields and in various open and closed systems with a series of aluminum and lead screens that were designed to test the absorption or penetration of each type of radiation. Radium gave off alpha and beta particles and gamma rays that did not seem to be particulate, whereas polonium gave off only alpha particles (Figure 2). The Curies were able to measure the proportion of each type of radiation in radium, but Marie’s thesis notes that the electrometer gave different results from those obtained by radiography or fluorescence induction because of the inherent differences in the instruments’ absomtion of the three tvues of radiation. The Curies also found that beta rays were magnetically deflected in the same direction as cathode rays, and that radium samples sealed in _ I
Figure 2. The behavior of radioact rays in a magnetic field. Alpha rays are SligMly deflected in the direction opposite to the more strongly deflected beta rays, which appear similar to negatively charged cathode rays. Gamma rays are unaffected. (Adapted fmm Reference 1.)
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glass built up electrical charges. Electrons had not yet been postulated a s subatomic particles by Rutherford, but the evidence was there in the Curies’ work: “Hitherto, the existence of electric charges nncombined with matter has been unknown.. .we are therefore led to make use of the theory [developed by Crookes and expanded by J. J. Thomson1 whicb is in vogue for the study of cathode rays. . . .We might similarly conceive that radium sends into space negatively electrified particles.” They were able to state, from their research and from that of their peers, that the beta ray was a highly deflected particle of high velocity and with negative charge equal to that of a hydrogen atom. But the energy source of the rays themselves was still unknown, and its character was giving rise to some peculiar observations. Not all of the Curies’ interpretations were correct, but some, even when incomplete, seem today to have been prophetic. Pierre Curie found that the radium salts were always warmer than their surroundings, and that 1g of radium gave off heat a t about 100 calories per hour. Marie wrote: “So great an evolution of heat can be explained by no ordinary chemical reaction. . .as the condition of the radium remains unaffected for years. The evolution of heat might be attributed to a slow transformation of the radium atom . . .we should be led to conclude that the quantities of energy generated during the formation and transformation of the atoms are considerable, and that they exceed all that is so far known.” All of the Curies’ radioactive solutions from the ore fractionation process glowed blue. Radium induced fluorescence, color changes, and even radioactivity in such ordinary materials as paper and glass. This induced radioactivity, which they discovered faded exponentially with time and avoidance of re-exposure, allowed the Curies to see exponential decay as a regular property of radioactivity they might otherwise have missed, given radium’s -1600-year half-life. The Curies weren’t alwavs right about the meaning of their LbseGations. They tried to apply their exponential decay idea to the temporary loss of activity they observed in radium salts dissolved in water and then redried or to salts that had been heated for long periods of time. Recovery of radioactive intensity followed an exponential formula similar t o the one for loss of induced
radiation in glass, and Marie Curie theorized that dissolving the radium salts i n water or heating them caused them to radiate more intensely and lose energy to their surroundings. The radium’s capacity to produce rays was supposedly “exhausted” by this, but would build up again once the external drain on emanation activity was removed. P e r h a p s s u c h a complicated scheme should have hinted to Curie that she was on the wrong track; she and her husband had already cut themselves off from the obvious solution hy declaring, “Mr. Rutherford suggests that radioactive bodies generate an emanation or gaseous material which carries the radioactivity.. . a supposition which is not so far justified.” Rutherford was right. Radon, the only naturally radioactive element found as a gas, is produced on solvation or heating of radium, hut a t the time of Curie’s thesis, its discovery was still a few years away. Although t h e Curies guessed wrong about the existence of radon, they paved the way for many of their colleagues’ discoveries by exercising a strict code of scientific ethics that had not been generally established. When a n American engineering firm offered them exclusive commercial patent rights to radium processing, the Curies turned down the offer in order to remain disinterested researchers. They also freely circulated samples of the purified radium crystals among their peers for independent study. The newspapers turned the Curies into front page news when their shared Nobel prize was announced in 1903 and never really abandoned them afterwards, for good or ill. The Curies tried to curb the intrusion of public fame on their daily work by refusing to be interviewed or make public appearances. Pierre had a particular aversion to any publicity or commercial “interest,” which he thought biased pure research. After his death, Marie avoided publicity only with some difficulty. His death was news, and she had trouble finding the privacy to mourn. She was given his professorship to continue, and that was news. She insisted on a daily routine in the laboratory, but was increasingly forced-hy depression and by the fatigue that, unbeknown to her, was caused by contact with such high radiation levels-to retreat to t h e country or to friends’ houses to recuperate. In 1910, just months before she received her second Nobel prize, she
was accused by the wife of a colleague, Pierre Langevin, of having had an affair with him. The newspapers raged with the story, and the university administrators considered dismissing Curie from the faculty. The accusation was later dropped, but the scandal had turned Curie into a recluse for several months and had threatened her scientific career. When i t was over, and she was awarded the Nobel in 1911, she began to concentrate on the construction of a new laboratory facility in Pierre Curie’s memory. World War I intervened; she and her older daughter Irene introduced X-ray equipment to the field hospitals behind the front lines and instructed military surgeons in the new technique for locating shrapnel in a wound before making a n incision. Curie also used her dwindling stock of radium to produce radon gas for encapsulation and implantation in cancer patients. She didn’t realize that siphoning off the gas for them contributed to her own cancer, from which she died in 1934. After the war, Curie was finally lured into public appearances by a n American female newswriter who told her that a public lecture tour of the United States would have Ameri-
can women running to donate funds to buy the new laboratory a gram of radium-which cost $100,000 at the time. Curie, in failing health, made the trip with her daughters, and was for years afterward known in American households a s “the radium woman.” Perhaps not surprisingly, her success on the tour was attributable more to her reputation as the discoverer of a cure for cancer-an incidental benefit of her research-than to her contributions to physics and chemistry. Nonetheless, she had finally learned how to use fame as a tool to accomplish her goals, and she raised the reputation of experimental research in the world of academic science. Fame has obscured some of Curie’s scientific accomplishments and exaggerated others over time, but the details of her work and her life are inextricably bound up in it. Deborah Noble
References (1) Curie, M. S. Radioactive Substances; Philosophical Library: New York, 1961. (2) Curie, E. Madame Curie; Doubleday: New York, 1937. (3) Pflaum, R. Grand Obsession: Madame Curie and Her World; Doubleday: New York. 1989.
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