SCIENCE/TECHNOLOGY
Scientists Honor Centennial Of The Discovery Of Radioactivity • Ramifications include radioéléments, the atomic bomb, nuclear medicine, radioactive dating, and solar neutrino studies Stu Boraian, C&EN Washington
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(ogether with the discovery of X-rays, the discovery of radioactivity opened up an entirely new scientific era that has dominated the past century," says nuclear chemist Gerhart Friedlander, retired from Brookhaven National Laboratory, Upton, N.Y. At last month's American Chemical Society national meeting in New Orleans, Friedlander and chemistry professor Paul J. Karol of Carnegie Mellon University, Pittsburgh, organized a symposium in the Division of Nuclear Chemistry & Technology that reviewed the century of progress since French physicist Antoine-Henri Becquerel discovered naturally occurring radioactivity in 1896. The symposium—called "Centennial of the Discovery of Radioactivity"— honored Becquerel's discovery. But it also focused on advances that have been achieved since then, such as the discoveries of artificial radioactivity and nuclear fission, and on current applications of radioactivity, including radioactive dating, nuclear medicine, and solar neutrino studies. Over the course of the century, radioactivity has proven to be a prolific source of Nobel Prizes for the researchers who have pursued its secrets. Becquerel discovered radioactivity in February 1896 when he noticed that a uranium salt spontaneously emitted a penetrating type of radiation that exposed a photographic plate. The significance of the discovery of "uranic radiation/' as he initially called it, was not apparent at the time, even to other
Participants at the symposium on "Centennial of the Discovery of Radioactivity" included: (front row, from left) Alfred P. Wolf Gunter Herrmann, R. J. Silva (of the Institute for Radiochemistry, Rossendorf Research Center, Dresden, Germany), Glenn T. Seaborg, Jean J. Fuger, Robert Guillaumont, Jean-Pierre Adloff, Leonard W. Fine, and Joseph Cerny (of the department of chemistry and Lawrence Berkeley National Laboratory at the University of California, Berkeley); and (back row, from left) Hans R. von Gunten, Petr Benes, Heino Nitsche, Steven W. Yates, Gerhart FHedlander, Joseph R. Peterson (of the department of chemistry at the University of Tennessee, Knoxville), and Paul J. Karol.
scientists, and the findings drew little notice. But the importance of radioactivity—a term introduced not by Becquerel but by Polish-born physicist and chemist Marie Curie—soon gained a higher profile when it was found that uranium-containing minerals were more active than metallic uranium. Marie and her husband, French physicist Pierre Curie, discovered the increased radioactivity in uranium oxide ores such as pitchblende. Their experiments led them to the conclusion that the radioactivity was coming for the most part not from the uranium itself but from unknown trace radioéléments in the samples. The first new radioélément discovered by the Curies was polonium, in July 1898. According to chemistry professor Jean-Pierre Adloff of the Center for Nuclear Research, Strasbourg, France, the Curies' claim for the existence of the
new element was unique in chemical history at that time in that they were not able to isolate and characterize the element, as would normally be required for such a proposal to be considered valid. Instead, the claim was based solely on the strong radioactivity of a polonium fraction isolated from pitchblende. Adloff believes the sample studied by the Curies likely contained only about 6 ng of polonium—far below the limits of sensitivity of the spectroscopic techniques available in the 1890s. In December 1898, the Curies also reported the existence of radium—in this case working with a sample that probably contained only about 30 pg of the element. Marie Curie's work to purify radium from pitchblende to determine its atomic mass and prove it was an element was later described in the book "Madame Curie" by the Curies' daughter, Eve Curie, and dramatized in the APRIL 29,1996 C&EN
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SCIENCE/TECHNOLOGY 1943 movie "Madame Curie," starring Greer Garson as Marie Curie, Walter Pidgeon as Pierre Curie, and Reginald Owen as Becquerel. For their work on radioactivity, Becquerel and the Curies shared the 1903 Nobel Prize in Physics. Becquerel received the honor for his "discovery of spontaneous radioactivity" and the Curies for "joint research on the radiation phenomenon discovered by H. Becquerel." Marie Curie later also received the 1911 Nobel Prize in Chemistry for the discovery of polonium and radium and chemical studies on radium. (In a tragic accident, Pierre had been crushed by a carriage and killed in 1906.) Marie died in 1934 of leukemia caused by her overexposure to radioactivity. Written versions of the presentations made by Adloff and several other speakers at the symposium appear in a recently published special issue of RadioBecquerel discovered "uranic radiation" in çhemica Acta. The issue is published as a book entitled "One Hundred Years After the Discovery of story is: "If you stumble on something Radioactivity" (R. Oldenbourg Verlag, that's curious, don't let it go. It may Munich: 1996). end up making you famous, even Symposium co-organizer Karol point- though it's not what you were after in ed out in his presentation that a French the first place. Niépce had published inventor apparently discovered radio- this work, but it sort of got left in obactivity more than 30 years before scurity because he didn't flaunt what Becquerel did—suggesting that the cen- he had done, whereas Becquerel really tennial symposium was, in a sense, tak- got excited about i t . . . and has 100% of ing place more than three decades too the credit for it." late. Retired physicist Samuel Devons of Karol says that Claude Félix Abel Columbia University's Nevis LaboratoNiépce de St.-Victor, a French inven- ry, Irvington, N.Y., now 82, was a retor of photographic processes, also search student in the 1930s in physicist discovered that uranium salts ex- Ernest Rutherford's group at the Cavposed photographic film in the dark. endish Laboratory of the University of "It's the identical experiment that Cambridge, England. In the late 1890s, Becquerel did, but he did it in the Rutherford had found that radioactivi1850s instead of in the 1890s," says ty had two components—α particles Karol. "Niépce published six papers on and β rays. In 1902, Rutherford and chemist Fred it in the French Academy of Sciences journal. When Becquerel was reminded erick Soddy, working together at of Niépce's prior work, he completely McGill University, Montreal, discov disregarded it, claiming it had nothing ered the law of spontaneous decay— to do with radioactivity, even though that an atom could spontaneously de the evidence is convincing that it prob- cay into another atom—"which was al ably was radioactivity. An encyclope- most pure heresy in those days," says dia of chemistry in 1884 refers to Devons. "And they propounded the Niépce's work and his discovery of this law of conservation of radioactivity, that the atoms changed into one anoth unusual property of uranium." According to Karol, the moral of this er at invariant rates, which was totally 56
APRIL 29,1996 C&EN
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contrary to the whole docof chemical theory that the atom was immutable. . . . For five years, roughly from 1900 to 1905, Rutherford and Soddy, in my estimation, set in motion a complete transformation of physics and chemistry and the relation between the two of them. . . . That was breathtaking in its
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In 1911, Rutherford proposed the nuclear model of the atom, the discovery that he is perhaps best known for. On the basis of a nowfamous experiment in which α particles were deflected from thin foils, he discovered that most of the mass of atoms was contained in a small, positively charged nucleus. In 1919, he disintegrated nitrogen atoms with α partides, showing that it is possible to break up a nu cleus. Hydrogen was liber ated, making this the first observed case of deliberate artificial transmutation of one element into an other. And in Rutherford's laboratory in 1932, atoms were also disintegrated with artificially accelerated particles, instead of with radioactivity, as had been the case up to then. Scientists today think of chemists as dealing with molecules and physicists as dealing with atoms, says Devons. However, "in Rutherford's day, it was more or less the reverse. The atom be longed to chemistry, and the molecule belonged to physics. The molecule was the smallest bit of matter that existed by itself and bounced around, and that's what physicists dealt with. When you started poking inside, deeper down, you used chemistry. . . . So the atom was a chemist's atom—it was the atom of Dalton and Mendeleev. They owned the atom. It was not yet the physicists' business." For his work on atomic structure, Rutherford "could, without any diffi culty, have been given several Nobel Prizes," says Devons. But he won only one—the 1908 Nobel Prize in Chemis try, in which he was cited for "investi gations into the disintegration of the elContinued on page 57
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SCIENCE/TECHNOLOGY Continued from page 56 ements and the chemistry of radioactive substances." In 1921, Soddy also won a Nobel Prize in Chemistry, for establishing the notion of isotopes. Work by Rutherford has led to current ideas of how the atomic nucleus is structured. "The quantum theory was established by observing the line spectra of radiation emitted by atoms," says chemistry professor Steven W. Yates of the University of Kentucky, Lexington, "but it remained unclear as to whether the nucleus—which was not really proposed by Rutherford until 1911, years later than the quantum theory—was a quantized system with discrete energy states. It is not surprising that Ernest Rutherford, who dominated the study of the nucleus for 40 years, played a large role in unraveling this puzzle." Nuclei are now known to exhibit elementary modes of excitation similar to the rotations and vibrations that molecules undergo. Another major contribution to the field of radiochemistry was the discovery of deuterium by physical chemist
Harold C. Urey and coworkers at Columbia University. "They actually made the confirming experiments on Thanksgiving Day of 1931," says chemistry professor Leonard W. Fine of Columbia, who is considering writing a biography of Urey. "They published their first note in February 1932 and their first full paper in April 1932, and Urey won the Nobel Prize in 1934, so it was really a fast and furious episode. "Deuterium oxide became very important as a neutron modifier for nu-
clear weapons development/' says Fine. "There was a project here called the heavy water project that supplied all the deuterium oxide for the biochemists and, at the same time, was very important as a stepping-stone to nuclear weapons." In biochemistry, deuterium was almost immediately used as an in vivo probe in studies of intermediary metabolism. "Before that," says Fine, "the only way you could learn anything about metabolic processes was
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SCIENCE/TECHNOLOGY to hang an odd cluster of atoms off the side of a molecule, put it in one end, and see what molecular pieces came out the other end. Once you could tag a molecule [with deuterium], then you could really get some serious insight into what was going on, because most of the molecules we're talking about had hydrogen all over them, and if you start putting deuterium in place of hydrogen you can walk through the body without the body knowing that anything unusual is tak-
ing place. . . . Very quickly, there were huge changes in our understanding of metabolism/' In 1934, French physicist Frédéric Joliot and his wife, French radiochemist Irène Curie (daughter of Marie and Pierre Curie), discovered that new forms of matter could be created by bombarding materials with α particles and that some products of the resulting nuclear reactions were artificially (as opposed to naturally) radioactive. Chemistry professor Robert Guillau-
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mont of the radiochemistry group at the Institute of Nuclear Physics, Orsay, France, says this discovery was a major breakthrough in nuclear science be cause it identified a new mode of ra dioactivity and was the first time a new radioactive nuclide was synthesized and identified chemically on the scale of a few atoms. This was also about the time, he says, when the field of radio activity divided into its modern subdis ciplines—nuclear physics on the one hand and the chemically oriented areas of nuclear chemistry and radiochemis try on the other. Joliot and Curie discovered artificial radioactivity when they irradiated alu minum and boron with α particles from a strong polpnium-210 source, producing two new radioactive spe cies, phosphorus-30 and nitrogen-13, both of which decay by positron emis sion. After this, the number of artifi cially produced radioéléments increased very rapidly, with over 200 known by 1937. Joliot and Curie were awarded the 1935 Nobel Prize in Chemistry for the synthesis of "new radioactive elements." Efforts to produce new elements also led to the momentous discovery of nuclear fission in 1938 by German physical chemists Otto Hahn and Fritz Strassmann. Gunter Herrmann, a retired professor from the Institute for Nuclear Chemistry of the University of Mainz, Germany, says the whole procedure—in which the light element barium was formed when uranium was bombarded with neutrons—is described in eight lines of Hahn and Strassmann's laboratory notebook. Herrmann points out that the discovery occurred at a tragic moment, on the eve of World War Π, so that the phenomenon of fission was used militarily first and peacefully only later. Hahn received the 1944 Nobel Prize in Chemistry "for his discovery of the fis sion of the heavy nuclei." Hahn's Nobel Prize is a controversial one. His longtime collaborator, physi cist Lise Meitner, did not share in it al though she is today regarded as one of the discoverers of fission. Meitner, a Jew, left Germany in 1938 to escape Nazi persecution. In addition, Strassmann didn't share the Nobel Prize with Hahn, an issue that was brought up in the discussion at the symposium. Strassmann, who was 36 at the time of the discovery, didn't share the prize because "he was totally
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unknown/' says Herrmann. "However, Hahn, who was 59, had already won a great international reputation and had several times been nominated for a Nobel Prize for other work, mostly performed in collaboration with Meitner. Thus, he was well known to the Nobel Committee. The decision to award him [the prize] was not announced in public because Hitler had forbidden the acceptance of Nobel Prizes by Germans. But Hahn was informed through private channels." At the symposium, chemistry professor Glenn T. Seaborg of the University of California, Berkeley, discussed his career as a radioisotope hunter during the 1930s at UC Berkeley, which was the preeminent center for such research at the time. Seaborg, working for the most part with nuclear physicist John J. Livingood, was involved in the discovery of isotopes such as iron-59, cobalt-60, iodine-131, technetium99m, and cesium-137. The studies generally involved bombarding targets in a cyclotron and separating the resulting chemical products to identify new species. The isotopes discovered by Seaborg now have many applications, particularly in biomedicine. For example, ioPierre and Marie Curie discovered polonium and radium. dine-131 has been used to diagnose and treat thyroid disease, among many other applications. And techne- from hyperthyroidism—a condition tium-99m is used to diagnose thyroid, similar to one her sister had died bone, liver, spleen, lung, cardiovascular, from—she was diagnosed and treated with the isotope and went on to live brain, and kidney disorders. Seaborg points out that his mother until 1968. Seaborg also played a major role in was one of the first to benefit from use of iodine-131. Gravely ill in 1953 the discovery of transuranium eleAPRIL 29,1996 C&EN
6l
SCIENCE/TECHNOLOGY ments. Most notably, in 1941 chemistry instructor Seaborg, graduate student Arthur C. Wahl, and research fellow and chemistry instructor Joseph W. Kennedy discovered plutonium by bombarding uranium with deuterons in a cyclotron. The discovery was the subject of Wahl's Ph.D. thesis—"probably one of the most significant theses in the history of the world/7 says Sea-
borg, because plutonium-239 proved to be fissionable with slow neutrons and hence was suitable for the explosive material in an atomic bomb. The discovery was one of the findings honored by Seaborg's 1951 Nobel Prize in Chemistry. Seaborg's work helped lead to the plutonium part of the wartime Manhattan Project to develop atomic bombs,
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APRIL 29/1996 C&EN
and, ultimately, to the first detonation of an atomic weapon (near Alamogordo, N.M., on July 16,1945) and the second bomb dropped on a city (Nagasaki, Japan, on Aug. 9). The first atomic bomb used on people—at Hiroshima, Japan, on Aug. 6—was a uranium bomb. Among the many peaceful applications of radioactivity today is radioactive dating. "One of the most important dating nuclides is carbon-14," says chemistry professor Hans R. von Gunten, retired from Paul Scherrer Institute, Villigen, and the University of Bern, Switzerland. "Carbon-14 is produced at a practically constant rate by interactions of cosmic rays with compounds of the atmosphere (for example, nitrogen). It then participates in the biological carbon dioxide cycle. When an organism dies, carbon-14 starts to decay. The current activity in a dead sample can be compared to the known constant activity in living organisms [to date the sample]. The method can be calibrated with tree samples whose age was determined by tree-ring counting." Such techniques have made it possible to date accurately many archaeological, geological, and cosmological samples. Biomedical applications of radioactivity can be "traced" at least as far back as 1913, when physicist George de Hevesy first enunciated the principle of the radioactive tracer. "The first important use of a radiotracer in biomedical work occurred in 1935, when de Hevesy applied it to studying animal metabolism," says Alfred P. Wolf, senior chemist at Brookhaven National Laboratory and a pioneer in the development of positron emission tomography
out a nuclear medicine practice. . . . Today, technetium-99m—first described by [Italian physicist Emilio] Segrè and Seaborg—the iodine radionuclides 123 and 131, and thallium-201 account for the major fraction of radionuclides used in clinical nuclear medicine." PET imaging is a key tool in the nuclear medicine armamentarium. "It only suffers from two problems," says Wolf. "You have to have a cyclotron close by and, at this stage, it's still expensive. In 1976, when PET scanning
j Rutherford (left, in portrait from 1 about 1935) and Soddy (in above j photo from about 1920) discovered j laws of spontaneous decay and j conservation of radioactivity.
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really got started, there were really only three places that produced the compounds and had PET machines. There are now about 300 installations worldwide that are producing positron emitters and using them in clinical settings. So in a span of roughly 20 years, you could say it's exploded." Chemistry professor Heino Nitsche of the Institute for Radiochemistry at Rossendorf Research Center, Dresden, Germany, reported on the need for environmental monitoring of actinides such as
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(PET). "Nuclear medicine as such had its birth in 1938 in a paper on thyroid activity using iodine-128/' Nuclear medicine probes both function and biochemistry. Computeraided tomography and magnetic resonance imaging "are morphological procedures," says Wolf, "whereas with nuclear medicine you can look at flow, metabolism, and biochemical pathways. It's a mature medical science. No modern hospital can afford to be with-
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uranium and plutonium, which have been introduced into the environment by nuclear testing, reentry from orbit of nuclear-powered satellites, nuclear reactor accidents, uranium mining, and nuclear weapons production, among other sources. Nitsche says absorption spectroscopy has become the method of choice for obtaining information on actinides in solution. Laser-induced photothermal and fluorescence spectroscopies have made it possible to determine actinides at lower levels than ever before, in the parts-per-billion range. And X-ray absorption spectroscopy is becoming increasingly important for the study of actinide speciation, particularly for reactions at solid-solution interfaces. Improved techniques for detecting radioactive pollutants, particularly the actinides, are being developed at sites such as the European Commission's European Institute for Transuranium Elements, Karlsruhe, Germany, says institute Deputy Director Jean J. Fuger. One location where actinide and other radioactive contamination has been a particular concern is in Jâchymov (called St. Joachimsthal in German), in the Czech Republic, says professor Petr Benes of the department of nuclear chemistry of Czech Technical University, Prague. The pitchblende used by Pierre and Marie Curie to discover radium was from a mine in Jâchymov, and radium was produced industrially as early as 1906 in a Jâchymov factory. Mining and treatment of silver, uranium, and other ores have
FrédéHc Joliot and Irène Cune discovered artificial radioactivity.
caused radioactive contamination of the town, but remediation efforts are now making headway against many years of neglect. Another current application of radioactivity is in solar neutrino studies, which are being conducted to confirm theoretical models of energy production in stars. At the symposium, Friedlander delivered a presentation prepared by senior chemist Richard L. Hahn of the chemistry department at Brookhaven National Laboratory, who was not able to attend the meeting.
Neutrinos are produced by the sun and other stars in hydrogen fusion reactions, in which two protons combine, producing a deuteron, a positron, and a neutrino. Models of this and subsequent solar reactions can be used to predict how much neutrino flux from the sun will reach Earth, and solar neutrino measurements can be made at Earth's surface to test the validity of these models. The classic solar neutrino experiment is that of astronomy professor Raymond Davis Jr. of the University
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De Hevesy (left) and Wolf (shown in 1959) pioneered nuclear medicine and radiochemistry techniques.
of Pennsylvania, which has been going on for more than a quarter-century in the Homestake gold mine in South Dakota. "The reaction that one looks at is the capture of neutrinos in chlorine-37 to form argon-37, which is radioactive," says Friedlander. "Because neutrinos have very low interaction cross-sections, the trick is to fish out a few atoms of argon in 600 tons of perchloroethylene, which is the target material.,/ In the experiment, says Friedlander, "you're looking for a few atoms in a mass of maybe 1031 atoms of material. If s a real triumph for radiochemistry that it has been possible to not only detect but also measure these minute amounts of radioactivity and use these data as a probe for the nuclear reactions that go on in the center of the sun." Neutrinos are the only products of solar fusion reactions that are able to reach Earth. In the past few years, three other solar neutrino experiments have been started up—two in which the neutrino-induced conversion of gallium-71 to germanium-71 is detected, and one in which a real-time instrumental detector is used to measure neutrinos. All of these experiments have to be done deep underground to shield them from cosmic rays, which give rise to background processes that over-
whelm the very small solar neutrino levels. The instrumental detector measures electrons produced by neutrino interactions with a large volume of purified water. "Again, if s radiochemists and the exquisite sensitivity of radiochemistry for measuring the concentrations of very small impurities—by activation analysis and other methods— that make these experiments possible," says Friedlander. The Homestake experiment has consistently shown a deficit in neutrinos compared with the astrophysicists' model calculations of nuclear reactions in the sun. "These results have presented a significant puzzle," says Friedlander. Too few solar neutrinos, compared with theory, have also been observed more recently in the three other experiments. Most researchers believe the deficit may be caused by "neutrino oscillations"—the conversion of "electron neutrinos," the type produced in fusion reactions, into other types of neutrinos. Neutrino oscillations of this type are possible only if some neutrinos have a finite mass. It has been assumed until recent years that neutrinos have no rest mass, but there is nothing in fundamental theories that excludes the possibility of a small rest mass for some neutrinos. "It would be an extremely important result if one could verify that neutrino oscillations exist and therefore that some of the neutrinos indeed must have mass," says Friedlander. "This would really be new physics, new insights into elementary particles that are not obtainable in any other way." According to Friedlander: "The discovery of radioactivity was the opening salvo that led to the theory of relativity and quantum mechanics, and, in a very real sense, it was the beginning of the whole field of nuclear physics and particle physics. It has had a tremendous impact on basic science in the 20th century, but if s also had important practical consequences. The use of radioactivity is very pervasive throughout chemistry, biology, agriculture, and industry. "The use of radioactive tracers is just an everyday matter now. And the field of nuclear power as well as the use of nuclear weapons can trace their origins to that discovery 100 years ago. None of this would have been possible without it." •
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