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THE TRANSURANIUM ELEMENTS AND NUCLEAR CHEMISTRY I. PERLMAN' University of California, Berkeley, California
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IS BUT a very few years since uranium was commonly known as the last element in the periodic system. It may be recalled that all elements beyond bismuth (element 83) are measurably unstable as evidenced by their radioactivity. The existence in nature of these elements comes about from the accident that certain isotopes of uranium and thorium have lifetimes comparable with that of the age of the earth and these longlived isotopes serve as sources for the constant replenishment of the many radioactive species of elements between uranium and mercury. It is also an accident of nuclear properties that no isotope of elements above uranium has a sufficiently long lifetime to have persisted through geological time, although there is no doubt but that transuranium elements existed at some early time. Thus we see that the periodic table of the naturally occurring elements ends with uranium because we are living in a universe so long after its formation. In the early 1930's several important events occurred which made it possible to consider the artificialpreparation of transuranium elements. One of these was the discovery of artificial radioactivity by Irene Curie and F. Joliot. The implication of this discovery was that not only could an element be transmuted but the transmutation product could be chemically identified, making use of its radioactivity as an analytical tool. In principle, then, uranium could be transformed into higher elements and there was a good chance that the products could be identified through their radioactivity. Simultaneously with the discovery of artificial radioactivity more powerful means by which such transmutations could be brought about were discovered and developed. In 1932, J. Chadwick discovered the neutron and soon afterward E. Fermi showed that all elements could be made radioactive by neutron irradiation and that a neutron capture reaction often produced a nucleus which decayed to a radioactive isotope of the next higher element. This, then, is one path by which element 93 might be reached, for if uranium, element 92, were irradiated with neutrons, it is reasonable to expect that one of the products would decay to an isotope of element 93. The far-reaching consequences of the irradiation of uranium with neutrons is well known. The initial difficulties in the interpretation of the results were finally resolved in the discovery of fission as well as the discovery of the f i s t of the transuranium elements, neptunium. Even before neutron capture reactions were being explored a more versatile means of producing nuclear
transformations was in the offing. This was the invention of the cyclotron by E. 0. Lawrence, and its development to accelerate charged particles to energies that could penetrate the nucleus of the heaviest elements. Since the capture of a positively charged particle, such as the deuteron, immediately results in a higher element, it became feasible to bombard uranium and to produce transuranium elements. It should also be mentioned that until the nuclear chain reaction was added to the tools for promoting nuclear transformations, the cyclotron was the most powerful source of neutrons. Besides leading to the discovery of uranium fission, the work on the transuranium elements presents many other facets. There are the discoveries of these new elements themselves, which alone produce a fascinating story. The nuclear reactions by which plutonium is produced in quantity are of utmost practiral and scientific importance and the macroproduction of neptunium is likewise of interest to the scientist. The elucidation of the nuclear properties of the heavy isotopes now available is greatly increasing our knowledge of nuclear structure and nuclear stability since the isotopes of these elements make available nuclear types not obtainable in nature. With respect to pure chemical problems the transuranium elements provide rich ground for exploration. The chemical properties of each of the new elements are, of course, of great interest in themselves, but, in addition, these elements are proving to be part of a transition series analogous t o that of the rare earths. Since all of these elements were first produced in quantities that can only be investigated a t so-called "tracer concentrations" by the methods of radiochemistry, many of the chemical properties were first deduced with invisible quantities of material. This called for a considerable further development of these methods. In particular, the necessity for developing a method for separating plutonium from the products of the nuclear chain reaction constituted a compelling stimulus for extending our knowledge of the behavior of trace amounts of substances. Similarly, the methods of quantitative nltramicrochemistry, poineered by P. L. Kirk of the University of California, were considerably extended through the work with the transuranium elements. This followed from the necessity of determining many of the properties of these elements with isolated compounds or at conventional concentrations in solution when only microgram quantities were available. All in all, the work done with the transuranium elements, and associated problems such as the work with
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cbulomb field of the nucleus does not oppose the entry of the uncharged neutron as it does charged paticles, the reactions are readily promoted by low energy neutrons. In general, the radioactive nuclei so formed are unstable with respect to negative beta-particle emission, resulting in the element higher by one atomic number. When uranium was irradiated with slow neutrons it was reasonable to suppose the beta-decay process would result in an isotope of element 93. However, instead of a single radioactivity or two, a whole series of activities was observed. The investigators were led to assign these activities to a series of "transuranium elements." The experiments of 0. Hahn, L. Meituer, and F. Strassmann appeared to confirm this viewpoint since the chemical properties of the new isotopes were not those of uranium. For several years these supposed "transuranium elements" were the subject of much experimental work in widely separated laboratories. Finally, 0. Hahn and F. Strassmann, earlyin 1939,after exhaustive chemical work on one of the activities, concluded that since it could not be separated from barium it must be an isotope of barium. Scientists had to accept the amazing conclusion that the uranium atom undergoes cleavage or fission when subjected to neutrons, resulting in radioactive products approximately midway in the periodic table. It was N. Bohr who first suggested on theoretical grounds that it is the rare uranium isotope, UZs5,which undergoes fission by slow neutrons. Soon after, it was shown by H. von Halban, F. Joliot, and L. Kowarski and by L. Szilard and W. H. Zinn that secondary neutrons are emitted during or immediately after the fission of uranium. In principle, the secondary neutrons could themselves produce still more fissions and the possibility of the occurrence of a nuclear chain reaction was immediately recognized. The energy release of such a chain reaction would be immense because of the large amount of energy produced in each fisssion. As we all know, the attainment of the self-sustaining nuclear chain reaction with pure natural uranium, constructed in a lattice arrangement with a material such as graphite, has been successful. Such "piles," ILS they are called, are neutron factories of tremendous intensity. Going back to May, 1940, the year after the discovery DISCOVERY OF NUCLEAR FISSION AND of fission, E. M. McMillan and P. H. Abelson, pursuing TRANSURANIUM ELEMENTS the problem of the transuranium elements again, anThe important and far-reaching results that were to nounced the discovery of element 93. They were able come from the search for the transuranium elements to show by means of chemical work that, along with the could not even be dimly visualized a scant ten years ago. fission products, there arose from the neutron irradiAE already mentioned, the search for the transuranium ation of uranium a radioactivity of 2.3day half-life due elements was to lead to the discovery of fission, the to the isotope 93l" which is the decay product of the nuclear chain reaction, and to new and important UZa9formed by radiative neutron capture in U2sa. knowledge of the structure of the heaviest elements. Uaa n + Uaav 7 The histories of the transuranium elements and the fission process are so closely intertwined that it is per88Us" dNpalod haps worth while to review the chain of events that led 23 min. 2.3 days to the discovery of both. Starting in 1934 and continuing for several years, E. Fermi and his coworkers in This, then, was the first actual identification of a transRome found that neutron irradiation constituted a uranium element. Later in the same year, G. T. Seageneral method of inducing radioactivity. Since the borg, E. M. McMillan, A. C. Wahl, and J. W. Kennedy the fission products, has greatly enlarged in scope and intensity the body of knowledge encompassing both chemistry and physics associated with nuclear studies. Some of us would like to consider all of these studies that are related to nuclear problems and for which chemical training is an asset as a new field called "nuclear chemistry." A simple definition of nuclear chemistry has already been given as the branches of nuclear work in which chemistry plays an important part. This would include the study of nuclear transformations since the most widely used method for deducing the nature of the reactions is the chemical identification of the reaction products. It will be remembered that uranium fission was discovered by proving chemically that certain radioactivities belong to elements near the middle of the periodic table. As already mentioned, the behavior of very minute concentrations of materials measurable through their radioactivity has been termed radiochemistry, and this, too, is a branch of nuclear chemistly. The methods of studying these minute quantities have been extended to allow the accurate deduction of the chemical properties of elements present only a t concentrations of molar, or so. While many other examples of different aspects of nuclear chemistry could be given, two or three should be mentioned. One of these is the familiar application of radioactive substances as tracers or labelling agents, an extensive use of which is being made in the solution of reaction mechanisms. This inclusion may be temporary since, although the techniques are common to nuclear chemistry, the actual applications belong properly in their respective fields. Another is the study of chemical processes induced by radiation. This latter field has been termed "radiation chemistry" and is most closely allied to photochemistry. Still another branch is "hot atom" chemistry in which an atom left in an excited state from a nuclear reaction utilizes some of this excitation energy in promoting chemical reactions. With these definitions of the field encompassed by nuclear chemistry we may return to the transuranium elements, fol it is here that nuclear chemistry has found its richest outlet for expression.
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discovered the next transuranium element, element 94, as a result of the bombardment of uranium with deuterons. E. M. McMillan had suggested the name neptunium for element 93 since Neptune is the planet beyond Uranus, the planet for whichuraniumwasnamed. Following the same convention, p!utonium was suggested for element 94. As will be pomted out presently, the fission reaction, which was discovered as a result of the search for transuranium elements, became the means by which macroscopic quantities of both plutonium and neptunium were produced. Uranium also served as the starting material for the cyclotron production of element 95 while plutonium formed in the chain reacting pile hecame.the starting material from which cyclotron irradiation resulted in the discovery of element 96. Finally, the sum total of the large amount of chemical work done on the four transuranium elements, as well those just below uranium, has led to the viewpoint that actinium, is the prototype of a transition series of elements, as lanthanum is for the rare earth group. WPTUNIUM
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Through a modification of'the chemical process for separating plutonium it was found possible to remove the small amount of neptunium which is present. Several special runs of this kind have been made in the Hanford, Washington, plant resultingin the recovery of several hundred milligrams of neptunium. With this material it has been possible to obtain a thorough knowledge of neptunium chemistry. With respect t o its valency, the oxidation staks VI, V, IV, and I11have been identified and studied. It will he recognized that uranium also possesses these oxidation states. In the case of neptunium, however, there is a shift in the stability toward the lower oxidation states. In all respects, the chemistry of neptunium differs from that of rhenium, making it certain that these elements are not homologs in the periodic system. It is of interest t o note that Npaa7is the longest-lived synthetic isotope that is definitely known. The spkcific alpha radioactivity is only about 1000 times greater than that of uranium, which means that milligram quantities can be handled with comparative safety using little more than the usual precautions for toxic substances. As will be pointed out, work with the other transuranium elements grows in difficultywith succeeding elements as macroscopic quantities are intensely radioactive.
Returning to the work of E. M. McMillan and P. H. Ahelsou on neptunium, their experiments on the tracer scale of investigation showed that element 93 has a t least two oxidation states analogous to the VI and IV states of uranium. Theyfound that a more powerful PLUTONIUM oxidizing agent is required to oxidize neptunium from Element 94, the second of the transuranium elements, its lower to its upper state than is the case for the corresponding reaction with uranium. The similarity of was also second in order of discovery. This element, neptunium to uranium, its neighbor, rather than to named plutonium, was first prepared and recognized rhenium, the element thought previously to be its homo- late in 1940 by G. T. Seahorg, E. M. McMillan, A. C. log in the periodic table, was the first experimental evi- Wahl, and J. W. Kennedy. The isotope which they dence that the 5f electron shell is that being filled in the prepared by cyclotron bombardment of uranium was heavy element region. later proved t o be of mass number 238 and the half-life While a great deal can be learned from studies using of this alpha-emitter was estimated to be about 50 tracer quantities, there are many properties of an ele- years. The discovery was made by first identifying ment which can only be determined satisfactorily a t and separating a new beta-emitting neptunium isotope macro-concentrations or with visible amounts of iso- of 2.0-day half-life. Upon decay of this neptunium, lated compounds. The use of an element having no alpha-particles began to appear and these could be sepastable isotopes for studies of this kind depends upon the rated into a chemical fraction correctly interpreted as existence of a long-lived isotope and the means for pro- that of element 94, plutonium. ducing that isotope in quantity. An isotope of neptunium which fulfills these requirements is the 2.25 X lo6-year alpha-emitter, NpZa7,discovered by A. C. Wahl and G. T. Seahorg early in 1942 as a result of the bombardment of uraniumpiith fast neutrons in the Berkeley was also produced, but be60-inch cyclotron. The primary product was the pre- I n the same irradiation PuZa8 cause of the five hundredfold longer half-life the alphaviously known 6.9-day Uaa7which, through its decay, produced the long-lived NpZa7. The nuclear reactions particles were not discernible. The earliest work with tracer quantities of Puaas are : demonstrated that this element, like neptunium, posU4" + n Us' + 2n sesses multiple oxidation states with properfies like the corresponding states of neptunium. Here again, it was 8U2m LNpSa7 recognized that the periodic system was being extended 6.9 days with elements representing a transition series considerI n the nuclear chain reactors a small fraction of the ably different than that which would place the series neutrons have sufficient energy t o produce the n, 2n U-Np-Pu, etc., as homologs of W-Re-Os, etc. As will reaction, and in this manner Npaa7is formed in the ura- be pointed out below, the heavy element transition nium along with considerably larger amounts of Pua3@. series hears much closer analogy to the rare earths.
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When larger amounts of plutonium became available it artificially produced element and indeed the first isolabecame possible to show that plutonium has the oxida- tion of a weighable amount of any artificial isotope. tion states VI, V, IV, and 111, like neptunium and uraUsing this plutonium and later hatches made in the nium, but that the trend toward stability of the lower same way, it was possible to check all of the crucial oxidation states noted for neptunium is further accentu- points of the separation process under production plant concentrations and to make changes where necessary. ated in the case of plutonium. The isotope PuZ3Owith 24,000-year half-life is that The scale of operation in order to achieve the high conwhich is produced in the nuclear chain reactors in large centrations was extremely small-that is, much of the quantities. This isotope is of great importance because early work was done on the microliter scale using meit can be made in quantity and because, like UZa5,it chanical manipulators under the microscope. The availability of the relatively large amounts of undergoes fission with slow neutrons. From this property it can itself support a nuclear chain reaction either plutonium from the chain-reacting uranium piles has as a power plant or a bomb. The fissionability of PuaSD made it possible to make a complete investigation of its was discovered by J. W. Kennedy, G. T. Seaborg, E. chemical properties, using methods which can be conSegre, and A. C. Wahl who first prepared submicrogram sidered to he those of ordinary chemistry except for the quantities by the neutron irradiation of uranium with health precqutions which are necessary. A large number of compounds of plutonium have been prepared and the Berkeley cyclotron. 'Early in 1942, groups and individuals working at . their properties determined. It may be said that the several universities on the plutonium problem were chemistry of plutonium today is as well or better undekbrought together at the University of Chicago for the stood than is that of most of the elements in the periodic purpose of consolidating their effortstoward the actual system, even though its chemistry is very complex, as production of plutonium. The two principal problems can be judged by the multiple oxidation states. In fact involved proceeded in parallel: (I) the design of a chain all of the four oxidation states can coexist at equilibrium reacting structure to produce plutonium, and (2) the in solution and in appreciable concentrations. Much of design of a chemical process for separating the pluto- the complex oxidation-state equilibrium was worked out nium from the uranium and fission products. By mak- by R, E. Connick and coworkers, working with W. M. ing use of tracer t,echniques with cyclotron-produced Latimer a t the University of California. Pu238it WI~S possible to deduce a considerable -amount about the chemical properties of plutonium and to de- AMERICIUM vise certain separation processes that could conceivably ~h~ discovery of americium, element 95, presents an work, as well as to eliminate other approaches from fur- interesting study in that it, like P u ~WB.S by ~ ~obtained , ther consideration. Then began the. arduous task of first producing its radioactive parent, but in the case of developing in detail what turned out to be a long and americium the parent could not he detected. Working complex series of chemical reactions, and to conform the Metallurgical Labor&ry of the University of them to the limitations of plant operation. The most Chicago, G. T. Seaborg, R. A. James, and L. 0. Morgan worrisome feature of this method of development was obtained the isotope AmzP1from a sample of n r a n i m that the inferences based on the tracer scale of operation which had been bombarded with high energy helium might be misleading, and that when the concentra- ions (alphsrparticles) with the Berkeley cyclotron. tions of plutonium anticipated for the Hanford plant The nuclear reaction between helium and uranium can were encountered the process might fail. In addition, it form isotqpes of plutonium directly, and among these would be necessary to finally isolate the plutonium free should be the isotope PU241. I t was predicted that of carrier materials, and the problem of devising this P U m might decay by beta-emission, the product of part of the process could obviously not be treated a t all which would be Amzi1. By deducing further what the on the tracer scale. With these and other important chemical properties of this new element might be, a posconsiderations in mind, the unprecedented task of Pre- sible means of separating it from plutonium could he paring visible amounts of a new element with the cyclo- devised. ~ l t h the ~ beta-particles ~ ~ h of p U 2 4 1 were not tron was undertaken. Since it was realized that, at detectable, presumably because of low energy, an alphabest, only a few micrograms of plutonium could be pre- emitter which was identified as an'isotope of element 95 pared in this fashion, it was necessary at the same t,ime did grow into the plutonium fraction as predicted. The to develop the ultramicrochemical techniques in order means of production ,vhich was deduced is: to work with this amount of material at conventional U1" + ~He4+ PuP'I n concentrations. As the culmination of a series of in8tense cyclirtron bombardments of large amounts of Pula1 Am"1 uranium there was produced, over a period of a year, long about one milligram of plutonium. A number of chemConsiderably larger quantities of Amz4' were subseists in the laboratory concentrated the plutonium by quently made and the half-life was estimated as 500 laborious means and finally on August 18, 1942, B. B. Cunningham and L. B. Werner a t the Metallurgical years. With this americium it became possible to study Laboratory in Chicago isolated in free state a pure com- its chemistry further. It was found that americium has pound of plutonium. This was the first isolation of an a very stable trivalent state which resists oxidation to
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high neutron 5ux pile for an intel-e neutron irradiation. A part of the Amz4'was transmuted to CmZ42and by means of ultramicrochemical techniques Werner and Perlman were able to isolate curium as a relatively pure compound. The curium was separated from untransmuted americium by the newly developed methods of adsorption with ion-exchange resins. The adaptation of column adsorption to separations of this nature was largely a development of groups working a t the University of Chicago, Clinton Laboratories in Oak Ridge, and a t Iowa State College. The difficultyof separating americium and curium is comparable to that of separating neighboring rare earth elements, but this was accomplished with good yield using a resin furnished by the Dow Chemical Company. . With this minute amount of curium in solution it was possible.to obtain the absorption spectrum using microadaptors on the spectrophotometer. As mentioned, the spectrum is of great interest in regard t o the actinide hypothesis for the heavy elements since, according to this thepry, curium, like gadolinium, would not be exCURIUM pected to absorb light in the visible spectrum. Through An important element in the actinide hypothesis for analogy with gadolinium, one would expect to find the heavy elements is curium, element 96. It would he little or no absorption here, presumably because of predicted that curium, as the seventh member in the the stability associated with the closed half-shell of actinide series, would arrive a t a very stable configura- seven f electrons. This prediction has been amply tion with a completed half-shell of seven f electrons an- verified with this sample of curium, which showed alogous to gadolinium in the rare earth series. On this strong absorption in the ultraviolet region but none basisit would be expected that curium would possess the from about 4500 t o 11,000 A. Other work with pure curium on the ultramicrochemitrivalent oxidation state exclusively, that, like gadolinium, it would not absorb light in the visible region, and cal scale gives ample proof for its trivalent nature dethat, in general, it would be very much like actinium duced from tracer experiments. The isolatibn of curium presented great difficulties and the exclusively trivalent rare earths. Curium was actually the third of the transuranium not only because of the very minute amount available eIements to be discovered. From the irradiation of but also because the isotope formed has a half-life of P u with ~ helium ~ ~ ions in the cyclotron, G. T. Seaborg, only 5 months and therefore is intensely alpha-active. R. A. James, and A. Ghiorso were able to identify the One microgram of Cm2" gives off 7 billion alpha disintegrations per minute and converts itself to PuS38a t the isotope Cm242formed as follows: rate of about 0.5% per day. It most be worked with PuZa8+ He' Crn242+ n entirely in an enclosed system as the health hazard Cm242is an alpha-emitter with a half-life of about five would be prohibitive for any other type of handling. The water decomposition from the intense alpha activmonths. With thisisotope available it became possible to study ity results in constant gas evolution, which makes it the chemistry of curium on the tracer scale. Exten- most difficult to separate precipitates by centrifugation. Neptunium and plutonium Irere named for the sive investigations by S. G. Thompson, L. 0. Morgan, R. A. James, and I. Perlman confirmed the exclusively planets which lie beyond Uranus, the planet for which trivalent nature of curium in aqueous solution. It is uranium was named. Since this convention becomes carried by rare earth fluorides in precipitation reactions exhausted with plutonium another was adopted for and cannot be oxidized or reduced t o states which are elements 95 and 96. Names for these elements were drawn from analogy with rare earth elements according not precipitable. The isotope CmZ4= can be made in another way than to their corresponding positions in the actinide and lanthat mentioned, namely by the neutron irradiation of thanide transition series, respectively. Since the sixth element beyond lanthanum is called europium, the the 500-year Amz4'. The nuclear reactions are: sixth actinide element was named americium after the Americas. The seventh rare earth element is gadolinium named for Gadolin who did such notable work on the rare earths. Element 96 was, by analogy, named curium after Pierre and Marie Curie, the pioneers in the Some time ago a small amount of Amz4'was placed in a field of radioactivity. higher states, a t least under the conditions in which uranium, neptunium and plutonium can be oxidized. This behavior along with the more gradual stabilization of the trivalent states of the earlier heavy elements led Seaborg t o suggest the "actinide hypothesis" in which actinium, element 89, is the prototype of a transition series of elements analogous t o the position of lanthanum with respect to the rare earth elements. The deductions of chemical properties made on the tracer scale have been confirmed and extended as the result of the isola.tion of minute amounts of americium by B. B. Cunningham. The work with macro-concentrations of americium by Cunningham, Werner, and others has shown convincingly that the stable state of americium in solution iimuch like that of actinium and the trivalent rare earths. Because of the relatively short half-life of AmZ4l(500 years), the work with visible amounts involves exposure to great quantities of alpha-activity. The specific alpha-activity is about seven billion alpha disintegrations per minute per milligram.
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