4—
Pioneering in Transurania

According to Meitner and Frisch, the third of the uranium "isomers" of figure 9.5, the 23-minute activity, was indeed an isotope of uranium. According to accepted theory, it should have reduced its surplus of neutrons by emitting beta particles. Among the first to seek the product of such a transformation, a nucleus heavier than any previously detected on earth, was Edwin McMillan (plate 9.3). He had an advantage over others in having at his disposal the large activating neutron flux from the 37-inch cyclotron and a highly cultivated technique for the study of the radiations from nuclear disintegrations. Although his time had been taken up with cyclotron problems, he retained the independence he had shown in studying gamma rays while most of the rest of the Laboratory were building machines or exploding deuterons.^[90]

In his follow up of fission, McMillan characteristically examined the penetration of the recoiling fragments through the standard absorber, aluminum. He obtained a maximum range of about 2.2 cm, in rough agreement with slightly earlier experiments by Joliot. McMillan also examined the decay of the products left with the irradiated uranium, products unable to propel themselves through a single sheet of cigarette paper. He found an activity of 25 minutes, which he conjectured might be the same as the 23-minute body of the Berlin group, and he detected a strong activity of about 2 days' duration. He did not suggest a source for this long period, but promised further absorption measurements to fix the ranges of the recoiling fragments.^[91] Segrè took up the study of the nonrecoiling product and refined the periods to 23 minutes and 2.3 days. The first he identified with the heavy uranium of Berlin. Was the second a fission fragment or a transuranic? Segrè did his chemistry, made the 2.3-day body a rare earth (which it was), identified it as a fission fragment (which it was not), and declared it to be a light element. He searched for the alpha emitter that might reasonably be expected to terminate the beta transformations beginning with U²³⁹ . No luck. His conclusion, after discussing the matter with McMillan and Seaborg: the 23-

minute U²³⁹ becomes a long-lived, undetected element 93. And, underlined: "Transuranic elements have not yet been observed ."^[92] Once bitten, twice shy. As a member of Fermi's group, Segrè had erred in claiming a transuranic; as a cautious and institutionally insecure member of the Laboratory, he did not wish to repeat his mistake, and thus missed the transuranic he had. He bent his efforts to straightening out the decay chains of fission fragments.^[93]

Segrè's erroneous negative finding was immediately confirmed by John Irvine, a chemist at MIT, who concentrated U²³⁹ by taking advantage of the effect on chemical bonding of excited nuclear states. He was unable to detect any activity in a precipitate made by treating his enriched material with a compound of rhenium, which, following the old opinion of the nondiscoverers of the transuranics, element 93 should resemble. In his summary of the state of research on fission, completed in December 1939, Louis Turner of Princeton accepted the straightforward conclusion from the experiments of Segrè and Irvine. The elusiveness of element 93 and the hypothetical terminating alpha emitter nonetheless bothered him. He reduced his bother by the good guess that 94²³⁹ is the alpha emitter and U²³⁵ the great grandson of U²³⁸ . That at least kept the scandal in the uranium family.^[94]

Meanwhile Abelson, who had finished up at Berkeley with measurements of the wavelengths of K rays from radioactive substances near the middle of the periodic table,^[95] began to doubt the assumption on which Segrè had based his denial of transuranic status to McMillan's 2.3-day activity. The old alternative to likening transuranics to the elements beginning with rhenium and osmium remained: 93 and 94 could well resemble rare earths, and these "actinides," as they were later christened, should have a chemistry like uranium's. In his spare time at Tuve's laboratory, where he went in September 1939 to help with its 60-inch

cyclotron, Abelson showed that the 2.3-day body did not behave consistently like a rare earth. McMillan, too, had his doubts about Segrè's diagnosis. His further tests had shown that the 2.3-day activity remained firmly with the irradiated uranium and that its intensity exceeded that of all the long-lived fission fragments collected by recoil. Furthermore, when cadmium guarded the uranium target from assault by slow neutrons, the intensity of the fission products fell dramatically; whereas the intensities of the 23-minute and 2.3-day activities not only did not change appreciably, but remained in the same ratio, "suggesting a genetic relation between them," and the consequent identification of the longer period with element 93.^[96]

Abelson visited Berkeley in May, with orders from Tuve to "make every effort to settle the identity of the 2.3-day substance." Should it come from U²³⁵ , the possibility of a chain reaction even in separated uranium appeared doubtful. Abelson and McMillan joined forces and soon found a distinct chemical difference between the 2.3-day activity and rare earths. (The difference, the effect of the presence of an oxidizing agent on certain reactions, explained the erratic results of previous investigators, who had not controlled the oxidizing power of their solutions.) In respect of these reactions, it resembled uranium, and, since it had nothing in common with rhenium, McMillan and Abelson referred it to a possible "second 'rare earth' group of similar elements starting with uranium." It remained to show that the 2.3-day activity grew from the 23-minute U²³⁹ . Samples collected from the parent at 20-minute intervals all decayed with a period of 2.3 days. The decay, by beta emission, produced element 94. McMillan and Abelson supposed, with Turner, that 94²³⁹ transformed into U²³⁵ by alpha emission, which, in fact, it does. Their search for the telltale alpha particles did not succeed, however, and they inferred that, if 94²³⁹ were unstable against alpha emission, it must have a half-life of a million years. They overestimated by a factor of 400.^[97]

Seaborg knew about the work on element 93 as it progressed, and it made him "eager to work in this exciting field." He assigned Arthur Wahl the task of satisfying his eagerness. While Seaborg and Segrè sought new fission products in uranium struck by very fast neutrons, Wahl perfected chemical means for concentrating 93²³⁹ . By the middle of October, both projects were well advanced: Seaborg and Segrè found two chains of decays from Pd through Ag to Cd, and Wahl, following the procedure devised by McMillan and Abelson (oxidation by bromate ion), had isolated the 2.3-day activity from several samples of uranium irradiated with neutrons.^[98] But neither he nor his senior colleagues could find the suppositious alpha-emitting 94. They decided to try another route. During the summer of 1940, McMillan had invoked the traditional Berkeley bombardment and sent deuterons against natural uranium. He caught a glimpse of a second isotope of element 93 with a beta activity slightly more energetic than 93²³⁹ 's; and he also saw a sign of its alpha-emitting descendent. After Kennedy had made a special thin-walled counter to follow this descendent, Seaborg wrote McMillan, who had left Berkeley for war work at MIT, that he, Kennedy, and Wahl would be "very glad to collaborate with you on the isolation of the new isotopes of element 93 and uranium found in the bombardment of uranium with deuterons." McMillan replied that he would be very pleased to have Seaborg continue the work.^[99]

McMillan had more than a curiosity about the secrets of nature in the continuation of the search for element 94. His and Abelson's discovery had been announced to the press with the usual fanfare: "A development that may bring man a step closer to the release of atomic force and energy which brought the universe into existence;" "something which may quite conceivably prove more influential in the destiny of the world than any single battle of the current World War."^[100] The publicity prompted a

reprimand from Lyman Briggs, director of the National Bureau of Standards, who headed a committee that tried to keep potentially useful information about uranium secret from foreign powers. The potential usefulness lay in the possibility that 94²³⁹ might be fissionable like U²³⁵ . Although Alvarez had discussed "fishing" 94²³⁹ with McMillan in January 1940 and Louis Turner had written Lawrence in July proposing that the 60-inch be put to making enough 94 to test the strong likelihood that 94 could be fished, Abelson and McMillan "did not see any possible connection of our work on element 93 with the fission problem."^[101]

Neither the mistake nor the publicity would recur, McMillan wrote Briggs; henceforth all discoveries about 93 and 94 would be submitted to his committee for determination of their sensitivity. As an example of his good behavior and his findings, McMillan disclosed his provisional results about the products of uranium bombarded by deuterons: the unknown isotope 93^? produces an alpha-emitting body, presumably an isotope of element 94, with the chemical properties of thorium. Briggs replied that the "most important contribution you could make at this time" was to discover whether the alpha emitter fissioned with slow neutrons. "Even rough data will be valuable, and facilities do not exist elsewhere which will permit attempting the work." Only the 60-inch cyclotron could produce a strong enough sample of 93 to decay into enough 94 to offer the possibility of detecting its fission. Briggs's committee—which probably meant Fermi—estimated that with a sample as strong as the one McMillan and Abelson had used, McMillan should be able to see one fission every ten seconds, provided that 94 was about as fissionable as U²³⁵ .^[102] That sample had been extremely strong, about 10 mCi of 93, almost certainly, as Alvarez wrote Turner, "the most heavily bombarded substance in history." Its manufacture had so strained the 60-inch that Alvarez doubted that its like would be seen again before the 184-inch started up. "I don't think that we shall be able to fish 94 for some time."^[103]

At first fishing may not have been on the agenda of Seaborg's crew. On December 14, 1940, they prepared a modest sample of 93^? by irradiating uranium oxide with 175 µAh of deuterons in the 60-inch cyclotron. Wahl took on the purification of 93 as part of his doctoral work. His excellent preparation when interrogated by counters showed a beta and gamma emission sufficiently distinct from those of 93²³⁹ to point to the presence of a new isotope. An alpha emitter, apparently the descendent of 93^? , also showed itself. Unfortunately, the half-life of 93^? fell out too close for comfort to that of 93²³⁹ (2.3 days), which was also produced to some extent in the deuteron bombardment. From another sample prepared in January, the half-life of 93^? appeared to be 2.1 days, consistent with the increase of the alpha activity that grew from it. On this evidence and some shaky chemistry of the alpha emitter, "Things look[ed] good for element 94," Seaborg wrote McMillan on January 20, 1941. He added: "no one else knows about the most important of these results (the element 94) except Wahl and Kennedy. . . . The Committee [Briggs's] will want us to keep the results VERY SECRET!" On January 28, 1941, Seaborg, McMillan, Kennedy, and Wahl announced the discovery of 94^? in a letter to the Physical Review withheld from publication by Briggs's committee. For the "?", they offered the choice of 235, 236, and 238; it was, in fact, 238, made by (d,2n) on U²³⁸ , and so identified by Kennedy, Segrè, Wahl, and Perlman in the fall of 1942.^[104]

Confirmation that the alpha emitter was an isotope of element 94 required its chemical separation from 93. Wahl tried many methods before he took up with the powerful oxidizing agent, persulfate ion, on the suggestion of a Berkeley chemist, Wendell Latimer, who was not authorized by the Briggs committee to know anything about the matter. (McMillan regarded the introduction of this agent, which promotes the 94 ion to a state soluble in hydrofluoric acid, as the most important contribution of Seaborg, Kennedy, and Wahl to the discovery of element 94.) By the last

week in February, all the 93 had decayed into element 94. Wahl hit what remained with persulfate, dissolved it in the presence of fluoride ion, and purified it. "These experiments," wrote Seaborg, Wahl, and Kennedy, in an understated celebration of Wahl's work, "make it extremely probably that this alpha-radioactivity is due to an isotope of element 94."^[105]

Meanwhile another game with 94 was in play at the Laboratory. In December 1940, Segrè, then in New York, talked with Fermi about the possibility that 94 would fission with slow neutrons, a matter then also receiving the attention of Bohr and Wheeler. Lawrence, in New York as usual during the giving season, was persuaded by Fermi and Segrè to order enough 94 from the 60-inch to test the hypothesis. On January 9, Segrè and Seaborg made a little 93²³⁹ by neutron bombardment and, by comparing yields, deduced that more 94 could be made by neutrons than by deuterons for the same cyclotron time.^[106] Fermi calculated that a kilogram of uranyl nitrate would be target enough; Seaborg and Segrè calculated that the amount of 93 they would have to make to give the 1 µg of 94 they hoped to have would be so radioactive—some 250 mCi—that it would have to be manipulated at long distance or by remote control. After practice runs with uranyl nitrate sent by Fermi—Cooksey had not been very generous with Laboratory funds for the project—Segrè and Seaborg irradiated 1.2 kg of the stuff with neutrons from 3,368 µAh of deuterons from the beryllium target of the 60-inch. For the next four days, from March 3 to March 6 inclusive, they extracted the 93, wearing goggles and lead-impregnated gloves, working with remote controls, carting their improving sample from one piece of apparatus to another in a lead bucket suspended from long poles. The precious product was sealed in a shallow platinum dish under a layer

of duco cement. Kennedy joined the team to monitor the decay of the 93²³⁹ into 94²³⁹ .^[107]

At this moment, early in March 1941, Seaborg's enterprises came together. Wahl's technique for the separation of 94²³⁸ , which Seaborg had kept secret from his partner, the alien Segrè, who had not been formally authorized to receive the information by the Briggs committee, was, of course, applicable to the big sample of 94²³⁹ . "These results came just in time," Seaborg wrote McMillan, "to be of great help to me [!] in the 94²³⁹ project which I [!] am doing for the Uranium Committee."^[108] By the end of March, the 93 had decayed to negligibility. Kennedy, Seaborg, and Segrè brought the sample—about 0.5 µg of 94²³⁹ mixed with rare earths and other dross—near the beryllium target of the 37-inch cyclotron, irradiated it with neutrons, and had the satisfaction of detecting about one fission per minute per µA of deuterons, from which they guessed that 94²³⁹ has a cross-section for slow neutrons about one-fifth that of U²³⁵ . The thick sample did not lend itself to such measurements, however: most fission fragments stopped within it or its cement topping. Wahl used the newly found chemistry of 94 to thin it. On May 17, 1941, it again went under the 37-inch. Almost twice as many fission fragments were counted as would have escaped from an equivalent amount of U²³⁵ . There was no longer a doubt: 94²³⁹ , which the Berkeley team estimated to have a half life of 10,000 years when left to itself, can be fished by slow neutrons.^[109] During the summer of 1941, a large sample of 94²³⁹ was prepared at the cyclotron and purified by Wahl. Studies of its chemistry by Wahl and Seaborg, of its fissionability under fast neutrons by Segrè and Seaborg, and of its low rate of spontaneous fission by Kennedy and Wahl

confirmed the suspicions that it belonged to a heavy rare-earth group and might make a fine explosive.^[110] That proved decisive for the career of Glenn Seaborg and may prove so for the rest of the human race.