Processes
Until the late spring of 1937, experimentalists knew only one way for artificial radioelements to decay: by the emission from their nuclei of a positive or a negative electron. Theorists had observed, however, that a nucleus liable to produce a positron might also transmute by capturing one of the two atomic electrons—the so-called K electrons—closest to it. The heavier the nucleus, the stronger the pull on the K electrons and the greater the likelihood of capturing one of them. The possibility was first aired by that inventive interpreter of Fermi's theories, Gian Carlo Wick.[64] A means of detecting the process, should it occur, lay close to hand. An atom containing the stable nucleus created by K-electron capture would lack an electron in its innermost shell. An electron from the next shell is likely to fall into the hole and to emit an x ray, called a Ka ray, in the process. K-electron capture by an unstable nucleus of charge Z betrays itself by a Ka ray characteristic of element Z – 1. The higher the Z and the longer the life of a positron emitter, the more likely the competing K process is to occur. None of the naturally occurring radioelements, all of which have high Z , decay by releasing positrons. Hence the K-prospector looked as though it were among the longest-lived
[62] Johnston, Wolfgang, and Libby, Science, 113 (1951), 1–2.
[63] Lawrence, "Report [for 1939]," n.d. (15/18).
[64] Wick, Acc. naz. lincei, Atti, 19 (1934), 319–24 (4 Mar 1934).
emitters of positive electrons he could make as far up the periodic table as his means allowed.
Fermi's theory modelled beta decay and the competing capture process in analogy to the theory of electromagnetic radiation: just as an atomic electron can release or absorb a photon in leaving or reaching an excited state, so a proton can give rise to a positron or capture an electron while turning into a neutron. The analogy breaks down in that the energy of the beta particle created in the process is not equal to Emax , the difference in energy of the nucleus before and after the creation; rather, the energy may take any value up to Emax , as indicated in figure 8.2. To save the principle of energy conservation, physicists had supposed that a second particle is created in the beta decay, a "neutrino," whose lack of charge and vanishingly small mass (necessary to produce the asymmetry in figure 8.2) protected it from observation. With a neutrino mass of zero, the shape of the theoretical beta curve is determined chiefly by the value of Emax . Fermi's calculation, which took the strength of the interaction between electron and neutrino to be proportional to the amplitude of their fields, did not give quite the degree of asymmetry observed. Theorists at the University of Michigan, E.J. Konopinski and his professor, G.E. Uhlenbeck, came closer in the cases of P32 and Al28 by making the interaction proportional to the product of the amplitude of the electron field and the gradient of the neutrino field.[65]
In 1936 Christian Møller in Bohr's institute compared the predictions of Fermi and of Konopinski and Uhlenbeck (K-U) for the total probabilities per second of positron emission (l+ ) and K-electron capture (lK ) in a hypothetical susceptible nucleus of high Z . (l is the inverse of the period of the activity.) His result: on both theories lK is much larger than l + , the disparity being the greater the smaller Emax . Then he specialized to the curious results of Cork and Lawrence, whose "platinum" decayed with a period of 49 minutes by emission of positrons of Emax = 2.1 MeV. Neither Fermi nor K-U could give enough positrons to fit these data. Møller supposed that K-electron capture must have occur-
[65] Konopinski and Uhlenbeck, PR, 48 (1935), 7–12. Cf. Rasetti, Elements , 193–200, and (for Al ) Cork, Richardson, and Kurie, PR, 49 (15 Jan 1936), 208.
Fig. 8.2
The beta-ray spectra of some naturally occurring
radioisotopes. The ordinate indicates the percentage
of the total activity at the energy of the abscissa to
which it corresponds. Rasetti, Elements , 146.
red in the Berkeley experiments 9 times or 47 times as often as positron emission depending on whether events followed Fermi or K-U. He advised looking for x rays from the "iridium" formed from the "platinum" decay. Lawrence could not find the x rays and, as usual, thought he had caught out the theorists: "It looks to be a serious difficulty for the Fermi theory." Since Bohr's institute had no machine for making radioplatinum, experimentalists followed up Møller's lead by seeking Ka from the decay product of the heaviest available positron emitter. This was Sc43 (Z = 21, t = 4 hours), made by alpha particles from radon on (ancestor) calcium via (a ,p). A search for Ka from (descendent) calcium by J.C. Jacobsen failed. Calculations indicated that for Sc43 , lK /l+ = 5 according to K-U and 0.1 according to Fermi. Calculation and measurement in Copenhagen therefore favored Fermi.[66]
Berkeley had by then plumped for K-U on the basis of its apparently better fit to measurements of beta decay. Here the primary instrument of research was Kurie's cloud chamber and the
[66] Møller, PR, 51 (1937), 84–5, rec'd 9 Nov 1936; Cork and Lawrence, PR, 49 (1936), 788–92; Jacobsen, Nature, 139 (1937), 879–80, letter of 6 Apr; Lawrence to Segrè, 5 Apr 1937 (16/14).
primary researchers himself, J.R. Richardson, and Hugh Paxton. By using hydrogen, in which slow particles have a better chance to show their presence than in oxygen, then the usual medium in the chamber, they obtained close agreement with K-U for N13 , F17 , Na24 , and P32 . The cloud chamber men concluded that "[the K-U theory] completely describes the process of emission of a beta particle."[67]
As they drifted further along the periodic table, however, their conviction dissipated. Active chlorine, argon, and potassium could not be fitted to K-U unless each contained two unresolved activities. And nothing fit unless Emax were put higher than the limit to which, as judged by the eye, the experimental curve tended.[68] (They could not measure all the way to the maximum because they could not register enough of the very few fastest particles.) Ernest Lyman, another graduate student associated with the Kurie group, confirmed the disconfirming of K-U in the cases of P32 and RaE. As he observed, however, P32 and RaE have unusually long periods (14 and 5 days respectively), and might die in ways not dreamed of in the competing theories.[69]
Fermi himself had pointed out that substances like RaE might escape his theory. He called attention to their position on the so-called Sargent curves, a plot of logEmax against logl for the naturally radioactive substances. Its author, B.W. Sargent, who took up the project while at the Cavendish, divided the empirical points into two classes, each of which fell roughly along a straight line (fig. 8.3). For a given Emax , an element of class II, which included RaE, has a much longer life than an element of class I. Their decay apparently required an inhibiting change of nuclear spin. At a meeting of the American Physical Society in Seattle in June
[67] Kurie, Richardson, and Paxton, PR, 48 (1935), 167, and PR, 49 (1936), 368–81, rec'd 7 Jan 1936, 372, quote. Konopinski had worried about N and the difficulty of putting the theories to the test; Konopinski to Bethe, 10 May 1935 (HAB, 3).
[68] Kurie, Richardson, and Paxton, PR, 49 (15 June 1936), 203 (APS meeting, 20–1 Dec 1935); Newson, PR, 51 (1937), 624–7, rec'd 16 Feb.
[69] Lyman, PR, 50 (1936), 385, and PR, 51 (1 Jan 1937), 1–7, rec'd 27 Oct 1936. By 1939, experiment favored a mixture of Fermi and K-U (Walke, Rep. prog. phys., 6 (1939), 20–1), and theory gave little support to either (Breit, RSI, 9 (1938), 64).
Fig. 8.3
The Sargent curves relating the maximum energy of the electrons
emitted by natural radioelements to the decay periods. Sargent,
PRS, A139 (1933), 671.
1936, Lawrence introduced Laslett to talk about a singularly long-lived positron emitter, Na22 (t = 3 years), first made by Otto Frisch by (g ,n) on F19 , then in comparative plenty by Laslett by (d,a ) on Mg24 . Its exceeding longevity interested Willis Lamb, who was brought forth at the same meeting by his mentor, Oppenheimer. Lamb reported that his calculations showed that if Na22 did require a change in nuclear spin, it would be twice as likely to decay by K-electron capture as by positron emission according to Fermi's theory, and thirty times as likely according to K-U. He proposed as a test not looking for the Ka line of neon but counting the relative numbers of alpha particles (one for each atom of Na22 created) and positrons (one for each positron decay) in the process. K-electron capture was not, and has not been, observed in Na22 .[70]
[70] Sargent, PRS, A139 (1933), 671, and in Shea, Otto Hahn , 227–9; Frisch, Nature, 136 (1935), 220; Lamb, PR, 50 (1936), 388–9; Laslett, ibid., 388, and PR, 52 (1937), 529–30, rec'd 25 June, and Laslett to Lawrence, 26 June 1936 (10/32).
While Laslett wrote up his results, Alvarez entered the game and picked up the chips. He noticed that Sc43 lies on the first Sargent curve and looked around for a neighboring element with a radioisotope on the second. The cyclotroneers had been exploring the region. By the end of 1935 they had reached zinc and had studied at least one element, argon, carefully; but they did not stop to examine the transition elements below zinc closely enough to find the eligible positron emitters they had activated in them.[71] Nor did Livingood's surveys of 1936 or a direct search among the activities of copper pick them up.[72]
Harold Walke, who was so unsure of himself that he had to be careful, then looked closely at the activities of the first few elements in the fourth period. He found a strong positron activity induced on titanium; chemical separation pointed to an isotope of vanadium (later identified as V48 ) produced by (d,n). The period of decay, 16 days, placed the new activity on the second Sargent curve. That was the combination desired: a Z high enough, a life long enough. Alvarez attacked radiovanadium with McMillan close behind, opening loopholes, "following the job." Alvarez did not attend the Laboratory picnic on Sunday, June 20. That day he detected rays from the decaying vanadium with a penetrating power appropriate to the K rays of titanium. K-electron capture, long expected in theory, thus materialized in the laboratory. Alvarez made the ratio lK /l+ about 1, closer to Fermi's theory than to K-U. Lawrence praised the work as "especially significant."[73]
[71] Livingood, PR, 49 (15 Jan 1936), 206 (Zn); Thornton, ibid., 207 (As, Ni, Co); Snell, ibid., 207, and PR, 49 (15 Apr 1936), 555–60 (A).
[72] Livingood, PR, 50 (1 Sep 1936), 425–32; Lawrence to Segrè, 5 Apr 1937 (16/14). Alvarez, PR, 54 (1 Oct 1938), 486, miscredits Yukawa and Sakata, Phys.-Math. Soc. Japan, Proc., 17 (1935), 467–79, and 18 (1936), 128–30, rather than Wick with the first suggestion that K-electron capture might compete with positron emission. Cf. Rasetti, Elements (1936), 202–3, and Segrè in Trower, Discovering Alvarez , 11–2.
[73] Walke, PR, 51 (1937), 1011, talk at APS, Washington, ca. 1 May; Alvarez, PR, 52 (1937), 134–5, letter of 21 June; Walke, PR, 51 (1937), 439, Hurst and Walke, ibid., 1033, and Walke, PR, 52 (1937), 663, letter of 6 Aug (K, Ca); Allen and Alvarez, RSI, 6 (1935), 329; Cooksey to Lawrence, 17 and 21 June 1937, and Lawrence to Cooksey, 25 June 1937 (4/21).
Once the process had been seen everyone saw it. Livingood, now disciplined in collaboration with Seaborg and another chemist, Fred Fairbrother, a Leverhulme Fellow from the University of Manchester, found a positron activity in manganese. Later Livingood and Seaborg showed that this isotope (or rather isotopes: Mn52 and Mn54 ) decays by K-electron capture and that Zn65 , which Livingood had examined earlier, does so too. Then Otto Oldenburg, on sabbatical from Harvard, found a K process without positron competition in tantalum excited by neutrons (Ta180 , t = 8.2 hours).[74] In all this there was a difficulty, however, which McMillan pressed on Alvarez. Perhaps the K electron does not jump into the nucleus but out into the world, driven by a gamma ray originating from an excited state of the stable final nucleus? If the probability for "internal conversion" (the release of an atomic electron that absorbs the gamma ray) were sufficiently high, only the x rays and the converted electrons would appear in the radiations. To decide the question, the experimenter must determine the element from which the K ray emerges: if from element Z (Z being the atomic number of the radioelement), then internal conversion; if from element Z – 1, then K-electron capture.
Alvarez took up this problem with the positron emitter Ga67 , which Wilfred Mann had made by (d,n) on zinc. The radiation from Ga67 consists of electrons, gamma rays, and x rays characteristic of zinc. Ernest Lyman and another graduate student showed that all the electrons had about the same energy. Alvarez explained: a nucleus of Ga67 swallows a K electron and ends in an excited state of Zn67 , which emits a monoenergetic gamma ray that has a moderate possibility of internal conversion; homogeneous electrons demonstrate the conversion and the zinc x rays the filling of the holes in the zinc atom's electronic structure. Walke found a better demonstration with long-lived V49 (t = 600 days), which decays only by K-electron capture and only into the ground state of titanium. No gamma rays or ionizing radiations
[74] Livingood, Fairbrother, and Seaborg, PR, 52 (1937), 135, letter of 30 June; Livingood and Seaborg, PR, 54 (1938), 239, 391; Seaborg, Jl., 1 , 355–6 (23 June 1938); Oldenburg, PR, 53 (1938), 35–9, rec'd 22 Oct 1937.
complicate the picture. Like the V48 in which Alvarez had made his discovery, V49 appeared to die out more closely to Fermi's than to K-U's specifications. By 1939 K-electron capture had been recognized in some twenty isotopes, including two of element 43.[75] K-electron capture proved to be as common as theorists expected. Among other consequences of its ubiquity, it ruled out the possibility that platinum could be the source of the positrons seen by Cork and Lawrence.[76]
Some pieces of the platinum puzzle fit well with the study of another nuclear process, in which, like the detection of K-electron capture, the Laboratory pioneered. This was the behavior of isomers, forms of the same unstable nucleus differing in internal energy. A pair of isomers can decay in several ways: each might emit a beta particle, or the more energetic isomer may relax into the lower by throwing off a gamma ray, or both processes might occur together. Isomerism first came to light in 1921, when Otto Hahn deduced that the third member of the radioactive chain descending from uranium UX2 (Pa234 ) consists of two beta emitters, both of which, he thought, arose directly from UX1 (Th234 ). No other instance was found. It took theorists some time to devise an explanation. In 1934 the inventive Gamow thought to trace the difference between Hahn's isomers UX2 and UZ to the presence in one of them of a hypothetical antiproton-proton pair in place of two neutrons. Another idea, put forward by a student of Heisenberg's, C.F. von Weizsäcker, in 1936, preserved the upper isomer long enough to emit a beta ray by supposing that a big difference in spin discouraged it from dropping immediately to the lower. Still, the matter was neither clear nor persuasive; Hahn's partner Lise Meitner expressed skepticism about isomerism and Bethe, though accepting the phenomenon, hedged over whether the pair UX2 and UZ was an example of it. The Cavendish's Norman Feather and Egon Bretscher cleared the matter up early
[75] Oldenburg, PR, 53 (1938), 35–9, rec'd 22 Oct 1937; Alvarez, PR, 53 (1938), 606, letter of 15 Mar, and PR, 54 (1 Oct 1938), 486–97, followed up by Helmholz, PR, 57 (1940), 248; Walke, Williams, and Evans, PRS, A171 (1939), 360; Walke, Rep. prog. phys., 6 (1939), 22–3. The vanadium isotopes were misidentified as V and V .
[76] Lawrence recognized the problem; Lawrence to Bohr, 8 Jul 1937 (3/3).
in 1938. They made UX2 the excited metastable state and the only direct descendent of UX1 , and UZ the rare result of UX2 nuclei that could not restrain their gamma radiation. By an appropriate attribution of the complex beta rays, they showed that UZ belonged on the first, and UX2 on the second Sargent curve.[77]
Feather and Bretscher had the encouragement of the first unequivocal example of isomerism among artificially active elements. That was the work of Arthur Snell, the most complete and exact bit of radiochemistry accomplished at the Laboratory to that time (August 1937). He was inspired by the apparent existence of too many active bromines. Fermi's group had found two, with periods of 18 minutes and 4.5 hours, which they supposed to arise by slow neutron capture in the two stable bromine isotopes, Br79 and Br81 . Then a Soviet physicist, I.V. Kurchatov, found a third activity (t = 36 hr) in bromine hit by neutrons from a Rn-Be source, for which there was no obvious available antecedent in natural bromine. Kurchatov proposed that whereas Fermi reactions were of the ordinary type (n,g ), his occurred via the then still unestablished route (n,2n), giving rise to a suppositious active Br78 .[78]
Snell checked these results by trying to make the Italian radioisotopes Br80 and Br82 and the Soviet Br78 in other ways than by neutron bombardment of natural bromine. He examined no fewer than twenty-eight different reactions involving As, Se, Br, Kr, and Rb activated by deuterons, alpha particles, and neutrons. Among his most significant results: Br78 does exist—he made it by (a ,n) on the single arsenic isotope As75 and by (d,n) on Se77 —but its period (6 min) and its decay mode (positron emission) exculpated it from responsibility for Kurchatov's activity; Br83 , hitherto
[77] Gamow, PR, 45 (1934), 728–9; von Weizsäcker, Nwn, 24 (1936), 813–4; Meitner, in Bretscher, Kernphysik , 41; Bethe, RMP, 9 (1937), 225–6; Feather and Bretscher, PRS, A165 (1932), 530–1, 545–50. Cf. Flammersfeld in Frisch, Trends , 71–6.
[78] Amaldi et al., PRS, A149 (1935), 522–58, and Amaldi, Phys. rep., 111 (1984), 128ff.; Kurchatov et al., CR, 200 (1935), 1201–3; Livingston and Bethe, RMP, 9 (1937), 348–50. According to Golovin, Kurchatov , 25–6, Kurchatov proposed the existence of bromine isomers to the Soviet Academy of Sciences in March 1936.
entirely unknown, also exists (t = 2.5 hr); and the three previously known activities had to be shared among Br80 and Br82 . From his inventory of twenty-eight reactions, Snell could show that both Fermi activities belong to Br80 . And, to complete his happiness, he created Kurchatov's activity (Br82 ) by deuterons on selenium. With the advice of Bohr—this result dates back to April 1937 or earlier—Snell pinned the reaction on Se82 , since Se81 , which could have given rise to Br82 by the familiar (d,n) reaction, does not exist naturally. On this explanation, Snell gave the first example of a (d,2n) reaction.[79] The excitement of the discoverers of such arcana may be hard for outsiders to share. But in the breast of the nuclear physicist, they inspired "great joy."[80]
The joy came also to McMillan, Kamen, and Ruben, who were still laboring on Lawrence's dirty platinum when Snell completed his work. Platinum appeared to have three periods, all activated by slow neutrons and decaying by fast electrons, and gold and iridium behaved similarly. It seemed too much of a good thing. "The results of this work so far [McMillan's group wrote] do not seem to be capable of any simple explanation without the introduction of a fantastic number of isomeric nuclei."[81] Candidate isomeric nuclei turned up everywhere after the summer of 1937, and frequently for the first time in Berkeley; of the seventeen pairs of artificial isomers established by 1939, eleven were discovered or first confirmed by members of the Laboratory.[82]
Livingood and Seaborg were the most successful hunters (plate 8.2). Earlier investigators had not carried their separations of zinc activated by neutrons very far. Seaborg went further. He and
[79] Snell, PR, 52 (1937), 1007–22, sent 3 Aug; Livingston and Bethe, RMP, 9 (1937), 326, are agnostic about (d,2n); Livingood and Seaborg, RMP, 12 (1940), 38, accept it. Cf. Segrè and Helmholz, RMP, 21 (1949), 271–2, 291.
[80] Walke, as quoted in Cooksey to Lawrence, 17 June 1937 (4/21); cf. Seaborg, Jl., 1 , 296 (26 Nov 1937), on the excellence of Snell's work. The original three bromine activities were confirmed by Bothe and Gentner, Nwn, 25 (1937), 284, letter of 19 Apr, by photoexcitation.
[81] McMillan, Kamen, and Ruben, PR, 52 (1937), 375–7, rec'd 19 Jul.
[82] Walke, Rep. prog. phys., 6 (1939), 26, and Walke, Williams, and Evans, PRS, A171 (1939), 360–82. Some thirty-four isomeric pairs were known in 1940, and over seventy-five pairs by 1949. Frisch, Ann. rep. prog. chem., 36 (1940), 16–21; Segrè and Helmholz, RMP, 21 (1949), 255–9.
Livingood in consequence added two new nickels to their treasury of isotopes, cleaned up earlier misattributions by Livingood and by Robert Thornton, and identified isomers of Zn69 . In all there were but three zincs, which Livingood and Seaborg prepared in a total of eleven different ways. Their attributions have stood.[83] Then there were iron and its neighbors. With separated fractions from fifteen bombardments of iron, four of chromium, and two of manganese, all by deuterons, and several irradiations of the same with alpha particles and neutrons, Livingood and Seaborg straightened out a great many reactions and uncovered a pair of isomers in Mn52 .[84] The most interesting and complex of the isomers came to light during a lengthy study of activated tellurium, which attracted Livingood and Seaborg not only as another radiochemical puzzle, but also as a possible quarry for a useful radioactive iodine.
Their first irradiation of tellurium with deuterons took place on March 26, 1938. They moved on to iodine, then back to tellurium, glimpsing, losing, and finally establishing the existence of an iodine with the biologically useful period of 8 days. That brought them through the summer. In September Livingood left for Harvard; he set up an electroscope in his kitchen to examine samples mailed him by the indefatigable Seaborg.[85] There were too many telluriums. Joseph Kennedy, Seaborg's graduate student, helped to untangle them. He soon confirmed his collaborators' conjecture that the 8-day iodine, I131 , descended from not one but two telluriums, isomers of Te131 with periods of 1 hour and 1.2 days. But on further examination, the hour period shrank to 45 minutes and then split into two, of 30 and 55 minutes; and, in addition to the three telluriums that had replaced Kennedy's two, there were three more, of periods of 10 hours, 1 month, and several months. After
[83] Livingood and Seaborg, PR, 53 (1938), 765, and PR, 55 (1939), 457–63, rec'd 15 Dec 1938; Thornton, PR, 53 (1938), 326, and Livingood, PR, 50 (1936), 425; Seaborg, Jl., 1 , 412, 416 (3 and 15 Dec 1938). Alvarez, PR, 53 (1938), 606, and Mann, PR, 54 (1938), 649–52, also helped to purge Thornton's "zincs."
[84] Livingood and Seaborg, PR, 54 (1938), 51–5, rec'd 10 May, and PR, 54 (1938), 391, rec'd 11 Jul; Seaborg, Jl., 1 , 360–1 (3 Jul 1938).
[85] Seaborg, Jl., 1 , 333, 339–43, 347–8, 353, 359–61, 367, 373, 377–8 (26 Mar–6 Sep 1938); Livingood and Seaborg, PR, 54 (1938), 775–82, rec'd 7 Sep 1938, which itemizes five radioiodines, three of which were new.
three weeks of hard work in January 1939, Kennedy and Seaborg proved that the 30-minute (corrected to 25-minute) activity was a parent of I131 and the lower of a pair of isomers whose upper level was the activity of 1.2 days.[86]
To shorten a story already sufficiently long, Seaborg, Livingood, and Kennedy labored on the tellurium system until the end of December 1939, when they declared on the basis of sixteen different reconfirmed reactions that there exist precisely four radiotelluriums, three of which come in two isomers each. With a clever chemical technique, soon to be described, they showed which isomeric state was the lower; and, with the help of two graduate students, Carl Helmholz and David Kalbfell, who examined the specimens in a beta-ray spectrograph, they identified conversion electrons knocked out by the gamma rays emitted in transitions from upper to lower isomeric states.[87]
After Livingood, Seaborg took up with an isomer hunter of even greater resourcefulness, Segrè, who brought the experience of identifying isomeric Cu65 in cyclotron scrap.[88] Segrè required something in addition to Seaborg before he would begin their planned search for isomers of element 43: a better detector than Livingood and the Laboratory's other hunters of new activities employed. This detector consisted of an ionization chamber of a type used in Rome connected to a dc amplifier built by Lee DuBridge during a summer at Berkeley.[89] Segrè's electrometer (plate 8.3) later served in the detection of H3 , C14 , and plutonium. It began by registering a beta ray, no gamma ray, and an x ray from activated molybdenum, which Seaborg and Segrè diagnosed as an electron converted from a gamma ray with almost 100 percent efficiency and the associated K radiation of element 43. They sent a letter to the Physical Review announcing these results; but
[86] Seaborg, Jl., 1 , 364, 394, 403, 406–7, 424, 427–30, 432–6, 439 (28 Jul, 11 Oct, and 8 Nov 1938, 11–30 Jan and 2 Feb 1939); Seaborg and Kennedy, PR, 55 (1939), 410, letter of 31 Jan.
[87] Seaborg, Livingood, and Kennedy, PR, 55 (1939), 794, letter of 31 Mar.
[88] Seaborg, Jl., 1 , 361 (9 Jul 1938); Perrier, Santangelo, and Segrè, PR, 53 (1938), 104–5. Cf. Segrè in Nicolini et al., Technetium , 8.
[89] Seaborg, Jl., 1 , 361–4 (9–18 Jul 1938); Segrè to Lawrence, 7 Jan 1938, and reply, 7 Feb 1938 (16/14). The standard armamentarium of detectors in 1940 is described in Seaborg, Chem. rev., 27 (1940), 207–10.
their pleasure suffered an interruption when Lawrence told them that Oppenheimer thought so high a degree of conversion impossible. Lawrence asked them to withdraw their report. The frequency with which the Laboratory had had to retract published results had declined since Lawrence had gone full time into fundraising and administration, and he did not wish to risk a throwback. Seaborg and "a rather agitated Segrè," who had lately been the master of his own ship, complied. Ten days after this contretemps, the Physical Review published a letter from Bruno Pontecorvo describing a similarly high conversion in rhodium. Lawrence conceded that Segrè and Seaborg should resubmit.[90]
They soon confirmed their observations. They asked a graduate student, Philip Abelson, who had built a good spectrograph, to determine whether the x radiation they had found belonged to element 43 (it did), and they established that the lower isomer associated with the 6-hour activity had a life of at least forty years. These are the isomers of Tc99 (the lower has a life of almost a million years), the clinically important species of technetium.[91] The collaboration was close and demanding. Segrè participated in the chemical separations and Seaborg in the physical measurements, and they wrote up their results together.[92]
Since atoms of isomeric nuclei possess precisely the same chemical properties, it might not seem advisable to think about ways to separate isomers chemically. Segrè thought it could be done, however, by modifying the Szilard-Chalmers process, in which nuclei rendered active by an (n,g ) process (e.g., radioiodine) and knocked out of a compound (e.g., ethyl iodide) by the neutrons they absorb, are collected by combining them with suitable molecules (e.g., in a precipitate of silver iodide). His thought was at first received unenthusiastically because the recoil from the isomeric
[90] Seaborg, Jl., 1 , 367–8, 376, 381, 383, 390, 393, 396 (26 Jul, 1 and 30 Aug, 14 and 20 Sep, 8 and 14 Oct 1938); Segrè and Seaborg, PR, 54 (1938), 772, letter of 14 Oct (originally 14 Sep); Kalbfell, ibid., 543, determined the energy of the conversion electron.
[91] Lawrence to Sproul, 31 Dec 1938 (16/14); Seaborg, Jl., 1 , 399, 405, 412–5 (23 Oct, 12–14 Nov, 3–10 Dec 1938); Seaborg and Segrè, PR, 55 (1939), 808–14; Segrè, Ann. rev. nucl. sci., 31 (1981), 9.
[92] Seaborg, Jl., 1 , 366ff., 418 (20 Dec 1938), 424 (10 Jan 1939), 437–8 (1 Feb 1939).
transition seemed insufficient to free an atom from its chemical bonds. Ralph Halford, an instructor in the Chemistry Department, with whom Seaborg discussed the matter, thought that he might be able to effect a chemical separation. Segrè and Seaborg irradiated a liter of ethyl bromide, which, after treatment by Halford, gave a hydrobromic acid enriched in the lower isomer of Br80 . This isomer thus stood revealed as the 18-minute activity observed by Fermi's group, by Kurchatov, and by Snell.[93] Seaborg and Kennedy immediately applied the scheme successfully to tellurium, confirming the double origin of 8-day iodine; and then, together with Segrè, tried hard, but with few positive results, to separate isomers of several other metals, from manganese to platinum.[94]