2—
A Fruitful Business
The episode with the deuterons exposed weaknesses in the work of the Radiation Laboratory that were only partly corrected before its mobilization in World War II. The pressure for quick results to encourage financial backers continued, with consequent hype and hurry. Errors plagued output: as Lawrence anticipated in
[68] Among the first to follow up the demonstration by Chadwick and Goldhaber were Szilard and the chemist T. Chalmers, who recommended radium gamma rays on beryllium. Szilard and Chalmers, Nature, 134 (29 Sep 1934), 494–5, in Szilard, CW, 1 , 145–6.
[69] Cf. Feld in Segrè, Exp. nucl. phys., 2 (1953), 380–4.
answering Cooksey's condolences over the death of the disintegration hypothesis, the Laboratory's mission and method guaranteed mistakes. "I have gotten over feeling badly," he wrote. "We would be eternally miserable if our errors worried us too much because as we push forward we will make plenty more." He worried a bit about the consequences. As a palliative he proposed that meetings of theorists should always include leading experimentalists, who could certify the value of the data, "Theoretical physicists," said he, forgetting his own persistence in error, "so often are liable not to appreciate which experimental observations are trustworthy."[70]
The proportion of solid results did increase, however, owing partly to Lawrence's resolution, which he sometimes kept or imposed on others, to finish and write up one research project before rushing to another; and owing largely to the appearance in the Laboratory of experienced researchers, who could design and carry through their own projects. Their presence brought something of the Cavendish pattern to the Laboratory. Just as Cockcroft and Walton, Oliphant and Rutherford, Chadwick and Lea, Walton and Dee, formed the nuclei of small groups that tackled similar problems from different points of view, so now Franz Kurie and Edwin McMillan, National Research Fellows for 1933/34, helped break the Laboratory from exclusive preoccupation with Lawrence's programs for physical research and machine design.
Kurie spent the first six months of his fellowship year fashioning a fine cloud chamber on the principle of Tuve's instrument, which worked "well, damned well as a matter of fact."[71] McMillan spent the same time disenchanting himself with the research project that had brought him to the Physics Department and considering what he might do in the Laboratory. Both found rich research lines under the inspiration of a discovery that also established the purpose, and secured the financing, of the prewar Laboratory. This prepotent discovery was made in France by
[70] Lawrence to Cooksey, 12 Mar 1934 (4/19); to J.A. Fleming, 2 Jul 1935 (3/32), resp.
[71] Kurie to Cooksey, 4 Mar 1934 (10/21), quote; Kurie, RSI, 3 (1932), 655–7; Dahl, Hafstad, and Tuve, RSI, 4 (1933), 373.
Curie and Joliot as they ploddingly reexamined the reactions on which they had based their generous estimate of the mass of the neutron.
Induced Radioactivity
Lawrence was not the only one to suffer for his hypothesis in Brussels. Curie and Joliot ran into formidable opposition to their conception of (a ,ne+ ) reactions—the supposition that boron and aluminum transform under alpha bombardment, with the simultaneous emission of a neutron and a positive electron. In their view, (a ,ne+ ) paralleled (a ,p) and demonstrated that the proton consists of a neutron and a positron.[72] No one doubted the presence of the positive electrons: but, to avoid the heavy neutron and the complexity of the proton, most Solvay participants preferred to place the origin of the positron outside the bombarded nucleus. The subterfuge appeared to work for beryllium, which emits gamma rays as well as neutrons under alpha bombardment, for the gammas might later convert into pairs of positive and negative electrons. But as Curie and Joliot pointed out, this explanation could hardly hold for aluminum, which, according to their experiments, did not emit gamma rays under alpha bombardment and gave out very few (if any) negative electrons in comparison with its positives. Consequently they held to (a ,ne+ ), only to be shot down by Lise Meitner, who had found no neutrons from alpha irradiation of aluminum.[73] Her faulty observation, which she later retracted, was to the Joliots what Chadwick's assurance was to Lawrence. They went back to their laboratory to prove their opponents wrong.
They thought that they could strengthen their argument by showing that neutrons and positrons appeared together and in equal numbers regardless of the energy of the incident alpha particles. Altering the incident energy required inserting absorbers between the polonium source and aluminum target; showing the associated production meant registering the positron on a Geiger counter and a conversion proton (from the neutron) in a cloud
[72] Curie and Joliot, JP, 4 (1933), in Oeuvres , 444–54, esp. 452–3.
[73] Solvay, 1933, 173–7.
chamber. All went as expected down to a certain energy, at which the conversion protons stopped, but not the positrons. Here Joliot confirmed the suspicion that Thibaud had expressed at the time of the Solvay Congress, that some radioactive bodies can emit positive electrons.[74] Joliot next tried the experiment with alpha particles of full energy. After the irradiation he removed the polonium source altogether. Still the positrons appeared for their allotted three minutes.
Joliot had made the aluminum radioactive by hitting it with alpha particles. He had discovered a two-step process, an (a ,n) reaction resulting in the creation of a new, unstable isotope of phosphorus followed by a positron decay to a stable isotope of silicon. The two-step achieved the same end as the single, straightforward, old-fashioned reaction (a ,p) would have procured. The intermediate product in the two-step brought not only confirmation of the Parisian heavy neutron, but something much more important, and altogether new: artificially created radioactive substances, which could be identified chemically by the carrier technique developed to analyze the products of natural radioactive decay. With the help of his wife, Joliot demonstrated that the three-minute activity followed the chemistry of phosphorus and that the fourteen-minute activity produced by (a ,n) on boron followed that of nitrogen. They brought a vial of one of their new creations to old Madame Curie, then dying of leukemia. Joliot described the scene. "I can still see her taking [it] between her fingers, burnt and scarred by radium. . . . This was without doubt the last great moment of satisfaction in her life."[75]
News of the discovery did not provoke much satisfaction when it reached Berkeley via Time and Nature . Lawrence, Livingston, and Henderson spent the weekend of February 24/25, 1934, repeating the experiments of Joliot and Curie in their own way, with deuterons from the cyclotron in place of alpha rays from polonium. "To our surprise we found that everything we bombar-
[74] Breit to Tuve, 9 Oct 1933 (MAT, 12/"spec. letters").
[75] Joliot, quoted in Goldsmith, Joliot-Curie , 57; Curie and Joliot, CR, 198 (15 and 29 Jan 1934), 254, 559, in Oeuvres , 515–9, and Nature, 133 (1934), 201, 721 (reaffirming the neutron mass), in Oeuvres , 520–1. Cf. Amaldi, Phys. rep., 111 (1984), 109.
ded . . . is radioactive." And also to their chagrin. "We have had these radioactive substances in our midst now for more than half a year. We have been kicking ourselves that we haven't had the sense to notice that the radiations given off do not stop immediately after turning off the bombarding beam."[76] It was not that the effect hid near the limit of detection: for aluminum it over-powered the Geiger counter. That made missing it—and the Nobel prize awarded to Joliot and Curie the following year—particularly galling. Later Lawrence's junior collaborators recalled what they remembered of their feelings. Thornton: "We looked pretty silly. We could have made the discovery at any time." Livingood: "We felt like kicking our butts."[77]
According to the standard apologies, the Laboratory missed the discovery because the same switch operated the cyclotron and the Geiger counter, and so turned off the means of detection with the initiating beam. It may be doubted that the equipment was so peculiarly wired. And even if it were, the fact that no accelerator laboratory thought to make substances radioactive remains to be explained. The Cavendish had looked for delayed activity in aluminum, among other elements, during the 1920s, with natural sources of alpha particles; they had found nothing, because, since neither the neutron nor the positron had yet been noticed, they had no idea what to look for, and sought to detect short-lived proton or alpha emitters with scintillation screens. As one frustrated investigator wrote, more truly than he knew, "It is very unfortunate that time did not permit of further experiments with a wide variety of elements and with devices for the detection of radiation of other kinds." Despite their larger sources and greater knowledge, accelerator builders did not reopen the matter.[78] It was not a question of labor-saving switches, but of labor-saving thinking. One expected either transmutation to known, stable species, or reduction to fundamental pieces of nuclei, but not the creation of brand-new radioelements.
[76] Lawrence to Beams (2/26), to J. Boyce (3/8), quote, both 27 Feb 1934; Kurie to Cooksey, 4 Mar 1934 (10/21).
[77] Davis, Lawrence and Oppenheimer , 60.
[78] Shenston, Phil. mag., 43 (1922), 938–43, quote on 943; Blackett, PRS, A107 (1925), 357; Rutherford, Chadwick, and Ellis, Radiations , 312–3.
Joliot and Curie had raised the possibility that deuterons might create artificial activities in their announcement of their discovery in Nature . They gave C12 (d,n)N13 as an example. Four groups stood ready to follow up the suggestion: Cockcroft's, Tuve's, Lauritsen's, and Lawrence's. Cockcroft at first preferred his original projectile and made N13 by stuffing a proton into C12 . Later he and his associates confirmed (d,n) reactions on boron, carbon, and nitrogen at energies under 600 kV. Tuve did not interrupt his investigations of Berkeley's mistakes to follow up Joliot and Curie's suggestions. Lauritsen did. He sent preliminary results on (d,n) reactions for publication on the same day that Lawrence did.[79]
The difference in research objectives between Caltech and Berkeley deserves notice. Henderson, Livingston, and Lawrence examined fourteen elements, from lithium to calcium, under bombardment by 1.5 MeV protons and 3 MeV deuterons; they noticed signs of proton activation only in carbon and supposed the ubiquitous deuteron activation to arise via (d,e+g ) reactions. They gave few and only rough quantitative data, for example, a half-life of the boron activity of about two minutes. In an unpublished lecture, Lawrence conceded that none of the measurements could stand up to the "very significant experimental findings" of Crane and Lauritsen.[80] The Caltech group limited its initial studies to 0.9 MeV deuterons on beryllium, boron, and carbon, understood that the activities they created arose from (d,n) reactions, showed that the half-life of the activity made from carbon agreed with that of N13 as given by Joliot and Curie, had their colleagues Carl Anderson and Seth Neddermeyer confirm the existence of positrons in the decay of N13 by observations with the Caltech cloud chamber, showed that the gamma ray found at Berkeley probably came from electron-positron annihilation, and determined the half-life of the boron activity to be ten times as large as Berkeley made it. Where Lauritsen's group gave careful and reliable
[79] Joliot and Curie, Nature, 133 (1934), 201–2, and in Oeuvres , 521; Cockcroft, Gilbert, and Walton, Nature, 133 (1934), 328 (letter of 24 Feb), and PRS, A148 (1934), 225–40 (rec'd 26 Sep); Crane, Lauritsen, and Harper, Science, 79 (1934), 234–5 (letter of 27 Feb).
[80] Lawrence, "Outline of lecture on artificial radioactivity," n.d. (40/16).
information about a few features of the new terrain, Lawrence's characteristically bolted through an impressionistic survey.
The usual tendency in the Radiation Laboratory may have been strengthened in this case by the increasing difficulty in maintaining the hypothesis of deuteron disintegration and by Lawrence's desire to assimilate their earlier results to the great Parisian discovery and insinuate an anticipation of it. "Indeed, in the light of our recent experiments in which neutrons and protons were found to be emitted from many elements when bombarded with deutons, the possibility presented itself that in these nuclear reactions [!] new radioactive isotopes of many of the elements might be formed." So Henderson, Livingston, and Lawrence hinted in the Physical Review in 1934. Later and in private Lawrence may have claimed more. A representative of the Rockefeller Foundation recorded this remark: "[Lawrence] said that they had discovered artificial radioactivity before Joliot and Curie did, but wishing to be overly sure [!] of their results, did not publish and were taking time to repeat the work."[81]
Some Physics Fallout
Lewis had very probably been the instigator in the deuteron experiments.[82] An extremely clever man with a secure reputation as a chemist, he had little to lose by backing poor physics; a hasty man, guided by smell and inspiration, he was the worst sort of collaborator for Lawrence. Also, he had ideas about atomic and nuclear structure that differed in principle from those physicists entertained. He had sponsored a static atom, in which electrons stand at the vertices of a polyhedron centered on their nucleus, in competition to Bohr's dynamic-electron model. His colleague Wendell Latimer had extended the scheme to the nucleus, which he supposed to consist of as many alpha particles as possible joined together in equilateral pyramids. He had no place for neutrons; they come to life outside nuclei, by couplings of protons
[81] Henderson, Livingston, and Lawrence, PR, 45 (1934), 428–9 (letter of 27 Feb); Crane and Lauritsen, PR, 45 (1934), 430–2 (letter of 1 Mar); Amaldi, Phys. rep., 111 (1934), 115–18; Frank Blair Hansen, "Trip report," 3–13 Apr 1938 (RF, 1.1/205).
[82] Lawrence to Potter, Pierce, and Scheffler, 3 June 1935 (35/7).
and neutrons, couplings easily broken and reformed, in his opinion, so as to make hydrogen atoms, deuterons, mass-three helium, and so on.[83] It was a qualitative, tinker-toy world, in which one part—such as the strength of the stick holding the deuteron together—could be changed without doing violence to other parts. Even before the detection of heavy water, a student of Berkeley's nuclear family foresaw the exploitation of the cyclotron to check the chemists' physics. "It may be," he wrote, "the research by Professor Lawrence and Dr Livingston will offer means of proving or disproving it."[84]
Fowler was astonished at the eagerness and confidence with which Berkeley chemists did physics. "If they do any chemistry it's kept well out of sight."[85] The fiasco of his work with Lawrence did not discourage Lewis. In 1936 he offered an explanation of neutron scattering that was as disruptive to theory as exploding deuterons. Bethe reviewed the manuscript for the Physical Review : "I think it is an extremely instructive example of the dangers of purely qualitative arguments." Lewis's former confederates at the Laboratory would not follow him: "The effect [for which Lewis argued] is so feeble, and the instruments so barbaric (he doesn't want to hear about counters) that no one believes him here."[86] With this rejection, Lewis ceased to play a direct part in the work of the Laboratory.
Kurie did not allow himself to be drawn into the search for activities induced by deuterons. He took on instead the elucidation of the mechanism of neutron activation. He studied closely the forked tracks created in his cloud chamber during neutron irradiation of nitrogen. In contrast to Rutherford's prompt reaction N14 (a ,p)O17 , Kurie thought he saw the delayed reaction N14 (n)N15® B11 + a + Q , where Q designates the energy carried away in gamma rays and N15 is an "intermediate nucleus." Kurie reported this first piece of careful physics done with cyclotron
[83] Lewis, Valence ; Latimer, JACS, 53 (1931), 981–90, and JACS, 54 (1932), 2125–6; Kohler, HSPS, 3 (1971), 343–76.
[84] G.A Pettitt, California monthly, 27 (1931), 18–21.
[85] Fowler to Rutherford, 22 Mar [1933] (ER).
[86] Bethe to Buchta, 6 May 1936 (HAB, 3); Nahmias to Joliot, 28 Apr 1937 (JP, F25); Seaborg, Jl., 1 , 242; Lewis and Schutz, PR, 51 (15 June 1937), 1105.
beams and a good detector at a meeting of the American Physical Society in Berkeley in June 1934.[87] In the fall he gave a seminar on his work. "For the first time in my life [it] was not a recital of numbers but of ideas." Everyone seemed convinced, except Oppenheimer, who worried about the powerful gamma ray that, if the conservation laws held, must be emitted in the formation of the intermediate nucleus. "Robert says that the evidence is well explained by it but he 'wishes it were not so'"[88] Kurie published his hypothesis and measurements—the sort of paper Lawrence was "proud to have from the lab"—and it was not so. As Bethe laid down the law, in 1937: "This [intermediate excited nitrogen nucleus] has no justification either theoretically or experimentally, and has subsequently been discarded." Apparently Kurie had overinterpreted his tracks.[89] His work was not to end the Laboratory's stream of flawed physics.
At the same meeting in Berkeley of the American Physical Society at which Kurie spoke about delayed disintegrations, McMillan discussed preliminary results of his study of gamma rays excited by 1.15 MeV protons driven against fluorine. He had taken up the subject on the advice of Oppenheimer, who had two objects in mind. For one, the energy balance in nuclear reactions could not be struck without knowledge of the amount carried away by high-frequency radiation. For another, and of greater interest to Oppenheimer, gamma rays from some artificially induced reactions might well be more energetic than any from natural souces; if so, they would permit a check of the theory of pair production—the materialization of a gamma ray into a positron and an electron in the field of a nucleus—at higher energies than previously available. Oppenheimer and one of his students, Wendel Furry, had a calculation of pair production in such regions in hand.[90]
McMillan's experimental arrangement occupies figure 4.2. The Lauritsen electroscope consisted of a quartz fiber suspended from a wire and carrying on its free end a crosshair viewed against a
[87] Kurie, PR, 46 (1934), 324; Lawrence to Gamow, 19 May 1934 (7/25).
[88] Kurie to Cooksey, 18 and 27 Sep 1934 (10/21).
[89] Kurie, PR, 47 (1935), 98–105; Lawrence to Cooksey, 29 Dec 1934 (4/19); Livingston and Bethe, RMP, 9 (1937), 338, 339 (quote), 341.
[90] McMillan, PR, 46 (1934), 325, 868, 870.
scale in the microscope eyepiece. Foils of various metals allowed determination of the absorption of the gamma rays from the target as a function of atomic number Z , and thereby distinction of the portion owing to pair production (which increases as Z3 ) from contributions from the photoeffect and the Compton effect. McMillan took elaborate precautions not to be duped by contaminants. The best results came from fluorine, which produced fine energetic gamma rays, some 5.4 MeV, in the reaction F19 (p,a )O17 . Pair production by these rays agreed perfectly with the curvature of the tracks of the most energetic photoelectrons they produced in a cloud chamber at Caltech.[91] McMillan's were the first experimental results in nuclear physics obtained at the Laboratory and controlled by a quantitative theory that have stood up under bombardment from other investigators.
Tracer Business
While McMillan and Kurie went their independent ways and Lawrence's group continued firing deuterons, another capital discovery arrived from Europe. Fermi had reasoned that because they carry no electrical charge, neutrons should be able to gain admission to nuclei more readily than protons, deuterons, or alpha particles; and that they were the only way to activate nuclei heavier than phosphorus, where even Berkeley's deuterons could not penetrate.[92] A methodical man, Fermi began with hydrogen as a target, then lithium, and so on, at first with no luck. He was not discouraged before reaching fluorine, which released electrons when struck by the neutrons from his modest radon-beryllium source. That was on March 25, 1934. Fermi then mobilized his collaborators, Edoardo Amaldi, Franco Rasetti, and Emilio Segrè, who raced at California speeds to procure and bombard specimens of all the elements in the periodic table. By July they had reached uranium and detected no fewer than forty artificial isotopes of the
[91] McMillan, PR, 46 (1934), 871–2; Henderson, Livingston, and Lawrence, PR, 46 (1934), 38; Crane, Delsasso, Fowler, and Lauritsen, PR, 46 (1934), 531.
[92] Cooksey had suggested the effect Fermi sought the year before: "I suppose that the neutron in the H is the boy that when given an introduction in the company of a proton raises all this merry hell." Cooksey to Lawrence, 6 May 1933 (4/19).
Joliot-Curie type, but which decayed by emitting negative rather than positive electrons. Of particular interest, for reasons to appear, was Na24 , with a half-life of fifteen hours, which Fermi's group could create either from Al27 via (n,a ) or from Mg24 via (n,p).[93]
The discoveries of artificial radioactivity and of the capacity of neutrons to effect transformations beyond the reach of deuterons coalesced several disconnected ingredients of Lawrence's research, machine building, and fund-raising into an enduring whole. Even before Fermi's discovery, at the Solvay Congress of 1933, Lawrence had emphasized the importance of deuteron bombardment as a source of neutrons: with a current of only 0.01 µA the Laboratory had a source that appeared to be more powerful than any likely to be obtained from natural radioelements. One standard estimate, used by Fermi, gave 1,000 neutrons/sec as the output of one mCi of Rn-Be; another, used by Joliot and Curie, made the efficiency of nuclear reactions induced by alpha particles from natural radioactive sources about 10–6 or 10–7 ; hence 0.01 µA of deuterons, or 6.3·1010 particles/sec, would give rise to around 10,000 neutrons/sec, which would have required the radon from 10 grams of radium. This was an exaggeration. Later elaborate measurements by Amaldi and Fermi and by Amaldi, Hafstad, and Tuve raised the yield from a mCi of Rn-Be to 25,000 neutrons/sec and the conversion efficiency of 1 MeV deuterons on beryllium to 3·10–5 , whence 0.01 µA of million-volt deuterons would give as many neutrons as 70 mCi of Rn-Be. Fermi did his first experiments with 50 mCi of radon and at times used 700 mCi, which his supplier, G.C. Trabacchi, drew from a gram or so of radium at the Istituto di sanità pubblica in Rome.[94] Fermi's source of
[93] Segrè in Fermi, CP, 1 , 639–41; Fermi, Ric. sci., 5 (1934), 283, in CP, 1 , 645–6 (dated 25 Mar 1934), announcing (n,a ) reactions on F and Al; Nature, 133 (1934), 757, letter of 10 Apr, describing activities of two dozen elements; Fermi, Amaldi, D'Agostino, Rasetti, and Segrè, PRS, A146 (1934), 483–500, in CP, 1 , 732–47, esp. 746–7 (rec'd 25 Jul 1934). Amaldi, Phys. rep., 111 (1934), 130, considers the Rome group to have been "probably the first large physicists' team working successfully for about two years in a well organized way."
[94] Lawrence, intervention in Solvay, 1933, 68. Cf. Livingston, Henderson, and Lawrence, PR, 44 (1 Nov 1933), 782–3 (letter of 7 Oct); Fermi, Ric. sci., 5 (1934), 283, in CP, 1 , 645–6, and Nuovo cim., 11 (1934), in CP, 1 , 715; Amaldi, Fermi, Rasetti, and Segrè, Nuovo cim., 11 (1934), in CP, 1 , 725; Fermi, Amaldi,D'Agostino, Rasetti, and Segrè, PRS, A146 (1934), in CP, 1 , 733, 745; Amaldi and Fermi, Ric. sci., 7 (1936), in CP, 1 , 887, and PR, 50 (1936), in CP, 1 , 892, 937–8; Jaeckel, Zs. f. Phys., 91 (1934), 493; Paneth and Loleit, Nature, 136 (1935), 950; Amaldi, Hafstad, and Tuve, PR, 51 (1937), 896.
neutrons compared well with the cyclotron vintage 1933, but it could not compete in total output with later versions or with any other artificial source of a µA of deuterons above a million volts. Still, a natural source retained its usefulness in situations where constancy, reproducibility, and good geometry were especially important.
By April 1934 the Laboratory was busy following up Fermi's results, which greatly complicated inventorying nuclear reactions. "Because of the magnitude of the radioactivity induced by neutrons it was immediately apparent [Lawrence wrote Gamow] that we should study thoroughly the phenomena before trying to untangle other nuclear reactions produced by proton and deuteron bombardment." And there was another reason, as Lawrence wrote another interested party, with interests much different from Gamow's. "We are not unmindful [he told Poillon] of the possibility that we may find a substance in which the radioactivity may last for days instead of minutes or hours, in other words, a substance from which we could manufacture synthetic radium. The probability is not at all remote at the present time. I am very glad that we have a patent on the cyclotron."[95]
With the help of Henderson and Livingston, Lawrence got tremendous amounts of radioactive aluminum, copper, silver, and fluorine; owing to a new set of dees made wider at the center to accommodate a larger ion source, the cyclotron was putting out 0.7 µA of 3 MeV deuterons, which shook over 500,000,000 neutrons/sec from its beryllium target. The poor estimate then still accepted, 1 mCi of Rn-Be gives 1,000 neutrons, implied that the cyclotron had the value as a neutron source of half a kilogram of radium. (Rutherford then estimated the equivalent of Oliphant's machine as a tenth of a gram of radium).[96] When
[95] Lawrence to Poillon, 3 Mar 1934 (15/16A); cf. Lawrence to Kast, 13 Apr 1934 (12/32).
[96] Kurie to Cooksey, 4 Mar 1934 (10/21); Lawrence to Beams, 13 Apr 1934 (2/26), to Cooksey, 21 May 1934 (4/19), to Oliphant, 5 June 1934 (14/6); Livingston, Henderson, and Lawrence, NAS, Proc., 20 (1934), 470–5; Rutherford to Fermi, 20 June 1934, in Amaldi, Phys. rep., 111 (1984), 133.
Franco Rasetti arrived in Berkeley in September 1935 to investigate artificial neutron sources, the cyclotron could generate 9 µA of 3.5 MeV deuterons, and therewith ten billion neutrons per second. Rasetti was flabbergasted by the "enormous superiority" of an artificial method that yielded what, by the natural way and the standard conversion, would have required the radon from kilograms of radium. In Rome he had made a certain activity by placing a silver target on top of a 500 mCi source; in Berkeley he got the same amount of activity with the target fifteen feet from the cyclotron wall.[97]
Lawrence did not find it necessary to trouble his backers with the names of Joliot, Curie, and Fermi. In writing Poillon and Ludwig Kast, president of the Macy Foundation, he claimed, what was true, that he had been the first to induce radioactivity in a range of elements by deuteron bombardment and allowed them to infer that he had discovered artificial radioactivity. A similar invitation was offered in the news that "we have found that an analogous effect is produced by neutron rays." This skillful reporting brought $2,300 from the Macy Foundation and $5,000 from the Research Corporation for working up materials to make radioactive substances on a large scale.[98] In the summer of 1934, therefore, when the Depression had forced substantial cuts on the Physics Department and Sproul went begging for money for the University, Lawrence found himself in excellent financial shape to perfect his machine for a purpose that had not been dreamed of when he and his associates built it: the creation of new radioelements and their manufacture in quantities sufficient for biomedical research. Before biology, however, comes chemistry. The physicists and machine builders had to transmute themselves into part-time chemists to separate out the various activities created by their neutrons and deuterons and to judge the possible utility of their handiwork for biologists.[99]
[97] Rasetti, Viaggi, 3 (1936), 77–8 (Aug–Oct); Lawrence to Cockcroft, 12 Sep 1935 (4/5).
[98] Lawrence to Kast, 3 May 1934 (12/32), and to Poillon, 15 and 26 Mar 1934 (15/16A); Kast to Lawrence, 23 Apr and 5 June 1934 (13/32); Lawrence to Sproul, 20 Feb 1936 (20/19).
[99] Lawrence to Beams, 17 Sep 1934 (2/26); Pettitt, Twenty-eight years , 40–1, 63.
It remained to find something useful. The plum fell to Lawrence, in September 1934, just after he had lamented to Beams that "the nuclear reactions we have been studying are not particularly novel."[100] Perhaps ignorant that Fermi's group had made the active isotope of sodium, Na24 , by (n,a ) on aluminum and (n,p) on magnesium, Lawrence did the same, by deuteron bombardment of table salt. But whereas Fermi's group had merely reported the existence and half-life of the product, Lawrence, from his special perspective, immediately emphasized the properties that might make it useful for "the biological field." These included its convenient half-life (fifteen hours), its nontoxic chemical character, and the energetic gamma ray that accompanied its disintegration.[101] And, what Lawrence did not make explicit, the fact that the valuable Na24 was isotopic with ordinary sodium made it unnecessary to remove the radioactive atoms from their parent for application. The target and the converted atoms could be administered together.
The essential property of radiosodium for biological research and medical application was the gamma ray emitted by the excited magnesium atoms produced by the decay of Na24 . Lawrence established that the reactions at play are Na23 (d,p)Na24 , Na24® Mg24 * + e– , Mg24 * ® Mg24 + g ; and he estimated that the gamma ray had an energy of over 5 MeV. He was pleased with this result, which made the gamma ray from radiosodium more than three times as hard as the hardest ray from radium, and which showed that he was still capable of solid scientific work. But not precise work. As one of his students, Jackson Laslett, soon showed, his determination of the gamma ray energy erred by excess by about 30 percent.[102] (In fact, the energy of the most energetic gamma ray from Na24 is under 3 MeV.) A gamma ray of 3 MeV was none the less a very useful item in physical research; members of the Laboratory used it to study pair production and the photodisintegration of the deuteron. It also retained its
[100] Birge, History, 4 , xi, 12; Lawrence to Beams, 17 Sep 1934 (2/26).
[101] Kurie to Cooksey, 27 Sep 1934 (10/21); Lawrence to Livingston, 1 Oct 1934, and answer of 12 Oct (12/12), mentioning Fermi's work; Lawrence, "Notebook," 27 Sep 1934 (40/14), and PR, 46 (1934), 746.
[102] Lawrence, PR, 47 (1935), 25; Lawrence to Cockcroft, 12 Feb 1935 (4/5).
promise for the health sciences. Lawrence's hopes could easily sustain a reduction of 30 percent. He wrote in December 1934: "We have succeeded in producing radioactive substances that have properties superior to those of radium for the treatment of cancer, and probably before long we shall make available to our medical colleagues useful quantities of radiosodium."[103]
The rate of production rose quickly. Within two months of first production, more than a mCi of radiosodium had been made and improvements in manufacture were under way that would increase the rate a hundredfold. Two years later Lawrence could make 200 mCi a day, with a current of only 1 µA, and he looked forward to multiplying the yield a thousandfold. With 20 µA, a day's product of radiosodium emitted the equivalent in gamma radiation of 100 mg of radium.[104] These production levels made an impact. Fermi supposed that Lawrence had slipped by a factor of a thousand and had meant to announce a µCi; Lawrence silenced his doubts by sending him a letter containing a mCi of Na24 . Wilhelm Palmaer, president of the Nobel Committee on Chemistry, highlighted the promise of a cornucopia of radiosodium during the ceremonies in which Urey and Joliot and Curie received their prizes. In another happy omen, the Rockefeller Foundation, the greatest patron of biophysics in the 1930s and eventually Lawrence's most generous private benefactor, advertised radiosodium as the exemplar of the cost-effective service to mankind it liked to support.[105]
The salt to make the equivalent of a gram of radium cost less than a penny (in fact much less, since the Myles Salt Company of Louisiana donated crystals of rock salt), and the power for the eight-hour exposure in the cyclotron less than $2. Putting the same point a different way, Science Service headlined its report of the meeting of the American Physical Society of December 1936, at which the Laboratory's Paul Aebersold announced Berkeley
[103] Lawrence to Cooksey, 4 Nov 1934 (4/19), on pair production; Nahmias to Joliot, 27 June 1937 (JP, F25), on photodisintegration; Lawrence to E.B. Reeves, Commonwealth Fund, 7 Dec 1934 (10/18).
[104] Lawrence to Akeley, 14 Nov 1934 (1/12), and to Cooksey, 12 Sep 1936 (4/5); Lawrence and Cooksey, PR, 50 (1936), 1140.
[105] Segrè, Ann. rev. nucl. sci., 31 (1981), 7; Palmaer in Prix Nobel en 1935 , 38, and Nobel lectures, chemistry, 2 , 337; Rockefeller Foundation, "Trustees confidential bulletin," Dec 1937, "Atom smashing and the life sciences" (RF).
production levels, "Machines of science produce radiation equal to $5,000,000 worth of radium." The calculation: the biological effect of the neutrons from deuterons shot at beryllium targets in the cyclotron equalled that of the gamma rays from 125 mg of radium. The reporter estimated the price of radium at $40 a mg and the cost of the cyclotron at under $100,000. "Thus as a radiation source the machine turned in a 50-to-one investment." And more: to get an equivalent neutron yield from a Rn-Be source would have required 10 kg of radium. That should answer "people who urge more practical scientific research and bemoan the apparently wasted ingenuity of those scientists who probe the hearts of atoms."[106]
Radium was passé. Lawrence advised his correspondents against investing in any of the stuff. Radioisotopes from the cyclotron, he said, would soon drive down the price of radium and supplant it in clinical use.[107] This advertisement attracted the attention of Bernard Lichtenberg, director of the Institute of Public Relations in New York. It did not seem good public relations to him, and he complained to Sproul. A mild reprimand went forward. "In explaining the work of the Radiation Laboratory," Sproul's assistant wrote Lawrence, "make sure the listeners realize that radium and radio-active salts are not the same."[108]
Lawrence reserved his more extravagant claims for presentation to his backers in private. He usually spoke modestly about the Radiation Laboratory in public, and allowed his audience to imagine what the future might hold. A lecture given at several colleges and universities under the sponsorship of the Sigma Xi in 1935 is representative. "I hesitate to express views [about the future]," Lawrence said. "I leave it to you to estimate the advantages for radiation therapy and biological research of radioactive substances having practically any desired chemical and physical properties." Two years later, in a second round of Sigma Xi lectures given at ten institutions from Virginia Polytechnic to Oregon State College during May 1937, he could point to the prospects for
[106] Myles Salt to Lawrence, 26 Nov 1934, and Lawrence's response, 4 Dec 1934 (11/24); Science service , 23 Dec 1936, re Aebersold's talk.
[107] E.g., Lawrence to G.M. Schrum, University of British Columbia, 7 Nov 1936 (2/29), and to Earl R. Crowder, M.D., Evanston, Ill., 4 May 1937 (5/5).
[108] G.A. Pettitt to Lawrence, 11 Mar 1937 (20/19).
biological research of machine-made radiophosphorus (P32 ) and radioiron (Fe59 ), the former found by Joliot and Curie in 1934, the latter a discovery of Berkeley cyclotroneers.[109] And he gave his audience a new basis for estimating the possibilities of his products. He had fresh samples of radiosodium airmailed to him for each performance. He called up volunteers, fed them radiosodium, and followed the course of the activity in their blood with a Geiger counter he carried with him. This "vaudeville," as he called it, held attention; no ear-witness could doubt "that we can make really strongly active substances."[110] Berkeley colleagues, including Oppenheimer, served as guinea pigs in local demonstrations, and would-be cyclotroneers elsewhere copied the show for their purposes. Lawrence was pleased to provide the main ingredient for these performances.[111]
Some indication of the impressions desired and the advertisments volunteered may be gathered from a radio interview in the spring of 1939 to which Lawrence brought his hot sodium. He passed a Geiger counter over it. Click, click. He asked his interviewer, Hale Sparks, to put the counter behind his back. Click, click. "You mean to say [Sparks cried] that the radiation is actually passing through my body now?" "Yes." Sparks: "Then the cyclotron has an unlimited future despite its great achievements of the past?" "Yes, indeed."[112]
[109] Lawrence, "Artificial radioactivity" (July 1935), 16 (40/17), quote, and Ohio jl. sci., 35 (1935), 405; letter to S.B. Arenson, 24 Sep 1937 (40/17); Newson, PR, 51 (1937), 624–7; Livingood and Seaborg, PR, 52 (1937), 135.
[110] Lawrence to McMillan, 5 and 11 May 1935, quotes (12/30); to M. Henderson, 3 May 1935 (9/6); and to Boyce, 18 Dec 1935 (3/8); McMillan, PT, 12:10 (1959), in Weart and Philipps, History , 263.
[111] E.g., Robley Evans to Cooksey, 25 June 1937, and to Lawrence, 17 Feb 1937, and Lawrence to Evans, 28 Jan and 23 Feb 1937 (7/8); S.J. Simmons to Lawrence, 8 and 20 Oct 1939 (16/24).
[112] Lawrence to H.A. Scullen, 21 Apr 1937 (40/17), and to Cooksey, 12 May 1937 (4/21); "University Explorer," "Adventures in science," 15 Apr 1939, 16 (40/15); George Volkoff to R. Cornog, 10 Mar 1941 (5/2).