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]