The Error
At Berkeley preparation included improvements in the cyclotron, which made possible production of 0.02 µA of 3 MeV deuterons, and curing "vacuum troubles and other misfortunes" that kept the machine down for most of September.[27] When the streams began to flow, they called forth showers of neutrons from everything they hit, a confirmation most agreeable to the minds, but also threatening to the bodies, of the cyclotroneers.[28] (The Laboratory was so full of stray neutrons that an investigator quirky enough to have tested the fillings in his teeth might have discovered artificial radioactivity.) Another set of doubters then
entered the game. Richard Crane, a junior collaborator of Lauritsen's at Caltech, came to Berkeley, collected some heavy water, dribbled it on a beryllium target in Lauritsen's machine, and got "enormous quantities of neutrons." That was of course most gratifying, "in entire agreement with our expectations," Lawrence wrote Cockcroft, "though the precise interpretation is as yet ambiguous." The point of imprecision was whether Crane's neutrons came from the disintegration of the beryllium target or of the deuteron projectile. In either case, however, Lawrence thought that Crane's evidence favored a value of the neutron mass close to unity.[29]
Once the cyclotron returned to work, Livingston and Henderson found quantitative evidence of the deuteron's instability. They counted the number of "disintegration" protons (some 40,000 per minute registered in their ionization chamber) and then the number of "recoil" protons reported by the same chamber when covered with a wax-coated lead screen (12/min.). Their previous estimate of the probability of the conversion of neutrons to protons in wax suggested that 40,000 neutrons would make around 12 protons. Hence neutrons and protons appeared in equal numbers, which would be necessary if they came from the breakup of deuterons. They realized that this agreeable agreement had "profound theoretical implications" through its relevance to the value of the neutron mass, which they set at 1.0006. Their report, signed also by Lawrence, appeared on November 1.[30]
At Cambridge preparation included completing reports for the Solvay conference to be held in Brussels at the end of October. Cockcroft had responsibility for reviewing particle accelerators and Chadwick for the state of knowledge about neutrons. Lawrence contributed by sending Cockcroft information about the cyclotron and by bringing his latest evidence for disintegration to the Solvay meeting in person. The invitation to attend, at his own expense for travel, was a great honor; Lawrence was but the eighth American so distinguished since the conferences began in 1911
and the only one in 1933. He declared himself "surprized and tremendously pleased,"[31] though the invitation came very late, some six weeks before the meeting (he owed it to Peter Debye, a member of the Solvay scientific committee, who had visited Berkeley the previous summer and taken a fancy to the Sloan x-ray tube).[32] Lawrence went to this most august of physicists' gatherings (fig. 4.1), the first international meeting he had ever addressed, to correct the opinions of Chadwick, Cockcroft, and Joliot, who also had a candidate for the neutron mass. "I particularly want to make some rather extensive remarks on Cockcroft's report."[33]
Fig. 4.1
McMillan's apparatus for studying the absorption of gamma rays by various
materials. The rays scatter at right angles to the proton beam from the target
and enter through the mica window. McMillan, PR, 46 (1934), 868.
Cockcroft ended his report with an unenthusiastic review of Berkeley work. He accepted the alpha particles from the bombardment of lithium and presented the data about the 18-cm protons, but declined to entertain the hypothesis of disintegration. "It is rather superfluous to discuss further the nature of the transformations with proton emission until we have more experimental information." And how to get the information? From improved Cockcroft-Walton machines. The weak current the cyclotron brought to the target, one-thousandth the flux from the "direct" Cambridge method, might well wash out the advantage of the greater efficacy of its faster particles. "Our present information does not suffice for prediction."[34]
Lawrence's extensive comments centered on the cyclotron method and its latest achievements—hydrogen-molecule ions of over 5 MeV, deuterons of 3.6 MeV, a promise of protons at 3.5 MeV, evidence of the disintegration of heavy hydrogen in the fields of target nuclei, and numbers that made the neutron's mass unity. These last remarks made Lawrence himself the object of a bombardment. Heisenberg observed that if disintegration occurred in the electric field of a nucleus, the yield should decline for heavy targets since the deuteron's penetration, and hence the rate of change of force on it, must decrease with increasing atomic number (Z ); for (very) high Z the field would appear to the deuteron to change adiabatically and produce no disintegration at all. That being the case, added Bohr, we might suppose that the deuteron splits after entering a nucleus; but then the speed of the ejected proton should increase with atomic number, like the nuclear Coulomb field, contrary to Lawrence's results.[35]
Then came the experimentalists. Rutherford said that he had found no neutrons from lithium under deuteron bombardment. Chadwick reaffirmed the value of the neutron mass at between 1.0067, which he deduced from the hypothetical reaction B11 (a ,n)N14 , and 1.0072, which he had deduced from Li7 (a ,n)B10 . Joliot and Curie came forward with a neutron still heavier than Chadwick's. Their careful examination of decay products of
alpha-bombardment of boron and other light elements had disclosed quantities of positive electrons along with neutrons. They supposed that these particles came away simultaneously, according to the reaction B10 (a ,ne+ )C13 , and constituted when together the familiar proton. In place, therefore, of Chadwick's initial conception, that n = p + e– , they now proposed p = n + e+ . So much, and much more, was tied up in the question of the neutron mass. Calculations based on the transmutation of B10 made mn = 1.012. This big mass had the advantage of accounting for the stability of the hydrogen atom, since it prevented the spontaneous union of its proton and electron into a neutron; but it made the decay of the neutron into a proton, electron, and neutrino energetically possible.[36] The only difficulty that Joliot and Curie saw with their fat neutron was the conflicting experience in Berkeley. They were prepared to compromise: "It is not impossible that it will be necessary to suppose the existence of neutrons with different masses," theirs being the elementary one and Lawrence's a condensed combination of the elementary with an electron-positron pair.[37]
Lawrence responded to these challenges by invoking suppositious gamma rays, whose inclusion in the energy balance would lower the neutron mass. Chadwick denied the gammas and insisted, against Joliot and Curie, that the neutrons came from the more plentiful isotope B11 , with the mass he had assigned them. After this exchange the theorists could only feign hypotheses and await the outcome of the squabble.[38] As perhaps no one outside France expected, victory eventually fell to Joliot and Curie.
On his way back to Berkeley, Lawrence stuck his head into the Cambridge lions' den. Chadwick bared his claws, to such effect that his behavior needed explanation. It was found in the consideration that he had been the effective director of the Cavendish for some time and was too overworked to observe the niceties of philosophical combat. With the other lions, especially Cockcroft and Rutherford, who licked his chops over Berkeley's "broth of a boy," Lawrence got along well. They merely roared in a friendly way against the hypothesis of deuteron disintegration and pointed their paws at the possibility that Lawrence had contaminated his targets and tank.[39]
Back at home Lawrence mobilized Lewis and switched Livingston and Henderson from trying to withdraw a beam from the cyclotron to clearing up the enigma of the 18-cm protons. Livingston arranged a target holder that would make possible bombardment of many samples in succession to test possible contamination.[40] Working night and day through the Thanksgiving holiday, Lawrence and his group found the yield of protons from deuterons to be unaffected by their efforts to clean up their targets. "Perhaps before long the evidence will be such as to convince the most skeptical, including those at Caltech and even Chadwick."[41]
The Caltech team, Crane and Lauritsen, suggested several possible complications in the analysis of Berkeley's experiments (for example, that the neutron found in deuteron bombardment of lithium might come from Li7 (p)2a followed by Li7 (a ,n)B10 ), and could find no trace of the 18-cm protons.[42] Then Tuve's group, which had the only machine then capable of checking Berkeley's results above a million volts, entered the picture. Lawrence had visited the Carnegie Institution on the way back from Brussels. "I persuaded Tuve to investigate the origin of the 18 cm protons and
the hypothesis of the disintegration of the deuteron right away," Lawrence wrote Cockcroft. "I want to get the matter cleared up as soon as possible and it will be a great help if Tuve, with his independent set up, will investigate the problem."[43]
The experiments went forward everywhere at an American pace. Lewis prepared two samples of calcium hydroxide, one with ordinary and the other with heavy hydrogen. Under bombardment by 3 MeV deuterons the ordinary target showed nothing extraordinary, whereas the heavy target yielded a cornucopia of 18-cm protons. What could be clearer? The bombarding protons broke up the bound deuterons. Lawrence dispatched this "unambiguous proof of d[e]uton disintegration" to the Cavendish; "It would seem now that even Chadwick will agree."[44] The same message went to the East Coast, to Pollard at Yale ("these recent observations definitely rule out the possibility of impurities") and Beams at Virginia ("the deuton is energetically unstable and disintegrates into a proton and a neutron"); to all other physicists through the Physical Review ; and, of course, to the Research Corporation. "We have proved beyond any reasonable doubt that the deuton explodes when struck hard enough. . . . This first definite case of an atom that itself explodes when properly struck is of great interest, not only as a possible source of atomic energy, but especially because it is not understandable on contemporaneous theories. . . . [It] promises to be a keystone for a new theoretical structure."[45] The flaw in the hydroxide experiment, which we shall reveal in a moment, was no more subtle than the hint about cheap energy. So far did hope, ambition, impatience, and a need for benefactors drive Lawrence from the objectivity he would have claimed as the first virtue of the scientist.
When the Cambridge atom splitters returned from Brussels, they had a fountain of dilute heavy water and samples of almost
pure deuterium created in their absence by the finally successful, and consequently now esteemed, Harteck. His achievement came in good time, since Lewis's still had run dry and he could supply nothing until just before the Solvay Congress.[46] Now, with sufficient stock to hand, Rutherford and a research student, A.E. Kempton, returned to the master's old game, let alpha particles from polonium plunge through deuterium gas (the inverse of deuterons on helium), and found no evidence of fast disintegration protons.[47] Cockcroft and Walton sent deuterons against copper and gold and likewise detected no protons.
In the middle of December, Rutherford gave a speech at the Royal Society summarizing the latest evidence. He and Oliphant had at last found neutrons from deuterons on lithium, and Lauritsen neutrons from deuterons on beryllium; but whereas everyone associated these neutrons with nuclear transformations, Lawrence plumped for what Rutherford dismissed in a letter to Bohr as an "exothermal nucleus," and, together with Livingston, offered very shaky evidence that ruled out the reaction Li7 (d,n)2a as the source of the Cambridge neutrons.[48] Another Cavendish man, D.E. Lea, countered the hypothesis of the deuteron's instability by ascribing the hard gamma rays he observed from wax irradiated by neutrons to the spontaneous, endothermic formation of deuterons.[49]
Lawrence sought protection behind his big gun. Your conclusions are "hardly justified," he told Cockcroft, since the Cavendish experiments had run at under 600 kV.[50] Beginning at 700 kV things became more interesting. Lauritsen had then recently found many neutrons from fast deuterons on beryllium, carbon, and even copper. That, Lawrence crowed to Livingston, amounted to unquestionable corroboration of their experiments. "Chadwick will have to come down off his high horse now." The word at the Laboratory was that Lawrence had "clinched his mass of the neutron—though the evidence [as Kurie rightly objected] is not as clear as I'd like to see." Why did Rutherford not find fast protons from alpha particles on deuterium gas?[51] Lawrence tried to convince the reigning theorist in the business, Gamow, whom he had met at the Solvay conference; Gamow found the fog of conflicting experimental findings too dense to penetrate and offered to "come to California and try to split nuclei by pure theory."[52]