From Physics to Physic

The multiplication of cyclotrons in the United States began in 1934, with Livingston's little machine at Cornell (plate 6.4). No fewer than eight were commissioned the following year at Bartol,

Columbia, Illinois, Michigan, Princeton, Purdue, Rochester, and Washington. They came in two almost standard sizes: poles around 16 inches in diameter (Cornell, Illinois, Rochester, Washington) and poles around 35 inches in diameter (Bartol, Columbia, Princeton, Purdue). Michigan exceeded them all, and Berkeley too, before the 60-inch, with poles of 42 inches, became the standard for the cyclotrons of the later 1930s (table 6.4). As we know, Lawrence cooperated fully with the builders of these machines, most of whom were his students and associates. Neither he nor the Research Corporation had any interest in restricting the spread of their patented technology.

These first American cyclotrons were made for physics. An indication of their orientation is that Livingston designed the Cornell cyclotron for acceleration of protons and deuterons without giving priority to either; he had separate chambers available for each from the time his cyclotron began to operate in 1935. Although deuterons, the particles of preference at Berkeley, are particularly effective in making radioisotopes and generating neutron beams,^[75] protons are better for close examination and

interpretation of nuclear reactions and forces. Moreover deuterons, which have a lower resonant frequency in the cyclotron than protons, can be accelerated to relatively higher energies with relatively less bother about the always painful oscillating circuit. As medical and biological applications of cyclotrons became paramount in acquiring financial support for their construction, parameters were set to favor the production of deuterons. Most of the large cyclotrons in operation in 1940 were not suitable for the acceleration of protons. In this circumstance Hans Bethe saw cause for regret and even alarm.^[76] But with his colleague Living-

ston's cyclotron, which delivered 3 or 4 µA of 2.0 MeV protons (or 1.35 MeV deuterons), Bethe could test the predictions of yields of nuclear reactions in light elements computed by himself and his students on the basis of Bohr's theory of nuclear structure.

The only applications envisaged by Livingston in his first description of the Cornell cyclotron concerned nuclear problems, viz., the study of "all types of reactions" caused by "all available bombarding particles." At first Cornell used deuterons in the reaction Be(d,a )Li in order to infer the states of the beryllium nucleus from the energies of its ejected alpha particles; they subsequently exploited the reverse transformation Li(d,n)Be for neutrons for scattering experiments; and they ended, in the late 1930s, with careful measurements of deuteron-induced disintegrations of nitrogen, carbon, and oxygen.^[77] Their allegiance to the original purpose of their machine was secured by its size: it could not make radioisotopes in sufficient quantities for biomedical studies and foundation support.^[78] The same things are true—enforced devotion to physics—of the Illinois cyclotron. Like its twin at Cornell, it was used to generate secondary beams that themselves became the instrument or object of study, for example, gamma rays and neutrons.^[79] That was not enough for cyclotroneers, however, and, as we know, both Green and Kruger at Illinois and Livingston at Cornell made important improvements in the equipment of cyclotrons.^[80]

The first of the larger cyclotrons to be completed outside Berkeley was Princeton's. It was more powerful (several µA of 10 MeV deuterons) and more costly ($12,000 inclusive of motor generator) than Cornell's. It was adaptable to isotope production. Henderson and White emphasized magnet design and explicitly preferred deuterons; their machine, although financed by the university and

not by outside medical interests and dedicated, according to their first description of it, to nuclear physics, proved a useful manufacturer of radioelements, to which it came to devote much of its time. Like Cornell's cyclotron, Princeton's began to run within a year or so of its commissioning.^[81]

Rochester's cyclotron also took but a year to build. Its promoter, Lee DuBridge, "one of the earliest to see a great future for the cyclotron," had been a National Research Fellow at Berkeley, though not in the Radiation Laboratory. He built with medical support and the help of industry and the Research Corporation.^[82] His 20-inch machine was dedicated at first to proton work in the unexplored region above the reach of high-tension apparatus and the Cornell and Illinois cyclotrons. Working at 2 or 3 µA, he and his colleagues reaped a good harvest of the known reactions—p capture and (p,a ) disintegrations—in the "fast" proton range of 1–4 MeV, and they discovered a new process, (p,n), which sets in at about 3 MeV. With this success, they decided to enlarge their magnet's poles to 26 inches in the expectation of having protons of over 7 MeV. They reached 7.2 MeV and followed (p,n) reactions in at least thirty-five elements. As their experimental work continued Cornell's, so their theorist, Victor Weisskopf, continued Bethe's. As at Cornell, theory went hand in hand with experiment.^[83]

The enlargement of the poles, which occurred in 1937, made possible the production of 4.5 MeV deuterons. They sufficed to make radioisotopes in sufficient quantities for tracer work by physicians and biologists at Rochester, and for an investment of

$30,000 by the Rockefeller Foundation, to be split about equally between the physicists and the doctors. The physicists used their share to attract Van Voorhis, who later rearranged the radio frequency system to double the power reaching the dees. The proton bombardments and the physics program retained the ascendency at Rochester, and only one day a month was devoted to deuteron irradiation of materials for biomedical experiments.^[84]

The balance fell out quite differently for cyclotrons of greater power than Rochester's. Columbia and Michigan mark the transition. Columbia began in February 1935, asking Lawrence's opinion about the conversion of a general purpose 14-inch magnet. He advised a magnet and machine of the type—and hence for the purposes—that Livingston was then pursuing at Cornell. An opportunity then presented itself to build bigger, in the form of the navy's 500-kW Poulsen arc, not Berkeley's twin, but big enough to support pole pieces 36 inches in diameter, which NYU decided it could not use. That was in the summer of 1935.^[85] By the time the magnet with its 36-inch poles was in place at the end of 1936, the biomedical economics of cyclotrons had been discovered; Columbia University, in consideration of medical applications, put up money for the completion of the machine and the chairman of the physics department and a colleague from chemistry went to the Research Corporation for more, "to produce very usable quantities of artificial radioactive substances." They got what they wanted, some $1,850.^[86] By the winter of 1938/39, having shimmed away the bad asymmetry of the poles and perfected the quarter-wave transmission line, the Columbia group, which by then included Paxton, was ready to make P³² and Na²⁴ for biological tracing. Eventually they had a neutron flux equivalent to that from 50 kg of Rn-Be and could detect on the

roof of their fourteen-story physics building neutrons that had passed through their colleagues on the way up from the cyclotron in the basement. "The cyclotron really operates very beautifully now."^[87] Between the decision to make a cyclotron, taken in February 1935, and its completion in 1938, the purpose the machine was to serve, the basis of its financing, and the magnitude of its hazards had been transformed.

The Michigan cyclotron was very well supported from the beginning—from the summer of 1935—by the university's Rackham funds. The chairman of the physics department, H.M. Randall, and the machine's main builder, James Cork, who had spent 1935/36 on sabbatical in Berkeley, apparently had indicated to the university's medical school, whose collaboration they obtained, that the large cyclotron they planned would yield radioisotopes for clinical use. That was more than Berkeley had by then achieved, and it was doubtless with relief that Randall learned from Lawrence in the summer of 1936 that the 27-inch could make samples of Na²⁴ equivalent to 50 mg of radium. "You can proceed with the construction of a cyclotron with the definite assurance that you will be able to produce enough radiosodium and other radioactive substances for medical investigations."^[88] The Michigan cyclotron put out its first beam (several µA of 5 MeV deuterons) in August 1936, about a year after the physics department had assigned it first priority. Lawrence, who happened to be in town at start up, "got a great [and well-deserved] kick out of seeing the second large cyclotron in the world go into operation." A new tank installed with Thornton's help in the winter of 1937/38 gave an intense beam that reached to 200 µA in the summer of 1939. That more than met the debt to the medical people, who had been satisfied with 15 µA.^[89]

Apart from the 27-inch cyclotrons begun at Yale in 1937 and at Stanford in 1940, built on the cheap for experiments in physics,^[90] all the machines commissioned from 1936 on were at least as large as Michigan's and, like it, dedicated largely to biomedical work. Physics became a secondary or tertiary goal of cyclotron builders, according to their reviews of their discipline. Kurie, Chadwick, and Mann, for example, gave pride of place to radiobiology, chemical tracing, and clinical treatments in their accounts of uses of the cyclotron; and Mann, writing in Nature in April 1939, offered Kamen and Wilson's internal target, which multiplied the harvest of therapeutically relevant isotopes, as "the most important recent" improvement in the cyclotron art and a grand "stimulus to those who are in favour of the immediate construction of a cyclotron for medical purposes in Great Britain."^[91] Lawrence was also a stimulus in this cause. His recommendation to the British that their medical cyclotron have 50-inch poles ("[the 60-inch] is probably larger than necessary"), might suggest that machines of the Michigan class were not big enough to practice medicine. It appears that 50 inches was an arithmetical compromise between the excessive 60-inch and the adequate Michigan type: as Lawrence wrote Livingood, "a 40-inch cyclotron really fills the bill for most medical purposes." Or, to put the point backWard, since the 37-inch was adequate for therapy and isotope manufacture, a 50-inch would do.^[92]

Lawrence's optimum medical cyclotron, although costly, came to considerably less than the Crocker machine. Late in 1938 someone at the Laboratory took the trouble to estimate the differences in price: for the magnet, metal and installation, $40,000 for the 60-inch, $18,000 for the 50-inch; for the remaining parts, $40,000 versus $32,000; for labor, $8,000 and $6,000. Since something could be saved by making some parts in departmental shops and by gaining concessions from manufacturers, Lawrence quoted $50,000 in all as the ante "to do it right from the standpoint of clinical work." But if only $20,000 were available, it might just do.^[93]

The largest cyclotron started in the United States after the Crocker was almost its identical twin, erected by, of all people, Lawrence's again friendly rival Merle Tuve. The Carnegie Institution of Washington, with its high investment in high tension, could scarcely remain aloof from the new forces and opportunities in high-energy physics. "If you want an equipment for producing artificial activity [as opposed to one for exact physics], there is nothing that even faintly begins to compare with a cyclotron, nor has there ever been." So Tuve answered an enquiry from Bell Labs in the spring of 1940. By then he had been cyclotroneering for a year. During the spring of 1939, Brobeck, Cooksey, and John Lawrence joined interested parties in the Washington area to discuss the desirability of building a big cyclotron at Carnegie for service to local institutions. The consensus held that Carnegie could do no better than to build a 60-inch machine. As the director of Tuve's unit explained to the president of the Carnegie Institution, the machine could do physics for the Department of Terrestrial Magnetism and chemistry and biology for other units of the institution, local universities (Johns Hopkins, George Washington, Catholic) and the federal government (the Department of Agriculture, the National Cancer Institute).^[94] It would not practice medicine. "Medical research will be given no preference whatsoever and, in fact, will be undertaken only in the sense of

fundamental work in physiology and biochemistry." Physics would have a third of the machine time.^[95] The Carnegie philanthropies had their own resources and freedom to determine their own programs.

Once commissioned, the Carnegie cyclotron came into existence in the manner then standard. Tuve and his main associate, Richard Roberts, received an "enormous roll of blue prints" from Berkeley, "almost lost our mind[s]" reading through them, learned that a visit to Berkeley and Berkeley cyclotroneers was indispensable, hired two (Green and Abelson), and started shopping for steel.^[96] They differed from Berkeley in contracting with ARMCO for the magnet ($16,025), part of which was cast (as at Chicago, Columbia (pole tips only), Harvard, MIT, and Ohio State, among others), and with GE for water-cooled coils to run it (as at Columbia, Harvard, MIT, and Rochester). Nonetheless, the whole would resemble the Crocker magnet very closely, in weight as well as in shape, the coils like "the turret[s] of a battle ship."^[97] The analogy was not idle. Like all other cyclotron builders in 1939 and 1940, the Carnegie Institution faced the free-market competition of an arms race. "War orders have made loads of extra work in getting contracts that are within starting distances of estimates based on Berkeley costs."^[98]