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Preview and Overview
Beginning in 1934 and 1935 the Laboratory pioneered two sorts of interdisciplinary work centered on products of the cyclotron. The earlier, radiochemistry, enlarged its domain as the energy of the deuterons it chiefly employed increased. Up to the summer of 1935, when the cyclotron first bombarded at 4 MeV, direct activation by deuterons did not penetrate much beyond potassium, although for a time Lawrence thought he had transmuted platinum.[1] In late 1935, with the energy at 5 MeV, the activation extended to antimony; in late 1937, when the 37-inch operated at 8 MeV, and recovered for Berkeley the lead in energy usurped by Michigan a year earlier, it reached uranium.[2] Here the Laboratory had a monopoly.
Neutron activation could be practiced anywhere, with modest means, and throughout the periodic table. Here the cyclotron conferred the advantage of intense neutron beams obtained from deuterons incident on targets of lithium or beryllium. The range and complexity of neutron activation were much increased in October 1934, when Fermi inserted a block of wax for one of lead in a measurement of absorption. (He had in mind, perhaps, the finding of his collaborators that the intensity of activation by
neutrons depended in some cases on the material on which the irradiated substances stood, and the observation by Joliot and Curie that neutrons directed at a layer of paraffin made a particularly strong impression on an ionization chamber.) The silver thimble whose induced activity served Fermi as a measure of neutron flux responded more enthusiastically to neutrons passed through wax than to neutrons striking it directly.[3] He explained that nuclei susceptible to (n,g ) reactions capture slow neutrons more easily than fast ones. Between 1934 and 1936 Fermi's group identified more than 80 radioelements—about half the radioelements known in 1937—made by neutrons fast and slow.[4] The Rome work created both an opportunity and a complication at Berkeley. It indicated that everything could be activated by neutrons; and it required that experimenters learn to discriminate between the effects of fast and slow ones.
The study of the reactions induced by deuterons or neutrons at Berkeley produced more than new entries, some of great importance, in the lengthening list of activities. The Laboratory had to its credit the first artificial (d,p) and (d,n) reactions, which, however, it had to share with Caltech; the first experimental demonstrations of K-electron capture, whereby a nucleus decays by swallowing one of its nearest satellite electrons; a full demonstration of the Oppenheimer-Phillips mechanism of (d,p) transformations; and early examples of isomeric nuclei.
Usually teams of two or three worked at the experiments, but even those who labored alone had important help from other members of the Laboratory. Everyone depended upon the crew on duty to keep the cyclotron going. Many turned to Kurie and his collaborators and successors, for example, J.G. Richardson and Ernest Lyman, to determine in their cloud chamber the sign of the beta particles given off in decays under study. Since each staff member became an expert on a few elements—Kamen recalled
that when he started work in 1937 only bismuth and tellurium were unassigned—an explorer had to consult experts on neighboring elements, or McMillan, who became the authority on them all, when hacking through the tangle of reactions in the middle reaches of the periodic table.[5] And also in the middle reaches, particularly among the transition metals of the fourth and fifth period and the platinum metals of the sixth, the chemistry easily acquired by physicists no longer sufficed for clean separation of the elements of an activated target. Chemists became essential collaborators.
The first chemist to have an ongoing connection with the Laboratory, Glenn Seaborg, began in the spring of 1936, when Livingood, perplexed by the mess of activities in a tin target irradiated with deuterons, sought his help.[6] Their ongoing collaboration—Seaborg at the sink, Livingood at the electroscope—produced the largest quantity of information about nuclear reactions obtained by any group in the Laboratory. In the fall of 1938, when Livingood went to Harvard, Seaborg continued the collaboration by mail, struck up a new one with Segrè, and brought his students from Chemistry to help in the increasingly difficult chemical separations.
The collaboration of Livingood and Seaborg instanced not only the assimilation of chemists into the Laboratory, but also a fundamental change in the character of its nuclear chemistry. Their first try with tin was made in the old hit-and-run manner: Livingood bombarded with deuterons for three hours; Seaborg separated for a few more; Livingood observed the decaying fragments; and the two enriched the Physical Review with rough indications of three activities in indium (if they were not owing to tin contamination), two in tin, and two in antimony. There were perhaps twenty-four hours of experiment in all and three months from irradiation to manuscript. The business had, of course, to be repeated.[7] Gradu-
ally Livingood and Seaborg's experiments became more elaborate and reliable. Between February and June 1937, for example, they bombarded seven elements from chromium to antimony, some many times, with deuterons or neutrons; Seaborg split the activities into a great many samples, which Livingood filed and observed for as long as they showed life, sometimes for two years and more. Among the products disclosed that busy spring was Fe59 , which became the standard, if rare, tracer for iron in studies of the blood.[8]
Like radiochemistry, the second pioneering line, radiobiology, danced to the tempo set by the machine. In the fall of 1933, the cyclotron's beam of 3 MeV deuterons drove out so great a flux of neutrons from beryllium that, Lawrence wrote Poillon, "we are already worried about the physiological effects on us." Lawrence was then about to set off for the Solvay Congress in Brussels; en route he planned to consult Wood at Columbia about the physiological effects of neutron irradiation, which, he thought, might have a satisfactory side, "of great medical importance."[9] Lawrence supposed that neutrons might destroy cancers more effectively than x rays, because, as Chadwick had shown, neutrons pass more readily through dense materials like lead and bone than through light hydrogenous matter like paraffin and body tissue. This differential absorption would make images taken by neutron rays the inverse of those by x rays—what appears light in the one being dark in the other—and perhaps also confer some special therapeutic benefit. "There is [therefore] some justification for the belief that the discovery of neutron rays is of an importance for the life sciences comparable to the discovery of x rays."[10]
Losing no time, Lawrence discussed the potential of big neutron beams with Dave Morris, treasurer of the Research Corporation, vice president of the Macy Foundation, and, in 1933, Roosevelt's new ambassador to Belgium.[11] At Morris's suggestion, between
Brussels and Berkeley Lawrence talked over his expectations with Macy's president, Ludwig Kast, who saw enough in them to grant the Laboratory $1,000 by the time Lawrence's train arrived in San Francisco. Two other grants of over twice that amount followed progress in the aggrandizement of the beam. In January 1934 Berkeley's neutron ray, "more penetrating than either x rays or radium," was intense enough to penetrate the New York Times .[12] In February Lawrence talked about it with the University Explorer over NBC. But by then his energies were fully engaged in proving his unstable deuteron and he found it more comfortable to hint at atomic power than at cures for cancer.[13] The collapse of the Berkeley teachings about deuterons and the discovery of artificial radioactivity then shelved development of neutron sources and concern with neutron hazards.
The steady increase in energy and current of the deuteron beam reopened the question of safety, and the "discovery" of radiosodium put a premium on pushing intensity and danger further. In April 1935 H.F. Blum, of the University's Department of Physiology, who wanted to try the effects (which he expected to be catastrophic) of neutrons on tissue containing lithium, agreed to look into safety.[14] During the summer and early fall of 1935, John Lawrence, then very much concerned with safety, was in Berkeley recovering from an automobile accident. He followed up Blum's experiments and learned that, as his brother put it, "neutron rays are considerably more lethal biologically than x rays." "We are getting harmful doses of neutrons in a few minutes when we stand near the magnet."[15]
Exposure of laboratory personnel to penetrating radiation had long worried directors of laboratories with high-voltage x-ray equipment, especially Tuve, who began in 1929 to press for studies of the long-term biological effects of the rays from his Tesla coil. As he wrote Aetna Life, who were doubtless pleased to read it, he took "extreme precautions," setting the tolerable dose at under 0.1 roentgen a day and the total integrated exposure to less than 1 percent of what it took to kill a rat.[16] A roentgen, or "r unit," is the quantity of x rays that makes about a billion ion pairs (1 esu to be exact) when passing through a cubic centimeter of air. The second International Congress of Radiology, meeting in Stockholm in 1928, took the roentgen as its unit and during the 1930s most workers in the field adopted a dose of 0.1 r of x rays as the tolerable daily allotment.[17] What should be the limit for the more deadly neutron rays? John Lawrence first suggested no more than a quarter of the x-ray limit, in accordance with his preliminary measurements on the relative lethality of the two radiations to mice. Berkeley tended to be more generous than other places. MIT's Evans recommended a limit of 0.001 n unit/day (an "n" unit being the quantity of neutron radiation registering 1 r in a certain standard detector), Aebersold ten times that, McMillan four times Aebersold's limit. No national or international level of tolerance to neutron rays was established before the war.[18]
The discovery of the uncertain danger of the neutron background brought prudence to the Laboratory. In 1935 the dose four feet from the beryllium target was 0.2 r an hour, five times the allowable x-ray rate per day; a water barrier cut it down "quite a lot," but not enough to stop the Laboratory's neutrons from spoiling experiments in the Chemistry building.[19] The 37-inch had a more extensive shield, a wall of water three feet thick; Fermi thought this extravagant, but the National Advisory Cancer
Council, which authorized $5,000 for it, did not. The three feet of water reduced the radiation at the control desk by a factor of three; the addition of a layer of water 1.5 feet deep above the cyclotron drove it down by a factor of ten (plate 8.1).[20] As for the 60-inch, with four feet of water around the entire machine, the dose at the controls fell to an insignificant 0.001 r/d, or so the heads of the Laboratory claimed.[21] Aebersold calculated that a five-foot shield would be necessary to keep the exposure at the controls, some forty feet from the target, to 0.01 n/d (n units per day). The wisdom in the field was that the background activity in Berkeley exceeded norms in other cyclotron laboratories.[22] Further to prudence, the cyclotroneers carried ionization gauges in their pockets so as to meter what Time called the "new lethal death ray hurled by magnet[s]" and Science Service represented as a "deadly danger for young researchers." They also had their blood drawn "every so often."[23] Documentation of the seriousness of the neutron hazard returned Lawrence to his idea of neutron therapy.[24]
In 1936 the Laboratory began to acquire biologists and physicians just as it did chemists. John Lawrence returned for the summer and came back for good in 1937. Macy money and Rockefeller riches allowed him to build up a sizable group, which, by 1940, numbered himself as director; five doctors of different sorts, including a visiting fellow, but not counting Stone, who supervised neutron therapy on behalf of the Medical School; a nurse and laboratory assistant; and four operators of the 60-inch cyclotron. The biomedical staff distributed itself into three
research groups, one for neutron therapy, another for work on leukemia, and a third for biological tracers.[25] The enduring members, besides John Lawrence, were Paul Aebersold, who obtained his Ph.D. in 1939, after several years at the Laboratory on fellowships in radiological studies from the Medical School, for work on collimating neutron beams for radiation therapy; and Joseph Hamilton, who worked in radiology at the Medical School after receiving his M.D. there in 1936, came to the Laboratory as a Finney-Howell Fellow, graduated to research associate on NACC funds, and ended, after the war, as director of the Crocker Laboratory.
Neutron therapy began at the 37-inch cyclotron in September 1938. It seemed at first to offer some advantage over treatment by x rays. So did the ingestion of P32 in cases of chronic leukemia and polycythemia vera, and radioiodine for diseases of the thyroid. Radiosodium, in which so much was invested—cured nothing.[26] The 60-inch cyclotron, which could treat more patients than the 37-inch, improved statistics. It appeared that the neutron ray was a cruel disappointment, but that radiophosphorus and radioiodine afforded many sufferers true benefits. The tracer research also had notable successes, particularly in elucidating steps in photosynthesis. And, as in the case of technetium in radiochemistry, discoveries of importance in radiobiology were made outside the Laboratory by people using radioactive preparations made in Berkeley.