Back in Berkeley

When news of fission reached the Laboratory through the daily press about January 29, the staff could not be kept at the assembly of the 60-inch cyclotron, which had been its preoccupation. "All of us couldn't resist our curiosity to look into the matter a bit."

There were various approaches. Oppenheimer proved that fission is impossible. Alvarez wired East for further information, learned that Tuve's group had detected fission fragments, and immediately did the same, for the benefit of Oppenheimer and others who would not accept "so revolutionary [an effect] . . . until it had been confirmed in several laboratories."^[53] Alvarez and his collaborator, G.K. Green, in Berkeley as a National Research Fellow, saw the effects of thermal and fast neutrons and went beyond other hasty confirmers by estimating the time delay between irradiation and fission. They obtained an upper limit of 0.003 sec using a pulsed neutron beam modulated in Alvarez's manner; the limit was soon lowered to 0.001 sec by the intermittent neutron generator at Imperial College.^[54] They all missed the so-called "delayed neutrons," emitted a few seconds after cessation of the irradiation in a small percentage of fissions. These dilatory particles, ejected during the rearrangement of fission fragments, give a handle for the control of the fission process.^[55]

Green and Alvarez used Alvarez's preferred detector, a fast electronic affair, a thin-walled ionization chamber (on one of whose plates the uranium sat) connected to a linear amplifier and oscilloscope. Dale Corson, then in charge of the Laboratory's cloud chamber, teamed with Thornton to see fission fragments in a slower, but more evocative, way. They put uranium oxide on a collodion foil in a chamber filled with air, water vapor, and alcohol; irradiating the whole with neutrons procured from the 37-inch cyclotron, they obtained the nice result shown in plate 9.2. Assuming the short side tracks to belong to recoiling ions from the chamber gases, they inferred that the lower fission fragment must have had an atomic weight of at least 75 to have continued its course without apparent deviation. Two weeks after Corson and Thornton sent off the best of their picutures to the Physical Review

(they could choose among 25 obtained in 885 exposures to cyclotron neutrons), Joliot, a past master of cloud chamber technique, exhibited at the Paris Academy of Sciences the single photograph of a recoiling fission product that he had got in 902 exposures to Rn-Be neutrons. To physicists, the several quick observations of strongly charged, heavy particles constituted "conclusive evidence" of the phenomenon brought to light by Hahn and Strassmann after months of tedious, masterful analytical chemistry.^[56]

Seaborg heard about fission at the Journal Club meeting on January 30. "The news excited me very much. After the seminar, I spent hours walking the streets in Berkeley, chagrined that I had not recognized that the 'transuranic' elements . . . were not really 'transuranic' elements; I felt stupid." (He had judged the results of Curie and Savitch to be "rather strange.") His follow-up, or, rather, that of Joseph Kennedy, to whom he assigned the task, was not a chemical but a physical investigation, a search for the fast beta particles that they thought might be released in the successive transformations of the fission products. Kennedy spent some weeks at the quest, but to no avail. Nor could fast beta particles be detected concurrent with fission. They gave up and notified their colleagues of their failure.^[57]

In March, reinspired by the "elegant chemical separations" and "startling conclusions" of Hahn and Strassmann's definitive demonstration that uranium can give birth to barium, Kennedy and Seaborg tried another way to obtain information about the fission fragments. Once the chemical nature of one partner in a fission is known, the nature of the other follows from subtraction of atomic numbers; but a similar inference from an isotope of the one to an isotope of the other is not possible if free neutrons come out from a bursting uranium nucleus along with the fission partners. Kennedy and Seaborg looked for evidence of these neutrons by placing a Ra-Be source on the axis of a cylinder,

surrounding it with two concentric shells, the inner of uranium oxide and the outer of water, and counting the neutrons reaching the water's surface. They could not confirm their expectation that more neutrons should reach their detectors with the uranium in place than without it.^[58]

There was another and more urgent reason for desiring to know not only whether, but also how many, free neutrons may be emitted during fission. Should the number be large enough, the process of fission, once instituted by the energy of the physicist and his machines, might continue on its own, perhaps with explosive violence. That had been plain to the Columbia group the moment they had confirmed Frisch's experiment. "Here is real atomic energy!" Dunning wrote in his laboratory notebook on confirmation day. And on the morrow, in the same exclamatory style: "Secondary neutrons are highly important! If emitted would give possibility of self perpetuating neutron reaction." Many others saw the same: the omniscient Fermi; the busy Szilard and Szilard's sour partner Arno Brasch, who twitted him about the now worthless patents on chain reactions with which he had hoped to direct the course of events.^[59] At Berkeley they also understood. "We are trying to find out whether neutrons are generally given off in the splitting of uranium; and if so, prospects for useful nuclear energy become very real!" "It may be that the day of useful nuclear energy is not so far distant after all."^[60] Alvarez spearheaded the hunt, though not with his customary thoroughness. He directed his beam of slow neutrons into a bottle of uranium oxide and looked for fast neutrons arising from fission. He saw none.^[61] He also tried, with the help of a chemist, Kenneth Pitzer, to detect an increase in the temperature of uranium irradiated by neutrons. Again the experiments were inconclusive. They were resumed by Malcolm Henderson, back in Berkeley for the

summer, who detected heat enough to indicate that the fission fragments recoiled with an average energy of around 175 MeV, in agreement with other estimates made in other ways.^[62] No further efforts to detect fission neutrons at the Laboratory have come to light, although Lawrence said publicly that the possibility of obtaining energy from uranium depended upon the number of neutrons released in fission.^[63]

His lethargy in prosecuting this possibility, with which he had been flirting for several years, may be accounted for as follows. For a change, time was more precious than money. Lawrence preferred not to delay completion of the 60-inch cyclotron for the chance that the Laboratory would be the first to find and advertise fission neutrons. Nor was a cyclotron the best tool for the search. The groups who first established the multiplication of free neutrons during fission worked with Ra-Be sources. The earliest in the field was Joliot's. They compared the neutron intensity at various distances from their source of photoneutrons (Rag -Be) when it sat in tanks containing plain water and water strewn with uranium oxide (hence they measured primarily the effects of slow neutrons). Fermi and two of his associates at Columbia used the same technique, which was adapted from one the Rome group had invented to measure neutron absorption; while another pair at Columbia, Szilard and Walter Zinn, observed fast neutrons from uranium struck by slow ones. In Paris they overestimated the average number of neutrons emitted during uranium fission as 3.5; in New York they made it 2, and then, in a more careful measurement by a combination of the two Columbia groups, who pooled their radium and uranium, 1.5.^[64] One preferred the higher or

lower figure according to whether one hoped for nuclear power or feared a nuclear bomb.^[65]

The only important immediate contribution of the Laboratory to the unravelling of the fission process was made by a junior member, Philip Abelson, who, by combining an undergraduate chemistry major with graduate training in physics, constituted an untried one-man interdisciplinary team. In 1937, in the course of the customary search for new activities, he had confirmed the general scheme of transuranics then recently put forward by Meitner, Hahn, and Strassmann, and added a new activity, of 17 hours, whose place he could not find. Then, under the inspiration of Alvarez's work on K-electron capture, he thought to identify "transuranics" by their characteristic x rays.^[66] In March 1938, in "transuranics" prepared in quantity at the cyclotron, Abelson isolated an activity of 77 hours, which he identified with the 66-hour body that the Berlin group had fixed as element 95, and whose characteristic x rays he managed to detect with a cheap spectrometer he had constructed. By extrapolating beyond uranium the increase with atomic number of the penetrating power of L rays—characteristic radiation involving the second or L shell of the atom—Abelson convinced himself that the x rays he detected belonged to an atom of nuclear charge 95. The revelation of fission dumbfounded him.

After a day in the dumps, Abelson looked again and learned that the "L rays" of 77-hr element 95 were in fact K rays of 2.4-hr iodine (atomic number 53). The connection, as Abelson worked it out: the iodine, Meitner et al.'s 2.5-hr element 96, descends from the 77-hr body, which must accordingly be tellurium; tellurium's beta decay can knock out a K electron, setting up the production of iodine's K ray. The process, taken all together, constituted "an unambiguous and independent proof of Hahn's hypothesis of the cleavage of the uranium nucleus."^[67] Abelson's rays and Dale Corson's pictures caused Oppenheimer to adjust his evaluation of

fission from "impossible" to "unbelievable." And if the unbelievable should give rise to some extra neutrons? "A ten cm cube of uranium deuteride (one should have something to slow the neutrons without capturing them) might very well blow itself to Hell."^[68]

Abelson carried on. In three months of hard work, reprieved from crew duty, he found in the products of uranium irradiated by neutrons no fewer than five activities ascribable to antimony, seven to tellurium, and four to iodine. He thereby not only showed the complexity of the German transuranics, but also demonstrated that fission fragments were ordinary isotopes. Two of his telluriums and one of his iodines had the same periods and beta-ray spectra as activities then recently identified in the Laboratory by Seaborg, Livingood, and Kennedy. Abelson could therefore specify decay sequence, half-life, element, and, in some cases, the isotope of the products he studied. In accomplishing his work he mobilized the great experience of the Laboratory in nuclear chemistry, as represented by Segrè and Seaborg, and, of course, the Laboratory's prop and pride: Abelson attributed his success "in large measure to the huge neutron intensities of the cyclotron."^[69] The cyclotron deserved the credit, and more. Its neutrons made fission products strong enough to swamp the activities of the natural descendents of uranium, which regrew during chemical analysis of the irradiated samples. With their "enormously weak preparations," Hahn and Strassmann had great difficulty finding iodine among the uranium products and succeeded only after Abelson's demonstration. Their confirmation resulted in what they called the "definitive suppression" of their long transuranic chains.^[70]

And what about the Curie-Savitch body that precipitated the studies that led to fission? As Meitner and Frisch observed, a barium isotope arising from uranium should be accompanied by

an isotope of krypton that might decay through rubidium, strontium, and yttrium to a stable zirconium. Hahn and Strassmann immediately started to look for these elements, of which they soon reported sightings.^[71] Now yttrium has chemical properties similar to those of lanthanum, from which Curie and Savitch had not been able to separate R_3.5h . The exact investigation of the sequence from strontium on was left to one of Hahn's assistants, who found an yttrium of the necessary period, but declined to pronounce definitively that it decayed to zirconium and constituted the inspirational French "lanthanide." It did, and does.^[72]