Emilio Segrè and Spontaneous Fission
As the staff at Los Alamos began research in the spring of 1943, the most formidable problems it confronted were related to the new materials that would be used in atomic bombs. These materials, uranium-235 and plutonium, were largely unknown. Uranium-235 formed only a tiny fraction of natural uranium (less than 1 percent) and plutonium had been discovered only two years earlier at the University of California, Berkeley, Radiation Laboratory by chemistry professor Glenn Seaborg and his associates. One of Seaborg's associates was Emilio Segrè, who had been a member of Enrico Fermi's team at the University of Rome. Fermi and his colleagues originally thought that their bombardment of uranium by slow neutrons in the mid-1930s had produced elements heavier than uranium, or transuranic elements.
Further investigations by Otto Hahn and Fritz Strassman, German chemists at the Kaiser Wilhelm Institute for Chemistry in Berlin, however, had revealed that the uranium fissioned instead. The discovery of fission led in turn to the discovery of the chain reaction that, if sustained, would provide the energy for atomic weapons. Segrè, who had fled the anti-Semitic laws imposed by the fascist regime of Benito Mussolini in Italy, had found a job as a research associate in UC's Radiation Laboratory. There, he investigated the products of the bombardment of uranium by the cyclotron, then the most powerful "atom- smasher" in the world.
After plutonium was discovered by Seaborg at the beginning of 1941, Segrè established that the new element fissioned when struck by fast neutrons, opening the way to its use in an atomic bomb. As Los Alamos was being set up in the spring of 1943, he and his associates at Berkeley turned their attention to spontaneous fission in uranium and plutonium. This process, if proved, might cause an atomic weapon to predetonate, blowing the fissile material apart before it had a chance to undergo an efficient chain reaction.
The possibility of spontaneous fission was real. After Fermi suggested it and UC Berkeley chemist Willard F. Libby sought in vain for it in 1939, the Russian physicists G.N. Flerov and K.A. Petrzhak discovered it in natural uranium in 1940. Segrè had, consequently, to ensure that plutonium and uranium-235 would not have a spontaneous fission rate large enough to cause predetonation in the gun-assembled fission weapon planned.
Working with his graduate students - Owen Chamberlain, George Farwell, Gustave Linenberger and Clyde Wiegand, Segrè and two UC chemists, Arthur Wahl and Joseph Kennedy, measured rates of spontaneous fission in natural uranium and plutonium in 1942 and 1943. The plutonium was made by the 60-inch Crocker medical cyclotron at the UC Radiation Laboratory by the bombardment of uranium-238 by deuterons, the ions of heavy-water (deuterium). By June 24, 1943, they found that such plutonium had a rate no greater than five spontaneous fissions per kilogram each second, or 18 spontaneous fission per gram of plutonium per hour, an acceptable rate.
These measurements at Berkeley were very difficult; the detectors used were so sensitive that cellos playing in the next room were suspected of causing more counts during the daytime than nighttime. The lights left on in the daytime were found to produce photoelectrons that caused the disparity. Leaving a flashlight on at night made up the difference.
The coincidence of pulses from several alpha- particles arising from the radioactive decay of plutonium could also mimic spontaneous fission, and extraordinary measures were taken to prepare materials of the right thickness and to calibrate the ionization chambers used to detect fission fragments to exclude these and other signals.
Although the results with plutonium produced in the Crocker medical cyclotron were encouraging, several researchers suggested that plutonium produced in nuclear reactors by the bombardment of uranium-238 by neutrons might have an isotope, plutonium-240, that would be likely to fission spontaneously. If this were only 1 percent of the reactor-produced plutonium and it had a high-spontaneous fission rate, predetonation would be much more likely.
At Los Alamos, chemists already planned to make plutonium that very highly purified by removing lighter elements that might react with alpha particles from decay to produce neutrons that could predetonate the bomb. Plutonium-240, however, could not be chemically separated from plutonium-239 without building huge isotope separation plants similar to those under construction at Oak Ridge, Tenn., used to separate uranium-235 from uranium-238. To investigate the possibility of spontaneous fission in plutonium, Los Alamos Director J. Robert Oppenheimer invited Segrè and his group to move to Los Alamos to continue their experiments there.
In mid-June 1945, Farwell recalled, "We all packed up – bags and counters, detectors, electronics and all – and went off to Los Alamos." Linenberger rode shotgun in an Allied moving van that carried their delicate equipment, while Farwell and Segrè flew in a DC-3, arriving on June 18.
Because of the delicacy of their detectors, the group could not remain in the technical area around Ashley Pond, where most of the scientific activity of the Laboratory was concentrated. They sought a place far from disturbances that might upset their instruments and ended up in Pajarito Canyon, 14 miles away. Shielded from radiation by the distance and housed in an old cabin, they found the solitude they required. "It was a most poetic place," Segrè recalled. "We went there by jeep every day. There was a bed in it (the cabin). Somebody occasionally slept there."
The cabin Segrè's group used still stands in TA-18 and became known later as "Dwight Young's" cabin. In the early days, it had been part of Ashley Pond's dude ranch and was known as the Pajarito Club.
On June 17, 1943, word came to Los Alamos of a study of spontaneous fission in polonium by Frederic Joliot and Pierre Auger in occupied Paris. The rate they reported – one spontaneous fission in every 1017 atoms of polonium – would be sufficient to rule out polonium as an element in the neutron initiator then planned for atomic bombs, because the neutrons produced in the process would pre-ignite the chain reaction. If a similar rate was found in plutonium, it might rule out the use of that element as the nuclear explosive.
Although Los Alamos scientists believed the rate reported was too high, and probably due to impurities in polonium that were difficult to remove, Oppenheimer and the other members of the Laboratory's governing board agreed to give Segrè all the necessary facilities to pursue their research in Pajarito Canyon.
In the summer of 1944, Emilio Segrè group at Pajarito Site found that plutonium from nuclear reactors had an isotopic impurity, plutonium-240, that prohibited its use in a gun-type assembly. Since all of the plutonium that would be used in the atomic bomb would be produced in reactors, this meant that the vast investment in the Hanford production reactors built by DuPont would go down the drain unless implosion could be perfected.
The Laboratory was reorganized to accomplish this. New division, G for gadget and X for explosives, were set up to develop the nuclear and high-explosive components of the implosion device. The Laboratory's Governing Board was divided into administrative and technical boards to manage the growing effort. Even then, the Technical Board's tasks were increasingly assumed by lower-level interdivisional committees and conferences that coordinated the effort required.
The reorganization of the Laboratory was accompanied by a vast expansion in personnel, as no stone was left unturned in the search for a suitable design and the development of suitable components for the gadget. From roughly 1,100 personnel, Laboratory employment grew within a year to more than 2,500. Implosion meant an explosion of the Laboratory population.
As June 1943 ended, the future of Los Alamos' program for a plutonium bomb seemed in doubt. Only time would tell if plutonium could be used in nuclear weapons and, if so, how. The resolution of those questions was to have a pervasive effect on the new Laboratory and the world.
- History @ Los Alamos