Los Alamos:Part 3

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Continued from Los Alamos:Part 2, see also Los Alamos:Part 1 and Los Alamos:Part 4

Contents 1 The Plutonium Gun 2 Weapon Materials 3 Implosion Method 3.1 References

The Plutonium Gun

Military guns are simple in design and a known quantity by virtue of their long use by the armies and navies of the world. The decision to use a gun to assemble active material in a fission bomb came about because gun technology is so well known.

During the July 1942 conference hosted by J. Robert Oppenheimer at Berkeley, Calif., it was taken for granted that a large caliber gun would be used to shoot two pieces of uranium into a supercritical assembly. After the discovery of plutonium, the use of guns became more problematic. Light element impurities could cause predetonation if sufficient assembly velocity was not achieved. Despite this concern, the elegant simplicity of gun technology gave Oppenheimer cause for optimism as Los Alamos came into existence. And despite the simplicity of gun assembly, a great deal of uncertainty remained about the nuclear materials and what the final product would look like.

Because of such uncertainty, Oppenheimer took personal control of gun development. As he noted, "At the present time our estimates are so ill founded that I think it better for me to take responsibility for putting them [design specifications] forward." With two types of fissile material to use, Oppenheimer faced a crucial first decision. Should the Laboratory develop a gun capable of using plutonium, the more difficult material to use, and adjust the gun to use uranium? Or, should two different guns be developed simultaneously?

Oppenheimer chose to develop the plutonium gun, code named "Thin Man" and then make the necessary changes to accommodate uranium. He believed that uranium presented few metallurgical problems and any changes in the gun would be minor.

With the help of Richard Tolman of the National Defense Research Committee, Oppenheimer proceeded with an experimental program. Their most significant impact on this program was choosing two key persons: Edwin Rose and Charles Critchfield. Rose, an engineer and gun designer, observed that a gun weapon could be delivered by aircraft if much of the steel used in the normal construction of guns was eliminated.

Since a fission gun would be fired only once, much of the steel used to ensure safety after repeated firings was unnecessary.

Critchfield, a mathematical physicist, brought a wealth of ordnance experience to the Laboratory. Critchfield realized that the Naval Gun Factory needed as much as six months to build the full-scale guns needed by the Laboratory. He suggested that time could be saved by testing at reduced scale using 20mm anti-aircraft guns and 3-inch naval cannon. These could be procured immediately. When the full-scale guns arrived, only the most promising nuclear designs would need to be tested.

Oppenheimer continued his direct supervision of the gun program until June 1943 when Navy Capt. William Parsons became the first Ordnance Division Leader at the Laboratory.

Work on Thin Man continued until July 1944 when Emilo Segre's experiments on the spontaneous fissioning of plutonium proved that a gun could not be used to assemble this material. Oppenheimer made the decision to abandon Thin Man and redirect much of the Laboratory's resources to develop the implosion method.

After the reorganization, gun work focused on uranium assembly, code named "Little Boy." Oppenheimer's earlier decision in 1943 to concentrate on Thin Man on the belief that a uranium gun did not present major technical problems proved prophetic. Little Boy was developed with few major complications.

Weapon Materials

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 Segre', 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. Segre', 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, Segre' 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. Segre' 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 Segre' 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 night-time. 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 Segre' and his group to move to Los Alamos to continue their experiments there.

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.

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 Segre' all the necessary facilities to pursue their research in Pajarito Canyon.

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.

Although Los Alamos was conceived in September of 1942 and occupied in April 1943, it was not until after the first plutonium arrived in Los Alamos July 10, 1943, that the first physics experiment was conducted at Los Alamos. On July 15, John H. Williams' Electrostatic Generator Group (P-2) observed neutrons from the fission of plutonium-239. Much of the intervening time was spent getting the necessary equipment up and running. The pressure tanks that enclosed the two Van de Graaff accelerators arrived during the course of the lectures and reviews defining the Los Alamos research program. On May 15, the University of Wisconsin "long tank" Van de Graaff produced its first beam. On June 7, the University of Illinois Cockcroft-Walton accelerator followed suit and three days later the "short tank" Van de Graaff, also from the University of Wisconsin, accelerated its first protons.

The "long tank" Van de Graaff had first priority because it would produce 1 MeV (megaelectronvolt) neutrons to cause fission in plutonium. An electron-volt is a unit of energy equal to the energy gained by an electron in passing through a potential difference of one volt. It could measure the number of neutrons produced per fission and the time between "fast" fissions of the type to be expected in a nuclear weapon. Up to that point, these quantities had been measured only in "slow fission" with thermal neutrons, and the fission of plutonium had been studied only through observation of the fission fragments (atomic nuclei) produced.

The Van de Graaff accelerators used at Los Alamos for these experiments were invented in 1929 by Robert J. Van de Graaff, a Princeton University physics professor.

Joseph McKibben, a postdoctoral physicist, and David Frisch, a graduate student at the University of Wisconsin, used deuterons (ions of heavy hydrogen) from the short tank to bombard carbon and other deuterons to produce fast neutrons and directed them at various materials to see which would reflect them best. By September 1942, they had narrowed their search to dense elements like lead, bismuth, tantalum, tungsten, platinum, gold and uranium, but the scattered neutrons from these elements were still very hard to detect. Before the short tank was moved to Los Alamos, McKibben and Frisch ran the accelerator around the clock to get the data they needed.

Four University of Wisconsin graduate students, Alfred O. Hanson, Morris Blair, David L. Benedict and James Hush used the long tank to measure fission cross sections (the probability that the neutron would cause fission) for uranium-235. Bombarding lithium targets to produce neutrons of up to 1.8 MeV to make these measurements, they found the fission cross-section to be about 1.66 barns. Although a uranium atom was not as big as a real barn, this was the name used by physicists for the unit of measurement for nuclear cross sections.

The group succeeded in getting the long tank into operation first and demonstrated that the number of neutrons produced in fast fission of plutonium was adequate to sustain a fission chain reaction. The neutron number was measured using an almost invisible speck of plutonium, about 142 micrograms, which had been produced in the cyclotron at Washington University in St. Louis. "Before we had to turn it over to the chemists," Richards recalled, "we were to measure as many of its physical cross sections as possible. In particular we needed to verify that it produced neutrons upon fission and to measure roughly the number of neutrons per fission. Many of us worked 18- to 20-hour days during this period, but we got the crucial measurements done. We arranged to take a few days vacation afterward. Some of us camped up in the beautiful Pecos Valley."

Not only was the number adequate to sustain an explosive chain reaction, but it was greater than the number of neutrons produced in the fission of uranium-235, to which the Los Alamos experimenters compared it. The experiment also showed that the delay in neutron emission in fast fission of plutonium was so small as not to threaten the possibility of an efficient chain reaction.

In the late summer of 1943, experimental work at Los Alamos was focused on the designs for two gun-type atomic weapons. One would fire a uranium "bullet" into a uranium "target," while the other would use plutonium bullets and targets and, to overcome problems that might be caused by impurities in plutonium, would fire the bullet at a higher velocity.

Implosion Method

It had also occurred to Richard Tolman, a professor of physics at the California Institute of Technology and vice-chairman of the National Defense Research Committee, that fissionable material might be assembled by detonating a high explosive around a hollow sphere and crushing it into a critical mass. Because of the difficulty of implementing this idea, however, few paid much attention to it. Robert Serber, a University of California physics professor, mentioned it in his indoctrination lectures at Los Alamos in April 1943 as one of "various other shooting arrangements" that had been suggested "but as yet not carefully analyzed."

Upon hearing Serber's lectures, Seth Neddermeyer, another professor of physics from Cal Tech, seized upon the idea enthusiastically. He recognized that blowing a sphere of uranium-235 or plutonium together in this matter would assemble these materials more rapidly than a gun could and proposed that it be explored. Oppenheimer agreed to a small program, which was set up on South Mesa.

The Ordnance Engineering Group (E-5) under Neddermeyer's direction, pursued experiments there and in Pennsylvania, at the Bruceton Explosives Research Laboratory of the NDRC. At Bruceton, "implosion charges" were fabricated for them by George Kistiakowsky of Harvard University, who was head of the project. Neddermeyer and Edwin M. McMillan, a University of California physicist who traveled there with him, were impressed that when a shell of explosives surrounding an iron pipe was set off, it closed the pipe. They returned to Los Alamos to repeat the experiment, varying the explosives, the pipe size and the arrangements, and studying the remains. A small plant was built at Anchor Ranch to cast the high explosives used in these experiments.

In September 1943, Oppenheimer asked John von Neumann, a Princeton mathematician who had been working on shaped charges, fluid dynamics and the computation of ballistic trajectories as a consultant to the Army's Aberdeen Proving Ground in Maryland, to look into the theoretical problems faced at Los Alamos.

Von Neumann agreed to spend some time as a consultant, working primarily in his office at the National Academy of Sciences in Washington, D.C., but with an occasional visit to Los Alamos. His first, in September, acquainted him with the implosion program. He suggested that shaped charges would produce an appropriate spherical detonation wave and pointed out that the method was not only likely to be faster than the gun, but that it would produce higher pressures and reduce the amount of active material required, making the bomb more efficient.

The Laboratory was galvanized by von Neumann's insight. Theorist Edward Teller scolded Charles Critchfield, who had been working on the project, for overlooking the greater efficiency to be expected from implosion, and Manhattan Engineer District Commander Leslie Groves chided Navy Capt. William Parsons for focusing on the "safer" gun method.

Kistiakowsky was persuaded to come to Los Alamos to head a new program to develop the high explosives. A diagnostic program, involving X-ray and photographic techniques as well as the "terminal observations" Neddermeyer had employed, was begun.

New ideas for diagnosing an imploding system, including the use of a betatron electron accelerator, magnetic fields, electric pins and natural sources of radioactivity to produce signals that would indicate the rate of collapse inside the sphere, were subsequently introduced.

Calculations showed that an inward-moving spherical shock wave would be disrupted by the interference of detonation waves from the high-explosive segments and by instabilities arising as the tamper material was pushed into the heavier nuclear core by the implosion. This led to a fuller understanding of the behavior of a symmetric implosion and greater doubt that it could be achieved. What was needed was an explosive lens to convert the detonation wave to a spherically convergent form.

Under Kistiakowsky's direction, a new site, Sawmill, off S-Site, was constructed between December 1943 and May 1944. James Tuck, a member of the British Mission at Los Alamos, had worked in England on the use of combinations of different explosives to "focus" detonation waves and headed a group to develop an explosive lens for the implosion gadget. After von Neumann suggested a workable design for the lens, Lt. Cmdr. Norris Bradbury, a Stanford physics professor assigned to the Dahlgren Proving Ground of the U.S. Navy Ordnance Bureau, was recruited in June 1944 to solve the problem of casting the high explosives for the design.

Even if the appropriate explosive lenses could be produced, they would have to be set off simultaneously to create a symmetrical implosion. After experiments with a variety of Primacord and electric detonators, Luis Alvarez, a University of California Radiation Laboratory physicist who had come to the Laboratory from radar work at the Massachusetts Institute of Technology, and his student, Lawrence Johnson, devised such a system in May 1944.

Although progress had been made, Kistiakowsky was skeptical about the success of the program in the spring of 1944. He predicted that by October they might be able to "recommend a design of the gadget that will have a finite chance of properly functioning," but added that in "November or December the test of the gadget failed. Project staff resumes frantic work, Kistiakowsky goes nuts and is locked up." The consequences of such a failure, however, would be devastating to the program.

In the summer of 1944, Emilio Segre's 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.

It was not clear, however, that the much more complicated implosion device would work. Before it could be used in combat, a test would be required.

Computers were people using desk calculators when Los Alamos began. By the end of the war, Los Alamos scientists were using the first electronic computer. John von Neumann was the primary agent of this change, which led to the Laboratory's strong program in computer science and technology, as well as making it possible to calculate the behavior of nuclear explosives.

Early calculations relating to the diffusion of neutrons in a critical assembly of uranium were made by Eldred Nelson and Stanley Frankel, who were members of Robert Serber's group in the Radiation Laboratory at the University of California, Berkeley, in 1942. When they came to Los Alamos in the spring of 1943, they ordered the same sorts of machines that they had used in California: Marchant and Friden desk calculators to make the calculations required in the design of nuclear weapons.

To perform some of these repetitive calculations, a group of scientists' wives were recruited to form a central computing pool. These "computers" included Stanley Frankel's wife, Mary; Josephine Elliott; Beatrice Langer; Mici Teller; Jean Bacher; and Betty Inglis. This became group T-5 under New York University mathematician Donald (Moll) Flanders when he arrived in the late summer of 1943.

The mechanical calculators tended to break down under heavy use by physicists and had to be shipped back to the manufacturer until physicists Richard Feynman of Princeton University and Nicholas Metropolis of the University of Chicago learned to repair them. Although Theoretical (T) Division Leader Hans Bethe at first objected that this was a waste of time, he relented when the number of working calculators diminished.

Dana Mitchell, whom Laboratory Director J. Robert Oppenheimer had recruited from Columbia University to oversee procurement for Los Alamos, recognized that the calculators were not adequate for the heavy computational chores and suggested the use of IBM punched-card machines. He had seen them used successfully by Wallace Eckert at Columbia to calculate the orbits of planets and persuaded Frankel and Nelson to order a complement of them.

In September 1943, von Neumann made the first of many visits to Los Alamos. A mathematician at the Institute for Advanced Study at Princeton, he had been asked by Oppenheimer to serve as a consultant in hydrodynamics, and during his visits he became aware of the work on implosion being conducted by Seth Neddermeyer and his group.

Von Neumann, who also was a consultant on explosives for the Army, pointed out that shaped charges could be used to produce a more uniform shock wave for this purpose. He subsequently developed Neddermeyer's one-dimensional theory of implosion with Edward Teller, the theoretical physicist from the Metallurgical Laboratory of the University of Chicago. When von Neumann had difficulty with the pure high-density incompressible phase of implosion, he suggested a test implosion to determine physical quantities that could not be calculated analytically. He subsequently formulated another model for computation, and Teller set up a group in T Division devoted to the theory of implosion.

Continued at Los Alamos:Part 4

References

• Adapted from the Wikipedia article, "Los Alamos National Laboratory" http://www.wikipedia.org/wiki/Los_Alamos_National_Laboratory August 11, 2003