Experimenting with tanks and barns

John H. Williams, group leader of Electrostatic Generator (P-2), above, and his group observed neutrons from the fission of plutonium-239. The Van de Graaff accelerators used at Los Alamos for these experiments were invented in 1929 by Robert J. Van de Graaff, a Princeton physics professor. The University of Wisconsin "long tank" Van de Graaff, left, produced its first beam May 15, 1943.

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 (million-electron-volt) 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 the products of American scientific technology. Invented in 1929 by Robert J. Van de Graaff, a Princeton University physics professor, these “electrostatic accelerators" used a very simple principle to build a large potential on a sphere. Electrons or positive ions carrying a small electric potential were fed onto a moving belt that carried them to a sphere, where the charge accumulated until several million volts were built up. The potential of the sphere was limited only by the breakdown of the insulating medium surrounding it.

At the University of Wisconsin, Ray Herb developed a pressurized container that could hold a non-conducting gas at high pressure around the sphere, making possible the accumulation of higher potentials. With this refinement the Van de Graaff generator replaced most other direct-voltage generators previously used for particle acceleration. In the energy range up to 4 MeV, the pressurized Van de Graaff delivered a steady parallel beam of particles, free from stray radiation, which made it an ideal source for nuclear studies in the range of energies for fast neutron research. Its unusually homogeneous beam energy could be focused on a small target and varied at will, so nuclear processes could be studied as a function of bombarding energy. The device was copied at a number of American research institutions, including the University of Minnesota, and much of the quantitative data on the nuclear properties of elements came from these machines in the late 1930s.

The two machines brought to Los Alamos had been used in nuclear weapons design research at Wisconsin. 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. One barn was equivalent to 10-24 square centimeters per nucleus.

The effectiveness of the Wisconsin machines made them the accelerators of choice for similar measurements at Los Alamos. The group that would use them was also primarily from Wisconsin, with one significant exception. Williams, a physicist from the University of Minnesota who had built a large pressurized Van de Graaff accelerator there, was among the first to arrive at Los Alamos and served as acting site director until the first Laboratory buildings were completed and the first scientists moved to the site. He was also one of the first members of the planning board formed at the beginning of the project to plan the technical program.

Williams was made the leader of P-2. Other members included Blair, Frisch, Hanson, Hush, Robert Krohn, McKibben, Rowland Parry, Worth Seagondollar, Richard Taschek and Clarence Turner, all from the University of Wisconsin, Carl Bailey from the University of Minnesota, Hugh Richards from Rice University and Po Povi Da of San Ildefonso Pueblo, who was a technician with the Special Engineering Detachment (SED).

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.

During the three months between April 15 and July 15, Allied forces overcame heavy German resistance in North Africa, taking 950,000 German and Italian lives in the process, and invaded Sicily. The United States, Australia and New Zealand opened a concerted offensive in the South Pacific with the invasion of New Guinea; Russia threw back a German offensive at the battle of Kursk about 300 miles south of Moscow and counter-attacked against the Germans on the Orel salient about 200 miles south of Moscow. If a nuclear weapon were to play a role in the war, it could come none too soon

-Robert W. Seidel

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