The Manhattan Project

The Manhattan Project In the months following the bombing of Pearl Harbor the Manhattan Project–the name given to the atomic bomb program because its original offices were in Manhattan–grew very quickly. And although the Army had been involved since June of 1942, it was just beginning to realize that someone was going to have to be put in overall charge. The man chosen was Leslie Richard Groves, a 46-year-old colonel in the Army Corps of Engineers. While he was a competent engineer, Groves was no scientist.

He did not understand the science behind building the atomic bomb, nor did he pretend to.He needed someone who would be able to supervise the scientific side of the project. After dismissing a number of candidates, Groves decided on who seemed like the most improbable of candidate of all–38-year-old J. Robert Oppenheimer. After he was officially given the job of laboratory director, Oppenheimer planned a campaign of “absolutely unscrupulous recruiting of anyone we can lay our hands on.”1 He used his charismatic personality to recruit some of the greatest scientific talent in the world to join the project.

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He then helped Groves find a location for his bomb-making laboratory, tentatively called “Site Y.” A number of southwestern sites were explored.On November 16, Oppenheimer, Groves, and representatives from the Army Corps of Engineers were looking at a site at Jemez Springs, New Mexico, a deep canyon about 40 mi. (64 km) northwest of Santa Fe. Oppenheimer did not care too much for the site, nor did Groves.

His main objection was that there was no room for expansion. Oppenheimer then innocently remarked about going back to Albuquerque via the Los Alamos Ranch School. Groves liked Los Alamos at once, and began moving quickly.He called Washington that very evening and began to buy the land. The Ranch School was having financial trouble as a result of the war, and so it was more than happy to sell out. Within a week, the land, the building, and other possessions of the school–including 1,600 books and 60 horses–sold for $440,000.

Los Alamos, or “the Hill,” as it was commonly referred to, officially opened for business on April 15, 1943. All bombs, and especially those being developed at Los Alamos, release energy in the form of light and heat.A certain amount of energy, called the binding energy, is required to hold the nucleus of an atom together. This energy is relatively small for light elements and steadily increases for heavier elements as far as cobalt, iron, and nickel. After that, in still heavier elements, it begins to decrease to the point that the binding energy of an extremely heavy atom, such as uranium, is less than that of many, much lighter elements.

A small portion of the mass of each particle is lost when it enters a nucleus so that a proton, for instance, actually weighs less inside the nucleus than outside. It must do this to fit in.To do this, it converts some of its mass into energy. The combined mass loss of all the particles of the nucleus equals the binding energy. There are two processes by which particles can be made to lose weight. One, called fission (the type of bombs dropped on Japan), happens when a heavy nucleus splits apart into two lighter nuclei.

These newly formed nuclei have a higher binding energy than their heavier “parent” nucleus; therefore, they demand a further weight loss on the part of their particles.The other process, called fusion, occurs when two light nuclei fuse together to form a single heavier nucleus with a higher binding energy. In both cases the particles must lose mass and release energy. Certain types of atoms with many protons and neutrons in their nucleus are radioactive; they are unstable and may break apart spontaneously. Other types, upon absorbing neutrons, break apart. In this process, the entire nucleus falls apart into two pieces, releasing energy in the process, but only after the nucleus temporarily increases its mass number by one.Two atoms, P-239 and U-235, undergo this type of division and release energy at the same time.

U-235 emits two or three neutrons in the process, while P-239 emits many more. Either of these two atoms may be used in an atomic bomb. After absorbing a neutron, an atom of these elements emits several more neutrons, making a chain reaction possible. If the surrounding structure is properly designed, the result is an explosion.

The amount of fissionable material needed to make an explosion is called the critical mass, or the trigger quantity.Because a chain reaction would begin immediately, the material cannot be place all together. It must be broken up and contained in pieces that are smaller than the critical size. These pieces are then brought together in one supercritical lump, and at the moment of detonation, neutrons are fired into it. Although the energy released by each fissioning atom is tiny, an explosion results from the cumulative energy of trillions of such atoms. However, the equivalent of this energy in mass is miniscule.

The bomb that leveled Nagasaki in 1945 released an amount of energy equal to that of a third of the weight of a penny. The basic principle of a self-containing chain reaction had first been demonstrated in an experiment devised by the brilliant scientist Enrico Fermi on December 2, 1942.On that day, at the squash courts of the University of Chicagos Stagg Field was the “atomic pile”–a nearly 500-ton pile of graphite bricks, stacked in 57 layers into which cubes of uranium or uranium oxide were embedded which two shifts of workers had labored for sixteen days to build. The twenty foot (6 m)-high structure had no blueprints, or even plans, except for what existed in Fermis head. “Long control rods, plated with the element cadmium, were set up so they could be inserted into holes in the graphite bricks and withdrawn when required. The graphite would slow down the neutrons emitted by the uranium and the cadmium would absorb them. As the control rods were withdrawn, however, fewer of the neutrons from the uranium, resulting in greater fission–more atoms split.At some point as the rods were withdrawn, fission would produce neutrons faster than the cadmium could absorb them.

The result would be a self-sustaining chain reaction.”2 Stuffed into the balcony overlooking the squash court were about forty senior scientists, while three young men were poised on a platform above the pile. They were dubbed the “Suicide Squad,” because it was their job to douse the pile with a cadmium salt solution if the experiment went out of control.

The only man on the floor was George Weil, a young physicist who would be the one to slowly pull the last control rod out of the pile. There was a safety rod controlled by a solenoid-activated catch designed to automatically fall into place and stop the chain reaction if neutron activity surpassed a preset level.There was a project leader, armed with an ax, ready to cut the rope so the rod would fall into place and hopefully stop the reaction if things went wrong. At exactly 10:37 am, the experiment began when Fermi instructed Weil to remove the last cadmium rod. The neutron counters were then activated. Fermi had his six-inch (15 cm) slide rule and was carefully calculating the rate of increase. It met his expectation, so he then instructed Weil to move the rod out another six-inches.

Once again the counter was activated. Fermi began calculating again, and he seemed pleased. The process continued for about an hour, when the safety rod was automatically released with a loud crash. The release was unexpected, but Fermi knew the pile was still subcritical. At 2 oclock that afternoon the safety rod was reset and the experiment continued. At 3:25 Fermi told Weil to pull the control rod out another 12 inches (30 cm).”This is going to do it,” he said. Moments later, Fermi announced to all that the pile had gone critical.

At 3:53 P.M. the control rod was reinstated. This was the first controlled released from the atomic nucleus.

Many consider that moment in the freezing squash court the key step in the development of the atomic bomb. After all that, the only thing left was the”engineering.” There were really two types of bombs being developed at Los Alamos. The first used U-235.

It was to be detonated by using a modified artillery gun inside a bomb casing to fire a lump of uranium onto a uranium target at 2,000 feet (610 m) per second. The impact would produce a nuclear explosion. The only problem was that U-235 was so rare that there would probably be enough of it to produce only one bomb over two years.The second type was the theoretical bomb built from the artificial element plutonium. No one there had actually seen the element, but they were reasonably sure that there was enough of it to make multiple bombs. By July, however, the scientists discovered that the tow subcritical masses of plutonium could not be brought together fast enough to prevent premature explosion, thus ruling out the simple “gun assembly method of detonation.” The solution, as suggested by one of Oppenheimers former students, physicist Seth Neddermeyer, was “implosion.” He proposed that the plutonium should be surrounded by a layer of high explosives that, when detonated, focused the blast so as to compress the plutonium instantly into a supercritical mass.

Being a much more complicated procedure, most of the scientists who first heard it, including Oppenheimer, did not think it could be made to work. The implosion theory became much more feasible when the mathematician Jon von Neumann showed them his calculations that it could be done, and would in fact require less of the precious fissionable material than the gun method. By early 1944 a crisis had developed at Los Alamos–they were having difficulties in getting adequate quantities of fissionable material. Ge …