The Making Of The Atomic Bomb

FAS Astronomers Blog, Volume 31, Number 9.

Physics drives the cosmos. Okay, there is a lot of chemistry going on as well. But astrophysics is the big topic, not astrochemistry. Physics played an important role in something that happened down here on the Earth during July and August just under eighty years ago. For the first time in history, the power of the cosmos came down to our small planet.

On July 16, 1945, a new kind of weapon exploded in the desert near Alamogordo, New Mexico. The first atomic bomb was detonated at a place called Trinity Site. Robert Oppenheimer described his reaction to the test by quoting from the Hindu scripture Bhagavad Gita, “Now I am become death, the destroyer of worlds.” In August 1945, two American warplanes dropped atomic bombs on the Japanese cities of Hiroshima and Nagasaki. Within a few days, the Japanese surrendered, and World War II was over.

The story of the development of the atomic bomb has been told many times over. A few selected articles and books can be found below. Most recently, a new movie, Oppenheimer, has just hit the major cinemas across the country.

The fact that an atomic bomb could be made at all resulted from the work of several physicists at the end of the 19^th century and the beginning of the 20^th century. Wilhelm Roentgen, Henri Becquerel, Marie & Pierre Curie, Ernest Rutherford, Frederick Soddy, and Bertram Boltwood discovered that certain elements decay into other elements through what we now call radioactivity. However, this was only the start.

There is a story that Leo Szilard read about a talk given by Ernest Rutherford, in which he said that radioactivity would have no practical purpose. A short time later Szilard was crossing a street in London. It suddenly dawned on him that if a radioactive material is bombarded with neutrons and more neutrons are released, a chain reaction could result releasing a huge amount of energy. Szilard patented his idea, but because he thought it could lead to the making of a weapon, he assigned the patent to the British Admiralty to keep it “out of sight”.

A few years later in 1938, Otto Hahn and Fritz Strassman bombarded uranium with neutrons, hoping to create a heavier element. However, they discovered that the experiment yielded the much lighter element barium. They were perplexed by this result and Hahn sent a letter describing their experiment to Lise Meitner, who had previously worked with Hahn and Strassman. Meitner and her nephew Otto Frisch were able to deduce that the neutrons caused the uranium to split into barium and krypton while producing energy in the form of more neutrons, which could theoretically continue the process.

With this, it appeared that an “atomic” weapon could be made. Szilard drafted a letter to President Roosevelt warning him of the possibility and arguing that if the United States didn’t investigate the building of a weapon, there was a chance that Germany would beat them to it and develop the weapon first. Szilard, not sure if Roosevelt would pay any attention to him, enlisted Albert Einstein to sign the letter.

Roosevelt soon initiated an Advisory Committee on Uranium, which morphed into the Office of Scientific Research and Development (OSRD). OSRD formed the Manhattan Engineering District in 1942 and General Leslie Groves was appointed to lead the effort. As work began to accelerate, Roosevelt consolidated everything into The Manhattan Project, which was the most secretive effort of the United States during the war.

The first step was to prove that all this was possible by creating a controlled uranium chain reaction. Italian physicist Enrico Fermi led the effort in the squash courts underneath Stagg Field at the University of Chicago. At the end of 1942, Fermi and his team built the first atomic “pile”, which was nicknamed Chicago Pile-1 (CP-1). The pile consisted of 40,000 graphite blocks infused with a small amount of uranium. The graphite slowed the neutrons. Rods of cadmium were also placed in the pile to absorb neutrons to prevent the pile from going critical too soon or running away and destroying much of Chicago.

On the afternoon of December 2, they slowly pulled the rods out from the pile and the “reactor” went critical – the number of neutrons was sufficient to keep the chain reaction going. Because of the secrecy of the experiment, President Roosevelt and others in the government were notified of the results with the message: “The Italian navigator has just landed in the new world.” By the way, the three instruments used to monitor the experiment were nicknamed Piglet, Tigger, and Pooh. Although, I don’t believe the House at Pooh Corner was anywhere in the area.

To make an atomic bomb, they had to utilize material that would create the appropriate type of chain reaction. It had to be quick, and it had to be something that could be triggered remotely. The search for such a material began with uranium. However, the most common uranium 238 tends to absorb neutrons and transforms to uranium 239. It, in turn, decays through two quick beta decays into neptunium 239 and then into plutonium 239. Plutonium does undergo fission and became the primary source material for the bomb. Uranium 235 will also undergo fission, but it must be chemically extracted from uranium 238 and this is a little more difficult than the process to create plutonium.

During the war as part of the Manhattan project, two remote sites were established. One was in Oak Ridge, Tennessee to extract uranium 235 from uranium 238. The other was in Hanford, Washington state designed to produce plutonium. Over time, these two sites slowly created enough material for the bombs to be tested and eventually used.

To keep the bomb-making part of the project under wraps and to bring everyone together into one location, the scientists and their families were sequestered in a remote area of northern New Mexico known as Los Alamos. Groves recruited physicist J. Robert Oppenheimer to lead the Los Alamos effort.

The challenge in creating the bomb itself was how to take two chunks of non-critical material and combine them in such a way that an uncontrolled chain reaction would result from a “critical” mass.

• The first method was the gun method where a pellet of material is shot into a sphere of material resulting in a critical mass. The bomb using this methodology was nicknamed “Little Boy.”
• The second was the implosion method where a hollow sphere of material is imploded in on itself also creating a critical mass. This bomb was nicknamed “Fat Man.”

Plutonium, as noted above, was the preferred material for both, and sufficient quantities were available. However, it proved to be problematic using the gun design and, initially, there wasn’t enough uranium 235 for a “gun bomb.” So, the Los Alamos scientists turned their attention to the implosion methodology, with which they could utilize the available supply of plutonium.

There was every confidence that Little Boy would work once more uranium 235 became available. But the implosion method of Fat Man was more of an unknown. Therefore, it was first tested in the early morning hours of July 16, 1945 at the Trinity Site in New Mexico. To allay everyone’s apprehension over what would happen, a few of the scientists placed a wager on the bomb igniting the Earth’s atmosphere. Although, Edward Teller had proposed the possibility a few years earlier and calculations showed there was very little chance of this happening. The bomb exploded as expected and the world was safe (maybe). Enrico Fermi did drop a few pieces of paper as the shock wave hit and, based on the distance they traveled, estimated the yield of the blast.

Soon after on August 6, 1945, a Little Boy uranium bomb was dropped on the Japanese city of Hiroshima. The Japanese refused to surrender. So, on August 9, 1945, another Fat Man plutonium bomb was dropped on Nagasaki. Japan reached out the next day and surrendered ending World War II.

From this moment on, the world was never the same.