Many scientists had discovered that an atom consists of a nucleus surrounded by electrons. This nucleus is further broken down into two types of particles, protons and neutrons, which are held together by an invisible strong nuclear force. James Chadwick (1891-1974), a British Scientist, made the very important discovery in 1932, of the neutron. Neutrons have no electrical charge; yet they can be used to split the nucleus of an atom. This process is known as nuclear fission.
In 1939, two exiled German scientists who had left Hitler's regime conducted an experiment using neutrons to split the uranium atom. This experiment was a very important one. The experiment shows that if two fragments of the split uranium nucleus flew apart at a very high speed, they would create other neutrons, which in turn would split other nuclei and generate enormous energy in a fraction of a second. This event that ensues is a chain reaction. If all the nuclei in a pound of uranium were allowed to split at the same time, the energy given off would be far greater than anything ever produced by man. .
There were several major problems that these scientists had to overcome when constructing the atomic bomb. It was very important that these scientists knew how to control the number and direction of the neutrons. They also had to work out how to control the power released through nuclear fission. Also, adequate supply of fissionable material had to be collected. The most important supply was uranium. This substance is mainly found in Canada and the Belgian Congo, and also plutonium, which was a recently discovered radioactive element that was man-made. All of these problems were solved during the Second World War when the United States developed the atomic bomb. .
In the summer of 1939, war broke out in Europe. Within a year, the German and Italian armies had controlled a large part of the continent. Killingray states: "Many scientists, some of whom were refugees from Germany and Italy, feared that Germany would develop an atomic bomb.