for the continued security of the world). Meitner and Hahn’s discovery pointed the way, though, and soon after, in 1939, a team at Princeton discovered a significant difference between the two different “flavors” of uranium. Uranium 238—the more stable version with three extra neutrons in the nucleus—proved to be much worse at absorbing neutrons and subsequently splitting than was uranium 235. If you wanted uranium 238 to undergo fission, you needed slow-moving neutrons, giving the nuclei a chance to absorb them, whereas uranium 235 was capable of latching onto high-speed neutrons.
The vast majority of uranium came in the 238 variety—99.3 percent of a typical chunk of uranium when dug out of the ground would be of this type. This was fine for generating nuclear power, because you could slow down the neutrons using special materials; but the slow neutrons were no good for the “all-at-once” fission required for a bomb. A bomb making use of slow neutrons would fizzle along rather than explode. So if a bomb were to be made from uranium, it would need to be mostly uranium 235, enabling the formation of a chain reaction with fast neutrons.
This proved to be a nightmare problem for anyone attempting to make such a bomb. It isn’t easy to distinguish between uranium 235 and uranium 238. There’s no point trying to chemically refine the uranium to separate the two different varieties (known as isotopes). The chemical properties of an element are determined by the electrons in the atom, and both isotopes have the same number of electrons. It’s only the number of neutrons in the nucleus, and hence the atomic weight, that differs. So to separate uranium 235, it was necessary to find a way to split off tiny amounts of a chemical with a very slightly different weight. It would take several years to discover a way to do this, proving one of the biggest difficulties faced by the atomic bomb project.
The first country to take the idea of a nuclear weapon seriously was Germany. In April 1939, the chemist Paul Harteck wrote to the German war office that nuclear fission would “probably make it possible to produce an explosive many orders of magnitude more powerful than the conventional ones…. That country which first makes use of it has an unsurpassable advantage over the others.”
The possibility of making such weapons was next picked up by Winston Churchill in the United Kingdom, while in the United States in August 1939, a letter from Albert Einstein warning of the dangers nuclear fission posed, written at the encouragement of the father of the fission reaction, Leo Szilard, was sent to the authorities, but it seems not to have raised much interest.
Initially, the difficulties of separating enough uranium 235 to make a bomb seemed insuperable. But in June 1940, American physicists Edwin McMillan and Philip Abelson, working at the Berkeley Radiation Laboratory, wrote a paper that suggested an alternative approach that would avoid the need for separating the uranium isotopes. If uranium 238 can be encouraged to absorb a slow neutron in a reactor, it becomes the unstable isotope uranium 239. This undergoes a nuclear reaction called beta decay, where a neutron turns into a proton, giving off an electron in the process (for historical reasons, the electron is called a beta particle in such circumstances).
The result of this reaction is the production of a new element, one that doesn’t exist in nature. This element was later called neptunium. But neptunium is also unstable and soon generates another electron, adding a second proton to the nucleus to become the element that would be named plutonium. This is a material that is as suitable for making a bomb as uranium 235. And because plutonium is chemically different from uranium, it is relatively easy to separate. Remarkably, the openly published Berkeley paper had shown the first step of how to use a nuclear reactor to make the principle ingredient of an atomic
The vast majority of uranium came in the 238 variety—99.3 percent of a typical chunk of uranium when dug out of the ground would be of this type. This was fine for generating nuclear power, because you could slow down the neutrons using special materials; but the slow neutrons were no good for the “all-at-once” fission required for a bomb. A bomb making use of slow neutrons would fizzle along rather than explode. So if a bomb were to be made from uranium, it would need to be mostly uranium 235, enabling the formation of a chain reaction with fast neutrons.
This proved to be a nightmare problem for anyone attempting to make such a bomb. It isn’t easy to distinguish between uranium 235 and uranium 238. There’s no point trying to chemically refine the uranium to separate the two different varieties (known as isotopes). The chemical properties of an element are determined by the electrons in the atom, and both isotopes have the same number of electrons. It’s only the number of neutrons in the nucleus, and hence the atomic weight, that differs. So to separate uranium 235, it was necessary to find a way to split off tiny amounts of a chemical with a very slightly different weight. It would take several years to discover a way to do this, proving one of the biggest difficulties faced by the atomic bomb project.
The first country to take the idea of a nuclear weapon seriously was Germany. In April 1939, the chemist Paul Harteck wrote to the German war office that nuclear fission would “probably make it possible to produce an explosive many orders of magnitude more powerful than the conventional ones…. That country which first makes use of it has an unsurpassable advantage over the others.”
The possibility of making such weapons was next picked up by Winston Churchill in the United Kingdom, while in the United States in August 1939, a letter from Albert Einstein warning of the dangers nuclear fission posed, written at the encouragement of the father of the fission reaction, Leo Szilard, was sent to the authorities, but it seems not to have raised much interest.
Initially, the difficulties of separating enough uranium 235 to make a bomb seemed insuperable. But in June 1940, American physicists Edwin McMillan and Philip Abelson, working at the Berkeley Radiation Laboratory, wrote a paper that suggested an alternative approach that would avoid the need for separating the uranium isotopes. If uranium 238 can be encouraged to absorb a slow neutron in a reactor, it becomes the unstable isotope uranium 239. This undergoes a nuclear reaction called beta decay, where a neutron turns into a proton, giving off an electron in the process (for historical reasons, the electron is called a beta particle in such circumstances).
The result of this reaction is the production of a new element, one that doesn’t exist in nature. This element was later called neptunium. But neptunium is also unstable and soon generates another electron, adding a second proton to the nucleus to become the element that would be named plutonium. This is a material that is as suitable for making a bomb as uranium 235. And because plutonium is chemically different from uranium, it is relatively easy to separate. Remarkably, the openly published Berkeley paper had shown the first step of how to use a nuclear reactor to make the principle ingredient of an atomic
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