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The Apocalypse Factory: Plutonium and the Making of the Atomic Age

Notes

Physicists also discovered that uranium-235 was splitting not just into barium and krypton but into many different pairs of atoms---xenon and strontium, and iodine and yttrium, and cesium and rubidium---into more than 20 different elements altogether. These fission products, as they're called, tend to be highly radioactive. That's because they have too many neutrons for the number of protons they contain. Uranium atoms, to keep their 92 protons together, have proportionately more neutrons in their nuclei than do lighter elements. As a result, when uranium atoms split, their fission products have a sort of neutron sickness---they have too many neutrons to be stable. To get back into balance, these fission products begin to convert neutrons in their nuclei into protons while releasing electrons and gamma rays. In this way, the fission products start crawling their way down the list of elements, away from hydrogen and toward uranium, until their neutron sickness is resolved.

The neutrons emitted by a fissioning uranium-235 nucleus travel very quickly---at about one-fifteen the speed of light. When a neutron traveling at this speed hits a uranium-238 nucleus, one of two things usually happen. The neutron can bounce off the nucleus and head in another direction. Or it can be absorbed by the nucleus, creating uranium-239. IOn fact, this capture of neutrons byu uranium-238 is the real reason why natural uranium ore does not explode. The absorption of neutrons by the heavier isotope of uranium squelches any possible chain reaction.

But if the fast-moving neutrons from fission are slowed down, something else happens. Slow neutrons are less likely to be captured by uranium-238 atoms. Instead, they bounce harmlessly off uranium-238 until they find a nucleus of uranium-235 to fission. That's the trick to achieving a chain reaction with natural uranium, Szilard realized. If some sort of moderator could be found that would slow down the fast neutrons f rom uranium-235, fewer neutrons would be captured by uranium-238, leaving more than enough to produce a chain reaction.

But how could neutrons be slowed down? Remarkably, Fermi had answered this question just a few years before. High-speed neutrons slow down quickly when they bounce off the nuclei of light atoms. So to find a good moderator, Szilard and Fermi, working together now as each recognized the potential of chain reactions, began making their way down the list of elements. Hydrogen atoms, with their single proton, work best at slowing down neutrons. But they occasionally absorb neutrons, slowing the chain reaction. The next heavier element, helium, does not absorb neutrons, but suspending uranium ore in a container of helium gas did not seem immediately practical. Lithium, with three protons, is a strong neutron absorber, so that would not work. Beryllium, with four protons, does not absorb many neutrons, but is highly toxic when inhaled. Boron, with five neutrons, absorbs neutrons like crazy. But what about carbon, with six protons and six neutrons? It captures neutrons at one-hundredth the rate of hydrogen. And a source of concentrated carbon was readily available: graphite, the "lead" in lead pencils.

"We always thought that they should have left us there," said Jeanie many years later, "that peaches were a better crop than atom bombs."

"Do I wish I hadn't discovered plutonium?" he once said. "No way. Once God had made a world that made bombs possible, there was no options. Both sides were going to make them. But if you ask me, `Do I wish the laws of nature were such that you couldn't make an atomic bomb?' God, yes."