September 05, 2006

Nuclear Fission

Otto Hahn and Fritz Strassmann, Concerning the Existence of Alkaline Earth Metals Resulting From Neutron Irradiation of Uranium, 1939

Lise Meitner and Otto Frisch, Disintegration of Uranium by Neutrons: A New Type of Nuclear Reaction, 1939

Do you feel you have a handle on how atoms behave? So did scientists in 1938. They knew that atoms have a nucleus containing positive protons and neutral neutrons of roughly equal mass, orbited by the right number of tiny, negatively charged electrons to make the atom as a whole neutral. They knew that the chemical properties of atoms are determined by the number of electrons in the outer shell, and therefore also by the number of protons in the nucleus. They knew that the positive charges on the protons must repel each other, so there must be a force strong enough at short ranges to overcome that repulsion and bind the nucleus together.

With this model, even radioactivity becomes understandable. Radioactive atoms typically release one of two types of radiation, designated alpha and beta radiation. Alpha particles turn out to be helium nuclei: two protons and two neutrons. Beta particles turn out to be electrons generated as a byproduct when a neutron changes into a proton. So, when an alpha particle is emitted, an atom drops two places on the periodic table, and when a beta particle is emitted, an atom goes up one place on the periodic table. (So, technically, lead could turn into gold if it emitted two alpha particles and a beta particle, but this is extremely unlikely.)

At the experimental level, this was all worked out by chemists as much as anybody else. It was they who separated out pure samples of various radioactive elements, then, after watching the radiation detectors ping for a while, used chemical reactions to separate out and identify new elements produced by radioactive decay. For example, after separating out samples of uranium, chemists ran them through a chemical reaction which precipitates out thorium but leaves uranium in solution, and discovered that the precipitate was radioactive, indicating that some of the uranium decayed into thorium.

After cataloging the nuclear reactions that occur with high enough probabilities for us to observe them in a single lifetime, scientists began firing neutrons (which aren't repelled by positive charge) at normally stable atomic nuclei. This often resulted in atoms emitting alpha or beta particles, or absorbing a neutron to become a new (possibly less stable) isotope. Enrico Fermi soon discovered that firing neutrons at uranium produced some interesting results. For example, one nuclear reaction involved an atom of uranium absorbing a neutron and emitting two beta particles. The resulting element behaved very much like osmium, so Fermi concluded that it belonged in the same column of the periodic table as osmium, but in the same row as uranium: element number 94, now known as plutonium. For this discovery and related work, Fermi was awarded the Nobel Prize in 1938.

Within a year, Otto Hahn and Lise Meitner proved him wrong!

Their discovery began, like so many others, with a puzzling observation. Hahn, a skilled chemist, had determined that either barium or radium had been produced from a sample of uranium bombarded with neutrons. He concluded that this must be radium, which is in the same row as uranium, and proposed that it was created by the absorption of a neutron and the emission of two alpha particles. However, the intermediate, thorium, had not been observed, and neither had the alpha particles. Even worse, he had been firing very slow neutrons, which should not have been able to give enough energy to knock out two alpha particles.

Lise Meitner, a physicist and long-time colaborator of Hahn, realized that the proposed nuclear reactions were highly implausible, and asked Hahn to perform some control experiments. Hahn proceeded to carry out the even more difficult chemisty to separate barium and radium. The result: the mystery element was barium, not radium! Producing barium from uranium by a chain of radioactive decays is about a million times as preposterous as producing radium, and you can tell that Hahn knows it in his paper. ("... which we publish rather hesitantly due to their peculiar results." "... drastic step which goes against all previous experience in nuclear physics.")

Lise Meitner and her nephew Otto Frisch came to the rescue with an explanation. As the number of protons in a nucleus grows, so too does the amount of repulsion that must be overcome by the strong nuclear force. The nucleus also becomes larger, meaning that the strong nuclear force, which weakens rapidly with distance, has less of an effect. The result is that very large atoms, such as uranium, are inherently unstable. With a little nudge from a stray neutron, the nucleus of a uranium atom can be distorted slightly from its normal spherical arrangement, which weakens the effect of the strong nuclear force enough to allow the distortion to grow, and the whole reaction accelerates to the point where the nucleus of the atom splits in two.

These results suddenly caused a reinterpretation of much previous work, including Fermi's. Plutonium can, in fact, be produced from uranium by a chain of decays, but what Fermi detected was actually the splitting of uranium to produce osmium. Hahn alone was awarded the 1944 Nobel Prize for chemistry, in a decision that was much contested. Fermi went on to help produce the world's first nuclear reactor and atomic bomb.


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