New Toys

For the first time since 1940, U.S. physicists could talk about atomic fission without looking over their shoulders. The subject of The Bomb itself was taboo at the annual midwest meeting of the American Physical Society in Chicago; but there was plenty more for the atomic scientists to talk about.

Fishing Probabilities. Swaggering little Enrico Fermi, who put the match to history's first atomic chain reaction, led off with a circumstantial account of how a chain-reacting pile works.

A typical pile is a 20-foot block of graphite (pure carbon) interlarded with lumps of fissionable uranium. The chain begins with the capture of a neutron by a uranium atom. When the atom "fishes" (splits by fission), neutrons released by the reaction fly off at more than 6,000 miles a second. To give the neutrons a maximum chance of being captured by other uranium atoms, they are slowed to "thermal" speed--roughly 3 m.p.s. Normally a neutron slows down to that speed after about 110 collisions with carbon atoms.

In controlling fission, the nuclear physicists' big problem was to calculate the probability that a given atom would capture a neutron traveling at a given speed. They found that in a certain type of pile the critical size at which a lump of enriched uranium begins to cook in a nonexplosive chain reaction is 1.5 kilograms (about 3 1/3 Ibs.). Theoretically, a pile might heat up to the temperature of the sun (over 6,000DEG), but no known container can withstand more than 1,500DEG. The physicists discovered that the simplest way to throttle down a pile was to thrust into it a cadmium bar, which stops neutrons cold.

Wave or Particle? It was abundantly clear at the Chicago meeting that the atomic scientists had found neutrons fascinating toys. Biggest news was the neutron's probings into the nature of matter.

The modern theory of wave mechanics, which holds that units of matter (e.g., electrons and protons) behave partly like particles, partly like waves, rests on a revolutionary electron experiment performed by Clinton J. Davisson and Lester H. Germer in 1927. Their experiment, done with crystals, was analogous to shooting a beam of electrons at a barrier punctured by two holes very close together.

To the great surprise of classical physicists, the electrons, recorded on a screen after they went through the holes, made a wave "interference" pattern. The experiment proved that, somehow, each electron went through both holes at once. The discovery, a great blow to the notion that matter is indivisible, led to the theory that a particle can be broken up into fields of energy which alternately reinforce and cancel each other, exactly like waves.

At Chicago, the atomic physicists reported that experiments with neutrons had spectacularly confirmed the wave mechanics theory. Like electrons, neutrons shot through a crystal produced a "diffraction pattern," which showed that they were really waves.

This discovery has some immediate applications. Blond, bushy-browed Walter H. Zinn, the discoverer, who looks like a happy Mephistopheles, thinks that neutrons can probably be used like X rays to examine the structure of molecules. Neutrons are light enough to be scattered by hydrogen atoms, which X rays do not detect; hence they can be used to study organic molecules, such as viruses, which mark the difference between living and inanimate matter.

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