Science: Topsy and Godiva

The payoff in nuclear energy comes when a mass of fissionable material "goes critical," i.e., when it begins to support a nuclear chain reaction. In the early days of the Los Alamos atom-bomb laboratory, critical points were determined by hand, by physicists who felt a little jumpy. The start of a chain reaction cannot be predicted dependably. Even a human hand moving near a mass that is barely subcritical can reflect enough neutrons into it to start the reaction and loose a cold and silent flood of death-dealing radiation.

The technique is less hair-raising...

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