Not-So-Clean Fallout

"Clean" thermonuclear bombs, like clean small boys, do not necessarily stay clean for long. The most familiar kind of radioactive fallout comes from the fission of plutonium or uranium 235, and from the so-called clean bombs that the U.S. Government has announced contain only small amounts of these troublemaking substances. The bulk of the bomb's bang comes from fusion of hydrogen, which creates no fission products.

But a bomb can be clean in one way and dirty in another. In Science, William H. Shipman and other scientists from the Naval Radiological Defense Laboratory, San Francisco, tell how they found large quantities of radioactive manganese 54 in the fallout from last year's thermonuclear tests at Eniwetok. Since Mn-54 is not a fission product, they concluded that it was formed when free neutrons from the explosion combined with iron or ordinary manganese, presumably in the bomb's structure. Figuring back, they estimated that "megacurie quantities" were produced.

Since each megacurie is roughly as potent as 2,200 Ibs. of pure radium, this is a large amount of radioactivity. Mn-54 has a rather long half life, 291 days, and since it is absorbed by living organisms, the Navy's safety men have added it to their list of dangerous fallout isotopes. They are now looking for plants and animals that may pick it up as it floats around the earth, and concentrate it in their tissues. They already know one plant, tea. that is avid for manganese and may concentrate the radioactive kind.

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