The Bomb Watchers

When U.S. nuclear sleuths, cruising in airplanes off the coast of northeast Asia, pick up radioactive dust from Soviet bomb tests, they give out no information whatever. Russian and British airborne atomic detectives are just as uncommunicative. But the Japanese, sitting innocently bombless between Soviet and U.S. test areas, can talk freely. Last week Dr. Yoshio Sugiura of the government's Meteorological Research Institute told a Kyoto meeting of the Japan Chemistry Society what he had deduced from "ashes of death" that fell in his own backyard.

Dr. Sugiura needed no airplane. Last November, just a few days after Japanese meteorologists detected air disturbances from Soviet tests in Siberia, he set two large porcelain dishes filled with water in the yard behind his Tokyo laboratory and let dust settle into them for 24 hours. He evaporated the water and got from each square meter 150 milligrams (.005 oz.) of dust. Most of it was ordinary dirt from Tokyo's grimy atmosphere, but the remainder was highly radioactive, and could be analyzed.

Telltale U-237. About 27% of the radiation came from U-237, a short-lived uranium isotope (half-life: 6.75 days) which does not exist in nature. Nearly all the rest came from elements with middle weight atoms, such as tellurium, zirconium and cerium. The content of the sample was roughly the same as that of dust that came from the great U.S. bomb exploded at Bikini on March 1, 1954.

Tipoff ingredient was the U-237. In the original atomic bomb of 1945 the active substance was U-235, the rare uranium isotope that fissions (splits) readily when struck by slow-speed neutrons. U-238, the abundant isotope of uranium, does not fission in this way, but when it is struck by high-speed neutrons from a sufficiently powerful detonator, it undergoes a variety of nuclear reactions. Some of its atoms split, splattering into middleweight atoms (fission products) and giving off enormous energy. Other U-238 atoms absorb a neutron, then eject two neutrons, turning into atoms of telltale U-237.

So the presence of U-237 as well as fission products in the dust that fell on Tokyo convinced Dr. Sugiura that the Soviet bomb of last November was a "super-U-bomb" like the U.S. Bikini job of 1954 (then popularly known as the hydrogen bomb). In short, it evidently got most of its energy from the fission of cheap, plentiful U-238.

Telltale Waves. Radioactive dust tells nothing about the power of the shot, but Japanese bomb watchers have another trick that gives a fair indication. They measure the power of the atmospheric wave set in motion by the explosion. The wave from the U.S. blast at Bikini (2,485 miles from Tokyo) rated .4 millibars in Japan, while the Soviet explosion (1,802 miles from Tokyo) rated only .15 millibars. These figures cannot be taken as directly proportional to the power of the explosions (shock waves can act odd), but observers in Japan estimate the biggest U.S. bang at 12 megatons, believe it was about twelve times as powerful as the biggest Soviet bang.

Japan's bomb watchers have not yet reached full conclusions about the Soviet tests announced last week (without details) by the U.S. Atomic Energy Commission. They are pondering two facts: 1) the rain that fell on Shikoku Island on March 24 was the most intensely radioactive that has yet fallen on Japan; 2) none of the government's 13 microbarograph stations recorded any shock wave at all.

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