W hen the Atomic Energy Commission allowed observers, including newsmen, to watch a thermonuclear test at Bikini atoll in late May, it told them practically nothing and got them out of the test area as quickly as possible. The rest of the tests were supposed to be secret.
They were not, however, kept entirely secret. Long-range atom-bomb watching has become a favorite sport in Japan. When nuclear tests are impending, whether by the U.S. or the U.S.S.R., all sorts of instruments are checked and tuned for superfine sensitivity. This year a survey ship, the Shunkotsu Maru, carrying scientists and scientific apparatus, has been cruising near the danger area in the Pacific, and tuna boats have been gathering radioactive dust. Japanese scientists relish the fact that they are the only ones in the world who make observations and report them openly.
By Earth, Air & Water. First atomic news that reaches Japan from a “high-yield” explosion in mid-Pacific is carried by earth waves, detected by seismographs. They are followed about three hours later by air waves, picked up by barographs. Then come ocean waves, which register on tide gauges.
On May 21, the day of the “open” shot, the Japanese scientists got the news promptly by earth, air and sea. A week later, on May 28, came waves from an otherwise secret explosion. Both explosions had about the same power, and both were apparently about half as strong as the big one the U.S. exploded on March 1, 1954.
For weeks after these explosions, the Shunkotsu Maru collected very little radioactive material, and Japanese scientists conjectured that the explosions, which took place high in the air, had tossed most of their “hot” residue into the stratosphere in the form of extremely fine dust. The explosion on March 1, 1954 behaved differently because it was a “tower shot” that stirred up millions of tons of quick-settling coral dust. First radioactive material from the May 21 explosion was brought home by the tuna boat Stiruga Maru. Analyzed by Dr. Kenjiro Kimura of Tokyo University, it proved to contain a familiar array of fission products—ruthenium, rhodium, tellurium, iodine, cerium, neodymium, etc.—as well as uranium 237 and neptunium 239. This combination of elements indicated that the explosion was the “fission-fusion-fission” type, which gets much of its energy from the fission of normally inactive uranium 238.
Slow-Falling Dust. Last week the Shunkotsu Maru reported radioactive dust from a third explosion that apparently took place on June 12 or 13. Analysis showed that it was also of the fission-fusion-fission type, but for some reason, perhaps small size or extremely high altitude, it did not stir up air or water waves strong enough to reach Japan. A small earth wave was detected on June 12, and a slightly stronger one on June 13.
Last week, as the scientists had predicted, radioactive dust began to settle out of the stratosphere over Japan. Coming into the troposphere, it mingled with the clouds and fell to earth as radioactive rain. Even before rain fell, the normal air radioactivity of 50 counts per minute rose to 400 counts at Tokyo. Rain at Matsue on the Sea of Japan registered as high as 89,000 counts. Japan’s weather bureau announced, to soothe the jittery public, that more radioactive rain was to be expected, but that it would probably not be harmful to humans.