Sept. 6, 1961 - The Atomic Energy Commission announced today that the Soviet Union had detonated a fourth nuclear device in the atmosphere. A brief announcement said the explosion took place this morning east of Stalingrad. Stalingrad, in southeast European Russia, is an industrial and agricultural area, but several hundred miles to the east there are areas of sparsely populated desert. Traces of radioactive fallout from the earlier Asian blasts have been detected in Alaska, it was disclosed today - 35 times the average daily level for August. Dr. Luther L. Terry, the Surgeon General, said there was no immediate health danger.