These neutrons can be used to make any element radioactive, Professor Szilard and his colleagues point out. It follows that the casing of the bomb could be selected with a view to producing, after the neutrons enter it, an especially powerful radioactive substance. Since each artificially made, radioactive element gives out a specific type of radiation and has a definite life span, after which it decays to one half of its radioactivity, the designer of the bomb could rig it in such a way that its explosion would spread into the air a tremendous cloud of specially selected radioactive substances that would give off lethal radiations for a definite period of time. In such a way a large area could be made unfit for human or animal habitation for a definite period of time, months or years. Take, for example, the very common element cobalt. When bombarded with neutrons, it turns into an intensely radioactive element, 320 times more powerful than radium. Any given quantity of neutrons would produce sixty times its weight in radioactive cobalt. If the bomb contains a ton of deuterium, 250 pounds would come out as neutrons. On the assumption that every neutron enters a cobalt atom, this would produce 7.5 tons of radioactive cobalt. That quantity would give out as much radioactivity as 2,400 tons of radium. Now, this radioactive cobalt has a half-life of five years, meaning that it loses half of its radioactive power at every five-year period. So after a lapse of that period of time its radioactivity would be equal to 1,200 tons of radium, in ten years to 600 tons, and so on. If used as a bomb-casing it would be pulverized and converted into a gigantic radioactive cloud that would kill everything in the area it blankets. Nor would it be confined to a particular area, since the winds would take it thousands of miles, carrying death to distant places. The radioactivity produced by the Bikini bombs was detected within one week in the United States. In that short time the westerly winds swept the radioactive air mass from Bikini, 4,150 miles away, to San Francisco. When it reached our shores, the activity was weak and completely harmless, but it was still detectable. That, by the way, was how we learned that the Russians had exploded their first atomic bomb. But, in the words of Professor Teller, one of the Los Alamos men who made the preliminary studies on the hydrogen bomb, “if the activity liberated at Bikini were multiplied by a factor of a hundred thousand or a million, and if it were to be released off our Pacific Coast, the whole of the United States would be endangered.” He added that “if such a quantity of radioactivity should become available, an enemy could make life hard or even impossible for us without delivering a single bomb into our territory.”