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The main trends of H-bomb development, however, are clear to all. An early step was to force the temperature of the fission detonator (atom bomb) as high as possible. One way to do this is to make the fission reaction more efficient. The early bombs "burned" only a fraction of their fissionable material. As they were improved, they burned more of it and reached higher temperatures. The improved bombs, even though not designed with hydrogen bombs in mind, were therefore more effective as detonators.
FUSION BOOST Another trick available to researchers is to place in the fissionable core a small amount of highly reactive tritium, perhaps mixed with deuterium. Both the isotopes are light gases, and so they can be highly compressed and confined inside the metal. They can also be dispersed through it in some chemical or mechanical way. When the detonator explodes in such a rig, the tritium reacts, turning into helium and raising the temperature of the explosion. Such "fusion-boosted" detonators are much discussed among hydrogen-bomb connoisseurs. The long series of "nuclear devices" that the Atomic Energy Commission tested in the Pacific and Nevada may have included many experiments with fusion boosting.
The purpose of the detonator, boosted or not, is to ignite the main charge of light elements. What this charge may contain has not been announced, and the possibilities are numerous. With some oversimplification, charges can be grouped in two categories: "wet" and "dry."
In a "wet" bomb, the main charge is made up of liquefied hydrogen isotopes: tritium and deuterium. The precious tritium is the most reactive. It combines readily with deuterium, and the energy that results raises the temperature sufficiently to make deuterium nuclei combine in pairs, forming helium and giving off more energy.
Since deuterium is comparatively cheap and easily obtained, a practical "wet" bomb should contain very little tritium. But even the best of this type is cumbersome and impractical. Liquefied hydrogen isotopes must be kept under high pressure at a temperature close to absolute zero. They must be carefully insulated. If held for long periods, they must be cooled mechanically to keep them from vaporizing and rupturing their container. Outside scientists say that the "device" exploded on Eniwetok in 1952 was "wet," and that it weighed, with its necessary insulation and cooling equipment, more than 65 tons. If so, it could not have been a droppable bomb.
"Dry" bombs (the March 1 explosion may have been the first of them) use chemical forces instead of cold and pressure to keep their volatile hydrogen crammed into a small space. Their main charge is lithium hydride, a chemical compound containing one atom of lithium and one of hydrogen. Since it is a stable solid that needs no unusual treatment, its use eliminates the troubles connected with liquid hydrogen. It is the key to what airmen call a "transportable" H-bomb.
THE BIG QUESTION