A Touch of Sun

To most of the U.S. public the hydrogen bomb was still a direful novelty last week, but to scientists there was little new about it. Long before the discovery of uranium fission they had known that familiar, plentiful hydrogen could make prime nuclear fuel. They had even demonstrated on a laboratory scale some of its nuclear reactions. They could not make the process work practically, but whenever they felt discouraged, they looked up at the shining sun whose radiation, derived from hydrogen, is the vital force of the world.

The sun, in sober fact, is a kind of hydrogen bomb that generates its life-giving energy by "fusing" hydrogen into helium. Man's new bombs will not use exactly the same reactions, but they will use similar ones yielding the same end product. When the first of them explodes, a little bit of the searing sun will have hit the earth.

Fission v. Fusion. The new-style "fusion" of hydrogen and the old-style "fission" of uranium have a family resemblance. Both depend on the odd and unexplained fact that atomic nuclei do not weigh as much as the sum of the individual nucleons (protons and neutrons) which they contain. It is as if a dozen apples in a paper bag did not weigh as much as the same apples spilled out on the kitchen table and weighed separately.

Even more oddly, some large nuclei (e.g., uranium) weigh more than two-middle-sized nuclei containing between them an equal number of nucleons. It is as if a bushel basket of apples lost weight when the apples were put into two baskets.

Why is the weight-loss so important? Because the weight that is lost turns fearsomely into energy, according to Einstein's famous equation: E equals mc^2. (For details, see chart.)

Uranium fission works by dividing large "bushel basket" nuclei. When a uranium nucleus splits in two, forming two smaller nuclei such as krypton and barium, the weight of all fragments added together is less than that of the original uranium. The weight-loss turns into free energy.

Hydrogen fusion works in the opposite way by forming "paperbag" nuclei out of smaller units. Deuterium (heavy hydrogen), for instance, has one proton and one neutron in its nucleus. When two deuterium nuclei are fused together, they form a helium nucleus (two protons and two neutrons) that weighs less than two deuterium nuclei. As in uranium fission the weight loss turns into free energy. It is this fusion of lighter nuclei into helium that will power the hydrogen bomb.

The Sun Does It. Scientists have long known that a fusion reaction takes place in the sun. Deep under the white-hot surface, the temperature stands at something like 20 million degrees centigrade. Under such extreme conditions all atomic particles are in violent motion. The nuclei of ordinary hydrogen (single protons) zip around with enormous speed. They jostle one another and slam against other nuclei --smashing some and joining others. A complex chain of reactions takes place involving carbon and nitrogen, but the final result is the fusion of hydrogen into helium.

Pre-Atomic Age scientists knew about this "thermonuclear reaction," but could not copy it because they had no way of approaching the temperature of the sun's interior. They found the way on July 16, 1945, when the first uranium bomb exploded at Alamogordo, N. Mex. For an instant the heart of the bomb was hot enough to make hydrogen fuse into helium. Ever since, a hydrogen bomb has been possible.

Designing one is not easy. The sun has plenty of time, but a man-made fusion reaction must take place during the small fraction of a second while the temperature generated by the fissioning uranium is still high enough. All the ingredients must be ready to react instantaneously.

Tritium & Lithium. Just what ingredients will be used and how they will be proportioned is, of course, a top military secret. But the general principles are known to many competent physicists, including the Russians.

A key ingredient will be tritium, a radioactive isotope of hydrogen which was discussed guardedly in the latest report of the Atomic Energy Commission. The nucleus of tritium has one proton and two neutrons. When it is struck by a highspeed proton (a nucleus of ordinary hydrogen), the two combine into helium and yield a great jolt of energy (see chart).

Deuterium (heavy hydrogen) may be used as a convenient source of reactive neutrons and protons. Another ingredient will probably be lithium, which has three protons and four neutrons in its nucleus. When joined by a proton, lithium turns into two helium nuclei. Lithium 6 (an isotope of lithium with three protons and three neutrons) may be used too. It combines with tritium to give two helium nuclei plus a free neutron.

All these ingredients, and probably others, will be arranged advantageously around the uranium, which will act as a detonator. The hydrogen isotopes are thin gases and hard to package, so they will probably be used in the form of chemical compounds. Lithium hydride, which may combine two desirable ingredients (lithium and tritium) in a single compound, would be handy for this purpose. Other tricks will be used to pack more hydrogen isotopes closely around the uranium.

Zigzagging Nuclei. When the uranium is set off and its temperature rises to millions of degrees centigrade, the whole mass of mixed or layered ingredients will turn into a welter of speeding, zigzagging nuclei, shot through with neutrons and powerful gamma rays. If the bomb is properly designed, many of the collisions will form helium, and each new helium nucleus will release its bit to the bomb's explosive energy.

How powerful will such bombs be? It has often been stated that they will have 1,000 times the power of uranium bombs. This figure may be too large; it may be too small. Theoretically, a pound of hydrogen turned into helium yields about seven times as much energy as a pound of fissioning uranium, and very large quantities can be used. Uranium bombs must not be too big or they will explode spontaneously. Hydrogen bombs would suffer from no such limit. Theoretically, a single bomb filling a whole ship could be exploded in an unsuspecting enemy's harbor. Such an explosion would rank as an astronomical event.*

The power of a bomb will depend not only on its size, but on its efficiency, which cannot be determined until at least one test bomb has exploded. There may be a limit beyond which it would not pay to add reactive materials. Very large bombs might burn as nuclear bonfires, wasting much of their effect upon space.

Spreading the Uranium. There may be much military importance in two side effects of the new fusion techniques. In fission bombs, a great deal of the uranium is scattered before it can react. But in a hydrogen bomb, even a small one, the ingredients packed around the uranium core could be induced to generate a large number of free neutrons. These would make more of the uranium react, thus stepping up the efficiency of the core's explosion.

Another limitation of the old-style uranium bombs is that the core must have a certain minimum size or it will not explode. Hydrogen bombs can be designed in such a way that a smaller core will detonate. So a stockpile of uranium (or plutonium) would go farther if built into hydrogen bombs than it would if used alone. This advantage would appeal to such nations as the U.S.S.R., if they have smaller hoards of uranium than the U.S.

Plenty of Bombs. For bomb builders the most convenient thing about the hydrogen bomb is the abundance of the ingredients. Rich deposits of uranium are rare, but enough ordinary hydrogen for hundreds of bombs could be drawn in a day from a bathroom faucet. Deuterium (heavy hydrogen) can be separated from natural hydrogen without much trouble. Lithium is plentiful too, and so are other elements that may be useful in a hydrogen bomb.

The key ingredient tritium (hydrogen 3) is radioactive and is excessively rare in nature, but it is not hard to make. One method is to bombard lithium 6 with neutrons in a uranium pile. The reaction yields tritium and helium, which can be separated by simple chemistry. This job could be done in the plutonium-making piles at Hanford, but probably will be done in a special pile built without difficulty for the purpose.

Scientists are confident that the U.S. will be able to test hydrogen bombs within a year or so. So will the U.S.S.R.

* Scientists are convinced that a hydrogen reaction will not spread through the scarce hydrogen in the atmosphere or the plentiful hydrogen in the ocean. To explode at all, a hydrogen bomb must have just the right ingredients, and seawater is a haphazard collection of many elements. Even a few scientists, however, will feel slightly nervous if the first test bomb is exploded at Eniwetok, so near the Pacific Ocean's hydrogen.

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