Sun in a Bottle - Charles Seife [22]
This transformation process releases energy, because the helium- 3 does not weigh exactly the same as the tritium did. The neutron that disintegrated weighed more than the proton and the electron that it turned into.17 There is mass missing: it was converted into energy, just as E = mc2 says. As the unstable tritium changed itself into the stable helium-3, it lost a little bit of mass and released a bunch of energy.
This is an example of a general rule. When a nucleus converts itself from a less-stable variety to a more-stable one, it releases a little bit of energy because some of its mass disappears. And nuclei always “want” to become more stable, just as a ball perched on a hill “wants” to roll down to the bottom. In the process of getting more stable, an atom releases energy, just as a ball rolling down a hill picks up more and more speed as it goes.
Marie Curie was seeing this process with radium. Radium-226 is a heavy atom with 88 protons and 138 neutrons. It is almost stable ... but not quite. On average, after 1,600 years, a radium-226 nucleus spits out an alpha particle (a helium-4 nucleus: two protons and two neutrons), leaving behind 86 protons and 136 neutrons—radon-222—and releasing a bunch of energy. This energy heats the hunk of radium, and it is why Curie observed that chilled radium would warm itself. It is also why a hunk of radium emits radon and helium. It so happens that radon, itself, is unstable; it decays into thorium, releasing energy, which, in turn, decays into another species and another and another, emitting energy at each step. Like a ball rolling down a bumpy hill, it keeps rolling and rolling until it reaches a stable place to rest: in this case, lead-206, which is much more stable than radium-226. The ball has rolled a long way down the hill. But it didn’t roll all the way down. There are, in fact, nuclei more stable than lead-206. At the very bottom of the valley are the most stable atoms of them all: the iron group.
Iron-56 (26 protons, 30 neutrons), nickel-62 (28 protons, 34 neutrons), and a few other nearby iron and nickel isotopes are the ne plus ultra of the nucleus world. They are the most stable elements of them all. They are at the very bottom of the valley. All other atoms “want” to be iron, just as a ball anywhere on the slope of a hill “wants” to be at the very bottom.
The landscape of nuclei is very much like a valley with a steep hill on one side and a shallow hill on the other (see the graph on page 47). At the very bottom of the valley is iron. Heavy nuclei, like radium and uranium, have more protons and neutrons than iron—they are high up on the shallow hill. To roll down to iron, they have to get lighter, shedding those extra protons and neutrons. Sometimes they do it in small steps, like radium does. Sometimes they do it violently, by breaking into two or more parts: fission. (Indeed, fission is little more than a way for heavy atoms to roll quickly down the shallow hill, losing mass and releasing energy in the process.) Light elements, on the other hand, are on the steep hill. They have to get bigger if they want to get into the valley of iron. This is what fusion is all about. Two light nuclei, if they slam together, can stick to one another to create a larger nucleus.
CURVE OF BINDING ENERGY: Iron is at the bottom of the valley. Light elements on the left and heavy elements on the right release energy when they move down the valley of iron by fusing or fissioning.
Fusion is a way for light atoms to roll down the steep hill toward iron. Since the fusion hill is much steeper than the fission one, a fusion reaction yields much more energy than an equivalent fission reaction. Fusion and fission are two sides of the same coin, but as Teller well knew, fusion is more powerful