Science Matters (18 page)

Fusion occurs when two small nuclei come together to form a single larger one. As with fission, it sometimes happens that the mass of the final fusion product is less than the mass of the ingredients that go to make it up. In this case, the fusion process can produce energy. The sun and other stars generate their energy through fusion in a process in which four protons (the nuclei of hydrogen atoms) come together after a sequence of steps to form a helium nucleus and a few extra particles.

Since the 1950s there have been extensive research and development programs in both U.S. and foreign laboratories to harness fusion as a source of electrical power. The general strategy is to hold nuclei together with intense magnetic fields, then raise their temperature to reproduce conditions that occur in the interior
of stars. Unfortunately, this program remains far from producing controlled fusion in a laboratory, much less an economically viable generating plant. We will discuss the current state of fusion research in the Frontiers section of this chapter.

In the spring of 1989, a brief flurry of excitement erupted when two scientists at the University of Utah claimed to have produced fusion in a desktop experiment. Dubbed “cold fusion” or “fusion in a bottle,” this process caught the imagination of the public because it might have provided us with an unlimited cheap source of energy. But cold fusion has faded from view as other scientists have been unable to reproduce the claimed results.

Nuclear chain reactions can run out of control. Take 25 pounds of uranium-235 and the mass will just sit there, giving off heat and spewing out neutrons. But place two 25-pound masses together and you can produce streams of neutrons that multiply into an unstoppable torrent—a nuclear explosion. The atom bomb uses this principle by separating two precisely machined hemispheres of uranium-235 and surrounding them with conventional chemical explosives. The first explosion pushes the hemispheres together into a single sphere that exceeds the critical mass.

The fusion of hydrogen into helium produces even more explosive power—in the hydrogen bomb. H-bombs are triggered by atomic bombs, which provide the heat and compression necessary to begin the fusion reaction. Hydrogen bombs thus unleash the same kind of energy as the sun. Atomic bombs can’t be much larger than a critical mass, but there is almost no limit
to the size of a hydrogen bomb. The more hydrogen explosive you start with, the larger the bang.

Most familiar nuclei are stable. Almost all the nuclei of carbon in your tissues and calcium in your bones are the same now as they were when they were made in the heart of a supernova billions of years ago. Some nuclei, however, do not share this property. In periods ranging from microseconds to times comparable to the age of the Earth, these nuclei spontaneously disintegrate, spewing out fragments when they do so. These nuclei are said to be radioactive, the process of disintegration is called radioactive decay, and the particles emitted during this decay constitute radioactivity. All isotopes of uranium are radioactive, as are many lighter nuclei such as carbon-14 and strontium-90.

The best way to think about the behavior of radioactive nuclei is to picture popcorn popping on your stove. Kernels don’t all pop at once. A few kernels pop, then a few more, spaced out over several minutes. No theory of nuclear stability exists that will always predict all the details of radioactive decay. However, we can measure the phenomenon with great precision.

Any given collection of radioactive nuclei behaves in roughly the same way as another, with individual nuclei decaying at different times. The overall rate of decay is called the half-life, defined as the time it takes for half of the nuclei in a given sample to decay. This means, for example, that if you have 100 nuclei with a half-life of one minute in front of you right now, you will
have about 50 one minute from now, about 25 (half of a half) in two minutes, 12.5 (on the average) in three minutes, and so on.

Half-lives of nuclei vary widely. Uranium-238 (the most common isotope of uranium) has a half-life of 4.5 billion years—about the same as the Earth’s lifetime. The shortest-lived of plutonium’s many isotopes, on the other hand, has a half-life of a billionth of a second, so its decay can be measured only with sophisticated electronic detectors. A range of values between these two extremes exists in nature.

Radiation, first discovered at the end of the nineteenth century, was mystifying to classically trained physicists and chemists. There seemed to be three different types of radiation, each coming from a different mode of decay. Scientists named these mysterious types of radiation for the first three letters of the Greek alphabet—alpha, beta, and gamma. We still use those names today, even though we understand a great deal more about all three types.

When a nucleus undergoes alpha decay, it emits a bundle consisting of two protons and two neutrons—the nucleus of a helium atom, also called the alpha particle. After alpha decay, the “daughter” nucleus has two fewer protons and two fewer neutrons than it had originally. This means that the nucleus can attract two fewer electrons with its electrical force, and after a time the two excess electrons wander off. What remains, then, is an atom with two fewer protons and two fewer electrons; an atom of a different chemical element has been created. Thus alpha decay changes both the mass and the identity of the nucleus involved.

Uranium-238, for example, decays by emitting an alpha particle, and the end product is an atom of the element thorium
(thorium-234, to be exact). Alpha decay, by changing the identity of the nucleus, changes the identity of the atom itself. Alpha decay (and, as we shall see shortly, beta decay) constitutes a modern version of the philosopher’s stone, the material medieval al chemists believed could change lead into gold.

In beta decay, one of the neutrons in the nucleus emits an electron and in the process converts itself into a proton. The daughter nucleus has almost the same mass as its parent, but has one more proton and one fewer neutron. Beta decay, then, changes the identity, but not the mass, of a nucleus. The beta particle was named before people realized that it was a plain old garden-variety electron, and you still see electrons referred to occasionally as “beta rays.”

One of the most intriguing beta decays doesn’t involve a nucleus at all, but a free neutron. Left to itself, a neutron decays into a proton, an electron, and a particle called the neutrino with a half-life of about 8 minutes. Neutrons in a nucleus can’t normally decay in this way. Thus we still have neutrons around, billions of years after the creation of the universe, only because they have been hiding inside of nuclei.

Finally, gamma decay involves a rearrangement of protons and neutrons inside the nucleus and the consequent emission of electromagnetic radiation in the form of a gamma ray. Gamma decay changes neither the mass nor the identity of the nucleus.

The story of radioactivity does not usually end with one decay. Typically, a radioactive nucleus decays by one process, producing a daughter that decays by another. The daughter of the second decay will decay in turn, and the process goes on through a long chain until it ends with a stable nucleus. This process is well illustrated
by uranium-238, a surprisingly common element in Earth’s crust (much more common than gold, silver, or mercury, for example). It decays by alpha emission to thorium-234, which decays by beta emission to protactinium-234 (91 protons, 123 neutrons) with a half-life of 24 days. This nucleus in turn decays by beta emission to uranium-234 with a half-life of 2 minutes, and uranium-234 emits an alpha particle to create thorium-230 with a half-life of 80,000 years. This series of decays continue, until it ends with the formation of lead-208, a stable nucleus.

One inevitable product of the decay chain that starts with uranium-238 is the radioactive gas radon-222. Radon decays by alpha emission with a half-life of about 4 days. This gas can seep up from the ground into your home, where its decay products may pose a significant health risk. Taking the long view, we recognize that the indoor radon problem is caused ultimately by the fact that uranium-238 was produced copiously in some super-novae several billion years ago.

The fact that an atom’s chemical reactions are largely independent of the nucleus leads to some very important ways of dating artifacts and rocks. The most familiar of these, the so-called carbon-14 dating method, is based on the fact that, in addition to normal, stable carbon-12, a certain amount of the radioactive element carbon-14 is always present in the environment. (Carbon-14 results from the collision of cosmic rays with nitrogen atoms in the upper atmosphere.) Because the chemistry of the two isotopes of carbon is identical, a certain amount of carbon-14 finds its way into all living tissues. When an organism dies, it stops taking in carbon-14, and its complement of these atoms
starts to fall off as they decay. We know how much carbon-14 there is in the environment, so we know how much carbon-14 was in a piece of organic material when death occurred. Given that the half-life of carbon-14 is 5,730 years, we can work out how long it’s been since that piece of material was taking in fresh carbon-14.

For example, if you find that a piece of wood contains only half the amount of carbon-14 it had when it was formed, you know that the tree from which it came died about 5,730 years ago. If the piece of material happens to be leather from a grave site or an elk shoulder blade used as a shovel, you have a pretty good idea of the age of the civilization that produced the artifact. This makes carbon dating an important tool in archaeology.

The same general technique can be used to date many rocks. The mineral’s atomic structure tells us how much of a given isotope must have been present at the beginning, and measuring the amount left (or, equivalently the number of decays that have occurred) will tell us how many half-lives it’s been since the rock was formed.

One common technique for dating rocks involves the beta decay of potassium-40 to argon-40 (half-life: 1.3 billion years). Potassium is an essential element in many common minerals, while argon is a gas that is not incorporated into rocks when they form. If we crush and heat a sample of rock, each argon atom that we detect must be the result of one potassium decay after the rock formed. Knowing the number of decays that have occurred, and knowing how much potassium was in the rock originally, we can work out how long ago the rock formed. The potassium-argon technique was used to date four-billion-year-old moon rocks brought back by the Apollo astronauts, and it is employed routinely to date rocks on Earth.

The independence of chemical reactions and nuclear reactions allows scientists to use radioactive tracers in fields as diverse as agriculture, geology, and medicine. The basic idea is simple: Scientists introduce a sample containing a minute amount of a distinctive radioactive isotope into a system and follow its progress. The system’s chemistry acts in its usual way on the sample, but the radioactive nuclei keep decaying, providing a “tag” that allows scientists to “see” the trace element as it moves, by chemical reactions, through the system.

Biologists commonly use tracers to follow the path of nutrients as they pass through the food chain. Physicians can watch iodine or thorium collect in the body and diagnose the presence of tumors. Earth scientists employ radioactive tracers to follow the paths of rainwater through groundwater reservoirs to lakes, streams, and wells. Oceanographers adopt the same technique to trace the direction and speeds of ocean currents. Any time chemicals shift from one place to another, radioactive tracers can help document those movements.

We can measure two different characteristics of radiation: (1) how many particles a source gives off, and (2) how much potentially damaging energy is absorbed from that radiation. The first quantity, measured in units called curies, characterizes the source of radiation. The second number is the most important from the point of view of health risks, since it measures the effects of radiation on materials that it encounters. The familiar Geiger counter, which produces a click when it absorbs energy from passing radiation, was designed to measure this quantity.

There are many ways of measuring radiation. You can measure the energy given off by a source, the energy absorbed by a target, or the biological effect of the radiation on that target. The unit that measures the last of these is called the rem (radiation equivalent in man) or, in the international system of units, the Sievert (Sv). (For reference, 1 Sv = 100 rem.) A typical dental X-ray might deliver 10 millirem (10 one-thousandths of a rem), while a dose of 750 rem is fatal. Fatal doses are normally encountered only in serious nuclear accidents or exposure to nuclear weapons.

At the other end of the scale, all living things on Earth are subjected to natural environmental radiation all the time. There is nothing sinister about this—the radiation was there long before the human discovery of nuclear physics and, for that matter, before there were humans at all. Coming from cosmic rays, radioactive isotopes in the air and ground, and even radioactive isotopes in our bodies, this dose typically amounts to 100 to 150 millirems per year for the average American. Add a slightly smaller annual dose from medical and dental X-rays and you have a total average annual radiation dose in the neighborhood of 250 millirem. There is evidence that living cells have developed repair mechanisms to deal with radiation at this level.

The prospect of harnessing fusion to supply our energy needs remains a dream of the scientific community. The central problem can be simply stated: how can you hold atoms together long enough, and raise their temperature high enough, to start a self-sustaining fusion reaction of the type that powers the sun? In
general, a hot gas will expand, and the problem is to keep it together until fusion reactions can take place.