chapter  9
29 Pages

Nuclear energy

In 1895, the German physicist Wilhelm Röntgen was studying the behavior of streams of electrons emitted from negatively charged electrodes, called cathode rays. He wanted to investigate the luminescence caused when cathode rays struck samples of various chemicals. In his experiments, Röntgen recognized that a new kind of radiation was emerging from the cathode-ray tube. This newly discovered radiation could pass through thick paper and even through thin layers of metals. At first, this new form of radiation was called ‘Röntgen rays’ in honor of the discoverer, but Röntgen himself used the universal mathematical symbol of the unknown, calling the radiation ‘X-rays.’ Henri Becquerel, a French physicist, had devoted his career to

studying luminescence. What interested him was a report that X-rays seemed to come from a fluorescent spot on the wall of the cathode-ray tube. Becquerel wondered whether there was a connection between X-rays and fluorescence. If the fluorescence of the cathode rays contained X-rays, then it seemed reasonable to suspect that X-rays could be produced in other forms of fluorescence. His question was, does a fluorescent material also produce X-rays? Because we can’t see X-rays, Becquerel needed a test for the

production of X-rays during fluorescence. X-rays ‘expose’ some

kinds of photographic films-the basis of X-ray examinations by physicians or dentists. Becquerel wrapped a photographic plate in dark paper to prevent light from exposing the plate. Samples of a fluorescent compound that Becquerel’s father had worked with, potassium uranium sulfate, were still on hand in the laboratory. To prove that the rays causing the plate to blacken really were X-rays, Becquerel had to demonstrate that they behaved in all respects like X-rays, not just in their ability to penetrate paper. He designed an experiment in which a small copper cross was placed between the uranium sample and the photographic plate. If the rays from the uranium sample passed through paper but not through copper, an outline of the cross should appear on the photographic plate. All that remained to perform the experiment was to put the cross between the mineral and the carefully wrapped plate, set this assembly in the sunlight to cause fluorescence, and develop the plate. On the day that Becquerel intended to try his experiment, the sky was completely overcast. Rather than take the whole assembly apart, Becquerel stored it, intact, inside a drawer in a laboratory cabinet. Five days later, he took the apparatus out of the drawer and developed the photographic plate, even though the experiment had never been completed as planned. When he developed the plate, he discovered that it had an image of the copper cross. The only explanation for the source of the image on the photographic plate was that some kind of radiation had to be coming out of the uranium mineral itself. Becquerel found that the uranium compound constantly and

ceaselessly emitted a strong, penetrating radiation. This property of constantly emitting penetrating radiation was termed radioactivity by Marie Curie. She showed that all uranium compounds were radioactive, the intensity of radioactivity being proportional to the uranium content of the various compounds tested. When Becquerel studied pure uranium metal, it proved to be more intensely radioactive than any other substance tested up to that time. It takes energy to expose a photographic plate. This energy in

Becquerel’s experiment was apparently some that the crystals had somehow stored. Think of this in terms of the energy balance:

EIN – EOUT = ESTORED

In the system Becquerel was studying, EOUT is energy emitted by the uranium compounds. Because the compounds had not been exposed to light, nor heated, nor treated with other chemicals EIN is zero. Therefore, there can only be energy coming out of the samples (EOUT) if there is a substantial amount of energy stored in them somehow (ESTORED). Another strange aspect of this radiation relates to the common experience that a hot object gradually cools until it is the same temperature as its surroundings. The amount of heat given off becomes less and less until no more is available to exchange with the immediate surroundings. This wasn’t the case for radiation coming from uranium. It went on steadily, at apparently the same intensity, for months. Further studies by scientists in many countries showed that more

than one kind of radiation was being given off from radioactive substances. Since the nature of the radiation was, at that time, not understood, the various kinds were named using the first three letters of the Greek alphabet: α-rays, β-rays, and γ-rays. γ-rays displayed an ability to penetrate materials, much like X-rays. β-rays were much less penetrating, and α-rays were scarcely penetrating at all. Becquerel recognized that β-rays were streams of rapidly moving electrons. Atoms consist of a nucleus surrounded by one or more elec-

trons that orbit around the nucleus. Electrons have a negative charge and are very light weight. The nucleus consists of two kinds of particles: protons, which have a positive charge, and electrically neutral neutrons. (Many other kinds of subatomic, nuclear particles have been shown to exist, but for our purposes we need only concern ourselves with protons and neutrons.) Since the mass of a proton or neutron is about 1,800 times greater than the mass of an electron, the nucleus contains almost the entire mass of the atom. Ordinary chemical reactions depend on what happens to the

electrons. The chemical behavior and identity of an element is determined by the number of its electrons. In a neutral atom the number of negatively charged electrons and positively charged protons must be equal, so the chemical identity of an element is also determined by the number of protons in its nucleus. Since the number of electrons might be changed by the atom’s gaining or losing some in a chemical reaction, the number of protons in the

nucleus is a more reliable indicator of the chemical identity of an element. The number of protons in the nucleus is called the atomic number of the element. The atomic number best characterizes the different varieties of atoms and best discriminates among the chemical elements. A second characteristic property, the mass number, represents the

sum of the numbers of protons plus neutrons in the nucleus. When it is convenient to lump protons and neutrons together, as in determining the mass number, we use the generic term nucleons. Thus the mass number is equal to the number of nucleons. In an electrically neutral atom, the number of protons must equal

the number of electrons. Since neutrons are electrically neutral, it is not necessary to restrict the number of neutrons to achieve electrical neutrality in the atom. It is entirely possible that nuclei of the same chemical element could have different numbers of neutrons. Atoms or nuclei having the same atomic number by different mass numbers are called isotopes. In nuclear reactions, it will be helpful to keep track of the pro-

tons and neutrons. We will use a notation that shows the chemical symbol of the element, the atomic number as a subscript, and the mass number as a superscript. As an example, uranium, U, has atomic number 92. Its most common isotope has mass number 238. This isotope would be represented as 92U238. Continued investigation showed that radioactivity is not like any

known chemical process. Chemists had learned ways to influence the rate or course of a chemical process: e.g., changing temperature or pressure, adding a catalyst, or exposing the system to light. None of these has the slightest effect on the rate of radioactivity. So, radioactivity could not be a chemical process, which means that it must not be occurring in the swarm of electrons-because chemical processes involve gaining, losing, or sharing electrons. If radioactivity is not occurring among the electrons, the only other place it can be happening is in the nucleus. Marie Curie defined radioactivity as the spontaneous emission of

radiation. She also showed that radioactive materials produce a prodigious amount of energy-millions of times more than any chemical process. On an equal mass basis, nuclear reactions release about ten million times as much energy as chemical reactions. This energy keeps pouring out in apparently endless quantity. We have

seen that a spontaneous change is one that involves a transition from a state of high potential energy to one of low potential energy. Thus we can immediately draw an energy diagram (Figure 8). To understand how to tap into this enormous and seemingly limitless supply of energy, we begin by inquiring where nuclear potential energy comes from. Since the nuclei of all atoms (other than hydrogen) have more

than one proton, and since electrical charges of the same sign repel each other, why do nuclei stay together? In an atom of uranium, with 92 protons, there would be a tremendous release of energy, from repulsion of particles of the same electric charge, if the nucleus could just fly apart. A force stronger than the tendency of like charges to repel each other must be holding the nucleus togetheras if we had somehow wrapped a tiny, nucleus-sized rubber band around the nucleus to hold all the pieces in place. A second force, one of attraction instead of repulsion, holds the

nucleus together. This second force is called the nuclear binding energy and overwhelms the weaker force of electrical repulsion. The nuclear binding energy acts only over very small distances, only when the nucleons are in contact. It is, in effect, the ‘rubber band’ that prevents nucleons from flying apart. For most nuclei, an external force that adds energy to the nucleus and helps overcome the nuclear binding energy is needed to break the nucleus apart; otherwise, it will stay together forever. But, a very large nucleus has a small chance that nucleons will find themselves separated by a distance beyond the reach of the binding energy, but still within the realm of the repulsive force between objects of the same charge. If that happens, the electric repulsion of similar charges ‘wins’ and

pushes the nucleons apart, overcoming the binding energy. This process we recognize as radioactivity. The more protons in a nucleus (the higher its atomic number),

the greater the repulsion among them and the more likely the nucleus is to experience radioactive decay. Additional neutrons in the nucleus reduce the proton-to-proton repulsion by increasing the size of the nucleus without adding to its positive charge. Too many neutrons destabilize the nucleus by making it so big that the short-range binding energy becomes less effective. A stable nucleus results from a finely tuned balancing act among the neutrons and protons. All nuclei having more than 82 protons are unstable. The relationship between the number of nucleons and the aver-

age binding energy per nucleon is called the curve of binding energy (Figure 9). The highest binding energy occurs at mass number 56, an isotope of iron. It is the most stable nucleus. At low mass numbers (<56) the curve rises very steeply. At high mass numbers (>56) the curve drops, but slowly. The binding energy per nucleon measures the stability of a nucleus. The higher the binding energy, the more stable the nucleus. Nuclear processes release far more energy than do chemical processes because the ‘rubber band’

holding nuclei together is much more powerful than is the electric force that binds electrons to their atoms. The rising slope of the binding energy curve for small nuclei

indicates that the average binding energy of the nucleons would increase even if these nuclei had more protons and neutrons than they already do. At the crest of the curve, for nuclei with twentysix protons, the nuclear binding energy and the electrical repulsive force are well balanced. Such nuclei are extremely stable. For nuclei with many protons, electrical repulsion overwhelms binding energy. For these large nuclei, the nuclear binding energy cannot hold the nucleus together. These large nuclei break apart in the process of radioactivity. The decreasing slope of the binding energy curve in the region of large nuclei signals that average binding energy of these large nuclei would increase-they would become more stable-if they had fewer nucleons, not more.