Understanding Radiation: Atomic Structure and Isotopes
All substances are made up from one or more of about 90 different kinds of simple materials known as “elements.” Among the common elements are the gases hydrogen, oxygen, and nitrogen; the solid nonmetals carbon, sulfur, and phosphorus; and various metals, such as iron, copper, and zinc. A less familiar element, which has attained prominence in recent years because of its use as a source of nuclear energy, is uranium, normally a solid metal.
The smallest part of any element that can exist, while still retaining the characteristics of the element, is called an “atom” of that element. Thus, there are atoms of hydrogen, of iron, of uranium, and so on, for all the elements. The hydrogen atom is the lightest of all atoms, whereas the atoms of uranium are the heaviest of those found on earth.
Heavier atoms, such as those of plutonium, also important for the release of nuclear energy, have been made artificially. Frequently, two or more atoms of the same or of different elements join together to form a “molecule.”
Every atom consists of a relatively heavy central region or “nucleus, “ surrounded by a number of very light particles known as “electrons.” Further, the atomic nucleus is itself made up of a definite number of fundamental particles, referred to as “protons” and “neutrons.”
These two particles have almost the same mass, but they differ in the respect that the proton carries a unit charge of positive electricity whereas the neutron, as its name implies, is uncharged electrically, i.e., it is neutral. Because of the protons present in the nucleus, the latter has a positive electrical charge, but in the normal atom this is exactly balanced by the negative charge carried by the electrons surrounding the nucleus.
The essential difference between atoms of different elements lies in the number of protons (or positive charges) in the nucleus; this is called the “atomic number” of the element. Hydrogen atoms, for example, contain only one proton, helium atoms have two protons, uranium atoms have 92 protons, and plutonium atoms 94 protons.
Although all the nuclei of a (liven element contain the same number of protons, they may have different numbers of neutrons. The resulting atomic species, which have identical atomic numbers but which differ in their masses, are called “isotopes” of the particular element. All but about 20 of the elements occur in nature in two or more isotopic forms, and many other isotopes, which are unstable, i.e., radioactive, have been obtained in various ways.
Each isotope of a given element is identified by its “mass number,” which is the sum of the numbers of protons and neutrons in the nucleus. For example, the element uranium, as found in nature, consists mainly of two isotopes with mass numbers of 235 and 238; they are consequently referred to as uranium-235 and uranium-238, respectively.
The nuclei of both isotopes contain 92 protons-as do the nuclei of all uranium isotopes-but the former have in addition 143 neutrons and the latter 146 neutrons. The general term “nuclide” is used to describe any atomic species distinguished by the composition of its nucleus, i.e., by the number of protons and the number of neutrons. Isotopes of a given element are nuclides having the same number of protons but different numbers of neutrons in their nuclei.
In a conventional explosion, the energy released arises from chemical reactions; these involve a rearrangement among the atoms, e.g., of hydrogen, carbon, oxygen, and nitrogen, present in the chemical high-explosive material. In a nuclear explosion, on the other hand, the energy is produced as a result of the formation of different atomic nuclei by the redistribution of the protons and neutrons within the interacting nuclei.
What is sometimes referred to as atomic energy is thus actually nuclear energy, since it results from particular nuclear interactions. It is for the same reason, too, that atomic weapons are preferably called “nuclear weapons.” The forces between the protons and neutrons within atomic nuclei are tremendously greater than those between the atoms; consequently, nuclear energy is of a much higher order of magnitude than conventional (or chemical) energy when equal masses are considered.
Many nuclear processes are known, but not all are accompanied by the release of energy. There is a definite equivalence between mass and energy, and when a decrease of mass occurs in a nuclear reaction there is an accompanying release of a certain amount of energy related to the decrease in mass.
These mass changes are really a reflection of the difference in the internal forces in the various nuclei. It is a basic law of nature that the conversion of any system in which the constituents are held together by weaker forces into one in which the forces are stronger must be accompanied by the release of energy, and a corresponding decrease in mass.
In addition to the necessity for the nuclear process to be one in which there is a net decrease in mass, the release of nuclear energy in amounts sufficient to cause an explosion requires that the reaction should be able to reproduce itself once it has been started.
Two kinds of nuclear interactions can satisfy the conditions for the production of large amounts of energy in a short time. They are known as “fission” (splitting) and “fusion” (joining together). The former process takes place with some of the heaviest (high atomic number) nuclei; whereas the latter, at the other extreme, involves some of the lightest (low atomic number) nuclei.
The materials used to produce nuclear explosions by fission are certain isotopes of the elements uranium and plutonium. As noted above, uranium in nature consists mainly of two isotopes, namely, uranium-235 (about 0.7 percent), and uranium-238 (about 99.3 percent).
The less abundant of these isotopes, i.e., uranium-235, is the readily fissionable species that is commonly used in nuclear weapons. Another isotope, uranium-233, does not occur naturally, but it is also readily fissionable and it can be made artificially starting with thorium-232. Since only insignificant amounts of the element plutonium are found in nature, the fissionable isotope used in nuclear weapons, plutonium-239, is made artificially from uranium-238.
When a free (or unattached) neutron enters the nucleus of a fissionable atom, it can cause the nucleus to split into two smaller parts. This is the fission process, which is accompanied by the release of a large amount of energy. The smaller (or lighter) nuclei which result are called the “fission products.” The complete fission of 1 pound of uranium or plutonium releases as much explosive energy as does the explosion of about 8,000 (short) tons of TNT.
In nuclear fusion, a pair of light nuclei unite (or fuse) together to form a nucleus of a heavier atom. An example is the fusion of the hydrogen isotope known as deuterium or “heavy hydrogen.” Under suitable conditions, two deuterium nuclei may combine to form the nucleus of a heavier element, helium, with the release of energy.
Nuclear fusion reactions can be brought about by means of very high temperatures, and they are thus referred to as “thermonuclear processes.” The actual quantity of energy liberated, for a given mass of material, depends on the particular isotope (or isotopes) involved in the nuclear fusion reaction.
As an example, the fusion of all the nuclei present in 1 pound of the hydrogen isotope deuterium would release roughly the same amount of energy as the explosion of 26,000 tons of TNT.
In certain fusion processes, between nuclei of the hydrogen isotopes, neutrons of high energy are liberated. These can cause fission in the most abundant isotope (uranium-238) in ordinary uranium as well as in uranium-235 and plutonium239. Consequently, association of the appropriate fusion reactions with natural uranium can result in an extensive utilization of the latter for the release of energy.
A device in which fission and fusion (thermonuclear) reactions are combined can therefore produce an explosion of great power. Such weapons might typically release about equal amounts of explosive energy from fission and from fusion.
A distinction has sometimes been made between atomic weapons, in which the energy arises from fission, on the one hand, and hydrogen (or thermonuclear) weapons, involving fusion, on the other hand. In each case, however, the explosive energy results from nuclear reactions, so that they are both correctly described as nuclear weapons. In this chapter, therefore, the general terms “nuclear bomb” and “nuclear weapon” will be used, irrespective of the type of nuclear reaction producing the energy of the explosion.