1. Introduction to Atomic Energy By R. F. Bacher Former Head of Bomb Physics Division, Los Alamos Laboratory, Professor of Physics, Cornell University R. P. Feynman Former Theoretical Physicist at Los Alamos Laboratory, Manhattan District Assistant Professor of Theoretical Physics, Cornell University Present day civilization is based on an enormous technical development. The industrial and agricultural advance represents an increasing control over the processes of nature. This control stems. from an ever-widening knowledge and understanding of the natural phenomena. As the advance of scientific knowledge finds its way into the development of practical control over more and more natural processes, there arise new social and political problems, each more bewilderingly complex and difficult than the one before it. The practical release of atomic energy accomplished during the war represents a control over a vast new range of natural phenomena. In its wake is the most difficult social and political problem which has ever arisen from technical advance. The purpose of this report is to explain the main aspects of the phenomena over which we have just gained control. All of the myriad processes which have been used in industry and agriculture, until the recent developments in atomic energy, have been the results of the regrouping of atoms into new patterns. The new development is the result of the discovery of ways to transform the atoms from one kind to another. This new freedom to change atoms, to transmute the elements, rather than just rearranging them in various chemical combinations, represents control over an entirely new class of natural phenomena. Only a very small fraction of the eventual potentialities of this control are clear at present. This report will try to indicate the relationship of the atomic phenomena to the military applications which have been made as well as to those of the peacetime applications which appear to be within reach in the rear future. More detailed accounts of these applications are given in other articles in this volume. MAIN CHARACTERISTICS OF NUCLEAR CHANGES It is important to make clear at the outset the essential characteristics of the new phenomena which distinguish them from those chemical and physical phenomena which have hitherto been the bases of all of our technical achievements. All objects, if sufficiently magnified, would appear to be made simply of a very large number of spherical atoms, arranged next to one another in some pattern. At present we know of only about 600 different kinds of atoms. The billions of different materials in the world are made only of these atoms, but they are arranged in billions of different patterns. Each of these atoms consists of a light-weight cloud, called the electron cloud, at the center of which is a very small, very hard, and very heavy core, the nucleus. The atoms are in constant motion and, when substances change, the atoms simply rearrange into new patterns. Ordinarily, only the outsides of the electron clouds some into contact with one another, and the atom as a whole does not lose its individuality. The cores are protected from contact with one another by their location at the center of the cloud. Thus, all of the processes which have previously been used in all industrial and scientific developments have depended only on the properties of the electron cloud and not, fundamentally, on the properties of the nuclei of atoms. Isotopes Among the approximately 600 known varieties of atom, many have the same kind of electron cloud and differ only in that the cloud surrounds a different kind of nucleus. Two atoms with different cores and similar electron clouds are called isotopes. Since the electron clouds determine the chemical and most of the ordinary physical properties of the atom, there was until recently no simple way to distinguish or separate the isotopes. It was not even recognized that the atoms with one kind of electron cloud differed at all. It was thought that there were only about 90 different kinds of atoms because atoms appear with only about 90 different kinds of electron clouds (distinguished only by the number of elementary particles, electrons, in the cloud). The atoms were named after the chemical elements in which they occur. One speaks of hydrogen atoms, iron atoms, uranium atoms, etc. The different isotopes of a single element have not (with some exceptions) been given special names but are distinguished by giving them each a number. This number represents the weight of the nucleus, as this is one of the simplest characteristics by which the nuclei differ. For example, naturally occurring hydrogen is a mix One of the atoms has a core ture of two different kinds of atoms. which is twice as heavy as the other. are known as hydrogen-1 or H-1 and hydrogen-2 or H-2 (also called deuterium). The element uranium has a number of different isotopes. Uranium which appears in natural mineral deposits contains essentially two different isotopes which are called uranium-238 (U-238) and uranium-235 (U-235). (A third, U-234, actually does occur, but only in very small quantities in natural uranium.) These two atomic nuclei weigh approximately 238 and 235 times as much, respectively, as the light isotope of hydrogen (H-1). In the applications to atomic energy development, the two atoms U-238 and U-235 will appear to have very different properties. But chemically the two forms of uranium look and act the same. Nuclear Transformations The new development is a result of the discovery of a way to change the atom core itself. We will therefore be interested in the ways that the nuclei change from one kind to another. Atomic nuclei can be changed if they can be made to collide with one another in spite of their protective electron clouds. This can be done by shooting one atom at others at speeds which are extremely high compared to the speed at which atoms are usually moving, in even the most violent of TNT explosions. When this is done (atoms moving at high speed are obtained from instruments like the cyclotron), the nuclei of two atoms collide and, in some cases, various small particles of nuclear material are knocked out. What remains forms a new nucleus. It immediately gathers around itself a definite number of electrons and thus forms an electron cloud of one of the ninety or so known varieties. It is therefore the isotope of some element and behaves chemically like any other isotope of that element. Radioactivity The new nucleus which is formed may not, however, be a stable, unchanging thing. After a period of time which can be anywhere from a fraction of a second to several billion years, depending on the particular nucleus, the nucleus changes into a new one and thus forms the isotope of a new element. When the nucleus so changes, it may emit some small particles or other rays. Some of these rays are similar to x-rays and are called gamma rays. They are the same as the rays emitted by radium, which have been used in the treatment of cancer. An isotope which is thus able to change from one kind to another is called an unstable or radioactive isotope. The change can be detected very easily by looking for the rays or particles which are emitted in the nuclear transformation. These radioactive isotopes, of course, behave chemically the same as the stable counterparts of the same element. Since their radioactivity can so easily be detected with modern instruments, the radioactive isotopes can be located among a vast majority of stable ones. This makes the radioactive isotope very useful to solve a great many biological and chemical problems. Such uses of radioactive isotopes as tracers will be described in much further detail in the fourth article of this volume. Neutrons Among the various particles which are knocked out of certain nuclei when they collide, one called the neutron is of particular interest. The neutron is similar to the nuclei of atoms, except that it carries with it no electron cloud. Thus it passes through material and can hit the nuclei of other atoms in spite of their protective electron clouds. Many kinds of atoms have been bombarded by neutrons. What usually happens is that the neutron is simply captured by the nucleus, and there is thus formed a new nucleus which is a radioactive isotope of the old element. This method of manufacturing radioactive elements will be a practical method for their large-scale production in the future. Nuclear Fission There are a few atoms of a particular kind called fissionable isotopes which do a particularly interesting thing when bombarded by neutrons. When a neutron enters the nucleus of a fissionable isotope, the entire nucleus splits into two pieces. The two fragments move away from each other with tremendous velocity, and the energy which is released in this way appears ultimately as heat. The fragments collect an electron cloud around themselves and thus form new radioactive isotopes of other known chemical elements, such as barium or iodine. It is this phenomena of nuclear fission which constituted the basis of the entire atomic energy development. If nuclear fission resulted only in the liberation of two new radioactive isotopes, albeit with a great deal of energy, only very small quantities of energy could be liberated on a practical scale in this way. This is because only a few atoms could be split by the very few neutrons which can be produced in a laboratory. The important thing about the fission process is that, besides the fragments and the energy, two or three new neutrons are also liberated as a kind of debris after the nuclear splitting. 1 1It is especially similar to and has the same weight as the nucleus of hydrogen-1. This important nucleus is often called the proton. It is believed that all other nuclei are simply conglomerations of protons and neutrons which are bound together by strong but, at present, little-understood forces of attraction. Thus, one neutron hitting the fissionable nucleus releases energy and two or three new neutrons. These new neutrons are now available to produce fission of other atoms of the same piece of material. This circumstance, that one neutron in producing fission releases several neutrons, provides the new instrumentality by which the energy stored in atomic nuclei has been made available. THE CHAIN REACTOR Suppose we have a large mass of some fissionable material. A neutron entering this mass will collide with one of the atoms and cause it to undergo fission, liberating energy and, say, two new neutrons. These two neutrons can now move through the block of material and eventually hit two other atoms of the fissionable material, causing them to split and liberating still more neutrons and more energy. In this way, starting with one neutron and assuming, for the moment, that each fission liberated just two secondary neutrons, we would at the end of the first fission have two neutrons, then four, eight, and so forth, in ever increasing sequence until an appreciable fraction of the atoms have undergone fission and large quantities of energy have been set free. In the atomic bomb we have an example of an uncontrolled chain reaction of this kind, with the entire chain of events taking place in an extremely short interval of time. In the plant at Hanford, Washington, we have an example of a chain reaction of this kind which is kept under control. Fissionable Elements Nearly all those isotopes which are more than about 220 times heavier than H-1 are fissionable when bombarded by sufficiently energetic neutrons. At present, U-235 and plutonium (Pu-239) are known to produce fission when bombarded by neutrons of any energy. For U-238, the neutrons liberated by the fission of one atom are not sufficiently energetic to produce (with high enough probability) further fission in additional atoms and thereby maintain a chain reaction. Hereafter when we speak of fissionable materials we shall mean U-235 or Pu-239. The preparation and purification of these fissionable materials form the heart of any atomic energy development program. How this can be done is described in more detail below. Conditions for the Maintenance of Chain Reactions Critical Size Chain reactions will not always occur even if fissionable materials are present. Certain other conditions must also be satisfied. Firstly, a chain reaction will occur only in a piece of fissionable material which 718674-46 |