by W Heisenberg
A ‘balance sheet’ of this series of reactions shows the following picture: First, there is a carbon nucleus, 6C12, which captures four protons, step by step. At the end, there remains the same 6C12 nucleus, with a helium nucleus, 2He4, plus the two positrons emitted in reactions No. 2 and No. 5. The summary of this ‘balance sheet’ can be put as follows: One helium nucleus and two positrons have been formed from four protons. The net result of the process, therefore, is expressed by the following summary formula:
4 (1H1) → 2He4 + 2 (1e0)
The helium nucleus consists of two protons and two neutrons; hence, its charge is two units smaller than the charge of the four protons. This difference is accounted for by the two positrons. Two of the four protons have thus changed into neutrons.
Since the masses of the protons and of the helium nuclei are exactly known, the energy balance of the entire process can be determined on the basis of the last formula. The energy liberated in such a process is 25·5 Mev., or—computed for moles and converted into kilocalories—600,000,000 kilocalories per mole. This is six times as much as in the process mentioned above.
That which takes place here can be expressed as follows: In the interiors of the stars, hydrogen is converted into helium by nuclear ‘combustion’, and this process liberates the vast energies continuously radiated by the sun and stars. It has been said occasionally, in jest, the sun is ‘heated by coal’. But this is not quite correct. The coal—carbon—acts here merely as a catalyst, and is not consumed in the reaction.
This example ought to be sufficient to show that immense stores of energy are liberated in nuclear reactions, if these take place in sufficient quantities of material. Let us further add that there is actually good reason to believe that older stars are poorer in hydrogen than younger ones, and that this fact points to a gradual consumption of hydrogen.
Now why is it that we have been unable to produce similar quantities of energy in the laboratory before the discovery of the fission of uranium, assuming for the purposes of this question that we had at our disposal a very powerful source of neutrons, consisting of about 100 grammes of radium mixed with beryllium? Compared with the sources normally available in the laboratory, this would be a very plentiful one, indeed. If we were to use this source of radiation to irradiate ordinary table salt (sodium chloride) for a whole day, an estimated 20,000 million chlorine atoms would be changed into radioactive sulphur atoms. This is a very high number, and the sulphur produced would be actually very strongly radioactive. But unfortunately, this quantity of sulphur is extremely small—just about one thousandth of a millioneth of a milligram. The energy obtained from it is correspondingly small in quantity—just six-millionths of a kilocalorie.
However, the largest cyclotron in operation at the present time can increase the neutron intensity to a point approximately a thousand times what used to be regarded as the upper limit. This intensity equals roughly that of a neutron source consisting of 100 kilograms of radium mixed with beryllium. In this case, both the quantity of the substance and the energy have been multiplied by approximately 1,000, and yet they remain extraordinarily small. At any rate, the energy yield is still just a minute fraction of the energy input required by the cyclotron in order to achieve this result. Therefore, a practical exploitation of nuclear energy will be possible only when the transmutation reactions take place spontaneously—and when such a reaction induces a second one and that one induces still another, and so forth, so that finally, as in chemical reactions, most of the nuclei have become transmuted in a chain reaction.
II. URANIUM FISSION AND CHAIN REACTION
In 1938, Hahn and Strassman discovered the process of fission in the uranium nucleus, which process has already been described on pages 134 and 135. A uranium nucleus is hit by a neutron, splits into two approximately equal parts, and in most instances, several neutrons are hurled forth. This reaction constitutes the foundation of modern nucleonics, since it permits the occurrence of the chain reaction, mentioned at the end of the preceding paragraph.
Now a few more details of this process of splitting, or fission, of nuclei. It releases a great deal of energy, in other words, the two fragments of the nucleus are hurled apart with enormous velocity. This energy—about 150 Mev. per fission—can be determined empirically by measuring the velocity of the fragments, or computed from the formula of mass defects (see page 72). Since this is a typically exothermic process (a process involving a yielding up of energy), it can take place also without an actual bombardment of the nucleus by neutrons. But the spontaneous fission of nuclei is such a rare phenomenon that technically it has no importance whatever.
Nuclear bombardment by neutrons is capable of producing nuclear fission more or less easily in the different elements at the end of the periodic table (Table III). Certain nuclei can be split even by slow neutrons (i.e. neutrons of thermal velocities). Every one of these is a nucleus with an odd mass number, above all the uranium nucleus 92U235 and the nucleus of the element plutonium, which will be discussed later. In these nuclei, the small amount of heating linked with the capture of a neutron is sufficient to produce fission. On the other hand, there are other nuclei which can be split by neutrons having a high velocity only. Thus, for instance, neutrons of at least 1 Mev. are required to produce a fission of the uranium nucleus 92U238. As American experiments with the huge California cyclotron have shown in the course of the past few years, if neutrons (or other nuclear ‘projectiles’) of more than 30 Mev. are used, it is possible to produce fission in the nuclei at the end of the periodic system as far down as tin.
When nuclei of the latter type are bombarded by slow neutrons, the energy of which is not sufficient to cause fission, the neutrons will be either reflected from the nucleus (mostly with a loss in velocity) or captured (see page 90). Thus, for instance, the nucleus 92U238 may change by the capture of a neutron into the nucleus 92U239, which then changes spontaneously, by beta emission, into 93Np239 (neptunium) and 94Pu239 (plutonium). The names ‘neptunium’ and ‘plutonium’ were suggested for elements 93 and 94 by the American scientists who first studied the properties of these nuclei. The processes just mentioned can be written as follows:
92U238 + 0n1 → 92U239 → 93Np239 + -1e0
93Np239 → 94Pu239 + -1e0
The plutonium nucleus, 94Pu239, thus formed, emits an alpha particle and changes into uranium, 92U235, although this is a rarer reaction. The half-life of this plutonium nucleus is about 24,000 years. As already explained, the capture of neutrons, as by the 92U238 nucleus, is a reaction the occurrence of which is more likely in the case of certain values of the neutron energy, where the incident neutron wave vibrates in resonance with the vibrations of the nucleus.
In the fission process, as a rule, a few neutrons are also hurled out from the nucleus; thus, for instance, in the fission of the 92U235 nucleus, two or three neutrons are emitted. This phenomenon, first verified as a fact by Joliot in 1939, makes possible the occurrence of the chain reaction necessary for the technical exploitation of nuclear energy. When a fission reaction takes place in a sufficiently large mass of—for instance—pure 92U235, the neutrons released by it collide with other 92U235 nuclei, causing fission in the latter, too, with the result that still more neutrons are liberated; these, in turn, produce fission in still other nuclei, and so on, so that finally, the entire mass of the substance is changed and a vast quantity of energy is released in the process. Owing to the high velocity of the neutrons, the whole process takes place in less than one-millionth of a second. It is, therefore, quite obvious that a sufficient quantity of pure 92U235 (or a sufficient quantity of pure 94Pu239) is an explosive of inconceivable power. Atomic bombs are made of these substances—and their destructive power is well known. A prerequisite for the occurrence of the chain reaction is, of course, that the mass of the ‘explosive’ should be sufficiently large, for if it is too small, the neutrons would escape through the surface before causing the fission of other nuclei. Therefore, a small mass or
quantity of the above-mentioned ‘explosives’ is totally harmless. However, as soon as their mass or quantity exceeds a certain magnitude, the explosion occurs immediately, and spontaneously. Therefore, the atomic explosion is started by combining small pieces of the ‘explosive’ into a big piece (by throwing them together), and the big piece thus formed explodes at once.
Oppenheimer was the scientific leader of the atomic bomb construction in America. The production of sizable quantities of the ‘explosives’ 92U235 and 94Pu239, calls for such a tremendous technical effort that only the vast industrial capacity of the United States of America was in the position to afford it. In Germany, their production was not attempted during the war, for the capacity of the already overburdened German industry would not have been sufficient for it.d
III. THE URANIUM REACTOR
The use of nuclear energy for peaceful purposes is more important than the manufacture of explosives. In order to harness this energy for peaceful purposes, it is essential to be able to produce a chain reaction which can be kept under control, so as to permit the withdrawal of just the right quantity of energy required at any given moment. Fortunately, it is possible to produce such a chain reaction in natural uranium, which is a mixture of two uranium isotopes—92U238 and 92U235, in the proportion of 140 to 1, and it is not necessary to enrich the natural uranium in the rare 92U235 first.
However, a chain reaction does not take place in pure metallic uranium, for the neutrons emitted in the fission reaction collide far more frequently with 92U238 nuclei than with 92U235 nuclei. They are reflected by the 92U238 nuclei, as a rule, without loss in velocity, and are eventually captured by one of these nuclei with which they resonate, so that they are lost for the chain reaction. But pieces of uranium can be encased in a moderator, a substance that slows down the neutrons. This permits the neutrons to be carried quickly past the resonance values and their speed to be reduced to thermal velocities. But despite the relatively small number of 92U235 nuclei, the thermal neutrons are more easily captured by them than by the 92U238 nuclei and, as a rule, they then produce fission in the 92U235 nuclei. The effect of the moderator thus is that the neutrons are seldom captured by 92U238 nuclei. If the moderator selected is a substance which absorbs thermal neutrons to a very small extent only, and if the apparatus is of a sufficient size to prevent the escape of too many neutrons through the surface before entering into reaction with the 92U235 nuclei, a chain reaction can be started. Practically, the only substances suitable for use as moderators are heavy water (D20) and absolutely pure graphite, the absorption coefficient of both of which is very small for thermal neutrons. In a uranium reactor (as an apparatus consisting of uranium and a moderator is called) built of pieces of uranium and heavy water, the following chain reaction takes place: A neutron, released by fission, leaves the piece of uranium—possibly after a few collisions with uranium atoms—and reaches the heavy water. There, due to collisions with deuterons, it loses velocity, and wanders around in the moderator at thermal velocity, until eventually it happens to collide again with a piece of uranium. It then produces a new fission in a 92U235 nucleus, as a result of which again two or three neutrons are liberated—and so it goes on. While the chain reaction in the atomic bomb is provoked by fast neutrons, the velocity of which is reduced by inelastic collisions to only a little below their original speed, the chain reaction in the uranium reactor is propagated by slow neutrons.
This chain reaction can be easily controlled and guided. The uranium is heated by the disintegration of the uranium atoms. The result is a spreading of the points of resonance in the 92U238, and so a greater number of neutrons are captured by these nuclei. The heating thus atomatically brakes down the chain reaction, so that the entire apparatus becomes stabilized at a certain temperature, the magnitude of which depends on its size and geometric design. Furthermore, it is possible to introduce, into the reactor, from outside, some substance that absorbs neutrons (cadmium is the most suitable material for this purpose) and consequently will act as an added moderator of the chain reaction and regulate the temperature artificially. Once a certain temperature is reached, the reactor will maintain it quite independently of the quantity of energy removed from it. If a considerable amount of energy has been removed—in consequence for instance of good thermal conductivity—the reactor cools off a little at once. The frequency of the disintegration processes immediately increases enormously, and the original temperature is re-established.
Figure 40A shows the interior of the model of a uranium reactor, composed of uranium and heavy water. It was installed, during the war, in a cellar carved in natural rock, in the village of Haigerloch in Württemberg, by a working team of the Kaiser Wilhelm Institut (Wirtz, Bopp, Fischer, Jensen, Ritter). The photograph shows a great many disks of metallic uranium, suspended by chains from a lid which can be lowered into a tank containing heavy water. The tank itself is enclosed in a thick layer of graphite, which, however, is hardly visible in this picture. Figure 40B is a schematic digram of the ura reactor; the shaded area is the graphite coating. The apparatus stood in a large water tank. A neutron sou hanging in the centre, for observation; measuring leads ( attached near the exterior. The apparatus was still a litt small to sustain a fission reaction independently, but a increase in its size would have been sufficient to start o process of energy production.
Figure 40A.—Interior of model of uranium pile.
The first uranium reactor large enough to yield energy built in Chicago, under the direction of Fermi, in 1942. ] built of uranium and graphite and began to react in Dece 1942.
When the chain reaction has begun, the uranium pile up nuclear energy in the form of heat: the uranium simpl hot. If it is desired to utilize the energy further technically, the heat must be removed in some way. This involves a number of technical problems which have still not been solved satisfactorily enough to permit the construction of an economically perfect power station based on nuclear energy. But this is just a question of time; sooner or later there are bound to exist power stations, stations for generating and transmitting heat, and naval engines, driven by nuclear energy.
Figure 40B.—Plan of uranium pile.
The uranium reactor has a feature which makes it difficult to use it as a source of energy, but on the other hand, makes it very useful in another respect: It is filled by an extraordinarily intense radioactive radiation, which is extremely dangerous to every living being in the vicinity of the reactor. For this reason, the reactor is shielded by walls many metres in thickness, constructed of concrete or a like material. The reason for the presence of these radiations is obvious: The interior of the uranium reactor is the scene of nuclear reactions, occurring on a vast scale, which produce radiation of all kinds (alpha, beta and gamma radiation). An effect, in particular, of the gigantic neutron intensity in the reactor is that substances inserted into the apparatus very quickly become radioactive by the processes discussed in Section 4 of our Sixth Lecture. Thus the uranium reactor can be used as an especially efficient ‘crucible’ for artificially radioactivated substances. In fact, this has hitherto been the technically most important application of the uranium pile. For instance, plutonium, so important today as an ‘explosive’ for military purposes, is produced exclusively by a transmutation of ordinary uranium in the uranium reactor. This brings us next, after the question of the generation of energy, to the second main problem of nucleonics, which may be called ‘ennoblement of matter’.
IV. ENNOBLEMENT OF MATTER BY NUCLEAR REACTIONS
More valuable materials are to be produced from those of a lesser value. New substances can be produced by nuclear reactions in very small amounts only. Therefore, their production is worth while only when they happen to be especially valuable. These extremely precious materials, which represent a considerable value, even in small quantities, are the radioactive materials. Their value consists in their radiation, which can be utilized in many ways and is considerable even when the quantity of the substance is very sma
ll. For this reason, the most important application of nuclear physics at the present time consists in an artificial production of radioactive materials.
Radioactive substances can be used for various purposes. They have been used, in medicine, for several decades, to irradiate malignant tumours, which experience has proved to be far more seriously injured by radioactive emissions than healthy tissue. In the majority of cases, of course, x-rays are used for this purpose. But whenever a complication arises that makes it difficult to approach the diseased spot without injuring other tissue, the use of radioactive preparations is preferred. However, since the natural radioactive substances are available for medical purposes in very limited quantities only—apart from radium itself, only mesothorium, discovered by O. Hahn, comes into consideration at all—it is hoped to achieve important progress in medical research by the production of artificial radioactive materials in sizable amounts, especially those which possess chemical properties different from those encountered in the natural ones.
Radioactive substances have still another use: Quite minute quantities of them are mixed with luminescent materials, which give out a constant luminescence due to the effect of radiation. Their best known application is in the luminous dials and hands of watches.
