by W Heisenberg
In conclusion, let us discuss a few examples which have played an especially important part in the progress of nuclear physics. Let us take these in their chronological order.
In 1919, Rutherford accomplished the first artificial nuclear transmutation, changing nitrogen atoms into oxygen atoms by bombardment by alpha particles. A proton (being a hydrogen nucleus, it is written 1H1) was liberated in this process, which can be expressed as follows:
7N14 + 2He4 → 8O17 + 1H1
The oxygen atom thus formed is the rare oxygen isotope of mass number 17. Figure 10 shows such a process in the cloud chamber.
Another important transmutation reaction led to the discovery of the neutron by Joliot-Curie and Chadwick, in 1932. A beryllium nucleus, 4Be9, was bombarded by an alpha particle, and a carbon nucleus, 6C12, plus a neutron was obtained, thus:
4Be9 + 2He4 → 6C12 + 0n1
As the third of these important reactions, let us mention the first transmutation of nuclei by artificially accelerated particles, accomplished by Cockroft and Walton, using protons, in the same year, 1932. The protons were accelerated by a high-tension apparatus of 600,000 volts. When such a proton hits a lithium nucleus, 3Li7, the following reaction occurs:
3Li7 + 1H1 → 2He4 + 2He4
Figure 27.—Transmutation of lithium nucleus into two helium nuclei by a proton (after Kirchner).
In other words, two alpha particles, or helium nuclei, are formed. Figure 27 (after a photograph by Kirchner) shows this process taking place in the cloud chamber. The end of the discharge tube, in which the protons are accelerated, is visible. From there they impinge on a piece of metallic lithium. We can see the tracks of two alpha particles start there and proceed in opposite directions. (The other visible track does not belong to this reaction.) A similar reaction is shown in Figure 28. Here a boron nucleus changes, in consequence of the absorption of a fast proton, into three helium nuclei, i.e.:
5B11 + 1H1 → 2He4 + 2He4 + 2He4
Figure 28.—Transmutation of a boron nucleus into three helium nuclei by a proton.
(Here, too, an alpha particle is visible, which does not happen to belong to this reaction.)
Finally, in view of the ancient history of these problems, let us ask whether we are able today to make gold out of mercury, which used to be the dream of the alchemists of old, and in what way this might be possible. To answer this question, it is sufficient to take a look at Table IV. It shows that mercury and gold are immediate neighbours, so that just one single step would be necessary to accomplish the transmutation of one into the other. In other words, by pure chance, the alchemists of past centuries happened to be on the right track when they endeavoured to turn mercury into gold. According to our present knowledge, mercury has 7 stable isotopes, with mass numbers ranging from 196 to 204, while gold has only one, with the mass number 197; this mass number is absent among the known isotopes of mercury. If the mercury isotope of mass number 196 is irradiated by neutrons, so that a neutron is incorporated in a mercury nucleus, the otherwise unknown nucleus of mass number 197 must be the result. This nucleus must be unstable, or else it would have been observed already. It will therefore change, by positron emission or K-radiation, into the stable gold nucleus of the same mass number. Thus the following two reactions occur, in this order:
(1) 80Hg196 + 0n1 → 80Hg197
(2) 80Hg197 → 79Au197 + 1e0
The mercury nucleus 80Hg196 has changed into the gold nucleus 79Au197.
Thus, fundamentally, the nuclear physicist would have no difficulty in producing gold out of mercury. However, this transmutation has never yet been actually recorded.
One might wonder why this transmutation has never been performed as yet. But the reason is that the profit would be far too small. Unfortunately, the mercury isotope 80Hg196 is extremely rare; it represents not more than about 0·1 per cent. of the natural isotopic mixture of mercury. If mercury is bombarded by neutrons, only one neutron in a thousand will happen to become incorporated in one of these nuclei. From the less rare isotopes we cannot produce gold, but either another mercury isotope or thallium. It is, of course, conceivable that our goal could be reached more easily by a bombardment by fast neutrons. The mercury isotope 80Hg198 is about 100 times more frequent than 80Hg196. If we succeeded in heating this mercury nucleus of mass number 198, by neutron bombardment, to such a high degree that it would emit two neutrons simultaneously, we would get the gold nucleus 79Au197, viz.:
(1) 80Hg198 + 0n1 → 80Hg197 + 0n1 + 0n1
(2) 80Hg197 → 79Au197 + 1e0
But in order to nip false hopes in the bud, let it be stated here that this method would indubitably be many million times more costly than the customary methods of getting gold.
7. THE TOOLS OF NUCLEAR PHYSICS
I. THE METHODS OF DETECTION AND OBSERVATION
The following sections of this book will deal with the tools and methods available to the nuclear physicist both for generating and observing the phenomena discussed in the preceding lectures. These procedures call for immense quantities of energy, and the most powerful instruments known to technical science must be used to supply them. Yet, the material results achieved even with these vast stores of energy are extremely small. Consequently, for these studies it is indispensable to have extraordinarily sensitive instruments, for the phenomena which are to be studied take place in one individual atom or, at best, in a very few atoms—in structures which are inconceivably small according to ordinary conceptions.
We shall begin with the instruments of detection and study. The oldest method is the scintillation method. When a very fast particle—an alpha particle for instance—impinges on a zinc sulphide screen, a reaction occurs there which produces a weak flash of lightning, a scintillation. It is therefore possible to observe the impact of each individual particle of sub-atomic order of magnitude, as the impact of bullets on a plastered wall can be observed, and we can also count the particles this way. It is, however, a poor policy to depend on the unaided eye, which is bound to grow tired little by little during the process of counting. This method is scarcely ever employed these days. It has been taken up recently by recording the weak flashes not with the eye but by electric amplification.
The ionization chamber supplies the fundamental principle on which most of the modern methods of observation are based. Let us attempt to describe this apparatus in a very rudimentary form: A gold-leaf electroscope consists of an earthed metal box containing an insulated metal rod with two gold leaves which spread apart when an electric charge reaches the rod. (Figure 29.) An inverted metal hood, charged by a battery to about 100 volts relatively to the earth, is placed above the electroscope and insulated from the latter. When a charged particle—an alpha or beta particle, or even a gamma-ray photon—enters the space between the electroscope and the hood, it tears electrons off the air molecules. These electrons attach themselves to other molecules and form negative ions, while the molecules thus deprived of electrons remain behind as positive ions. As shown in Figure 29, such ions are created all along the track of the particle. If the hood is positively charged, the positive ions stream to the rod of the electroscope, and the negative ions to the hood. Thus the electroscope becomes charged, and its leaves spread apart. Of course, the apparatus in this rudimentary form is not sensitive enough to detect individual particles. Indeed a far more delicate apparatus, such as a string electrometer, while able to detect a rather weak radiation, cannot register individual particles. The method of detecting small charges has been improved considerably in various ways and respects, according to the particular nuclear-physical purpose in view.
Figure 29.—The principle of the ionisation chamber.
The first apparatus of general applicability (for the detection of individual particles) was Geiger’s point counter (Figure 30). Fundamentally it is simply a vastly improved ionization chamber. In this apparatus, a metal rod is drawn to a fine point, and the chamber is given a fairly high voltage, with the result that a strong elect
ric field is created in the vicinity of the point. When a charged particle or a gamma-ray photon flies past and liberates electrons there, these liberated electrons are so strongly accelerated by the intense field that they, in turn, are able to tear electrons off air molecules. These electrons, in turn, are able to do the same thing, and thus the number of electrons liberated increases like an avalanche, and their number ceases to increase only where the field is weaker. But during this process, such enormous numbers of them are produced even by the effect of one single particle or photon that they can be detected by the means available to us.
Figure 30.—Point counter (after Geiger).
Under certain conditions, when the voltage is not too high, the multiplication of the number of electrons always increases by the same factor. Therefore we speak of a proportional region of the counter and also of a proportional count. The factor just mentioned may be of the order of magnitude of 1,000 or even higher. But if the voltage on the counter is increased beyond a certain limit, we pass beyond the proportional region. In that case, the electrons liberated by the particle start a genuine glow discharge, so that the result is a ten-millionfold or even a hundred-millionfold increase. In that case the discharge must be stopped, so that the counter may once again be ready for a new particle. In this resolving region, the amplification is independent of the number of the primarily liberated electrons. The amplification continues always until the very moment when a glow discharge begins to take place.
About fifteen years ago, this point counter was considerably improved by Geiger and Müller, and has become the counter which is still by far the most important observing apparatus of the nuclear physicist. In principle it is very similar to the original Geiger point counter, except that instead of a point, a thin wire is placed in its centre. (Figure 31.) Usually, it is filled not with air, but with a mixture of argon, under a pressure of 60 to 80 mm. of mercury, and alcohol vapour under a pressure of about 10 mm. of mercury. But there are also several variants. The wire is earthed through a very big resistance, and the outer cover has a potential difference of 1,000–1,200 volts relative to the earth.
Figure 31.—Geiger-Müller counter.
The situation here is similar to that in the point counter. At a lesser voltage, a proportional amplification, by the factor 1,000, takes place. When the voltage is higher, a glow discharge begins; the device is now operating in the resolving region. From the moment of the inception of the glow discharge, the wire, which may be connected to a condenser, becomes strongly charged, as does also the condenser, since owing to the very high resistance, an appreciable time must elapse before the charge flows to the earth. Therefore, during this period, both the wire and the condenser, C, connected with the wire, are at a certain voltage which can be amplified by methods commonly employed in broadcasting technique. As is customarily done in most of these measurements, a counting device, similar to a telephone counter, can be attached, or the voltage can be transferred to a loudspeaker, and thus one can count and register every particle that passes through the counting tube.
The number of the ions produced by beta and gamma radiation is small. Therefore, big amplification is required, and it is customary to work in the resolving region. Furthermore, since the beta particles are not very penetrating, thin-walled tubes are used for counting beta particles; on the other hand, for gamma-ray photons thick-walled tubes are used, so as to keep other types of radiation out as much as possible. When dealing with alpha rays, which produce far more electrons, big amplification can be dispensed with, and it is possible to work in the proportional region. The advantage of this method is that, if the counting device is properly connected, it does not react to other types of radiation. The latter produce weak potential impulses only. A special amplifier, called the thyratron, which transmits impulses above a certain wavelength only, permits the weaker impulses to be eliminated, so that only those produced by alpha radiation are counted. This is important mainly because, in addition to the radiation under investigation, all other conceivable types of penetrating radiation are roaming through space. In the first place, electrons are liberated everywhere, even in the counter itself, by cosmic radiation which cannot be screened sufficiently by any known means. Secondly, no existing substance is entirely free from radioactive impurities, so that even the material of which the counter is made tends to release impulses occasionally. Such limited effects are simply just inevitable with these measuring devices. When counting alpha particles, the counting tube must be equipped with a thin window of mica, to enable the particles to enter, since they would not be able to pass through anything thicker.
Another very important instrument of the nuclear physicist is Wilson’s cloud chamber, the operation of which was explained in our second lecture. The advantage of this device consists principally in the fact that it permits us to obtain a visual record of nuclear processes, thus showing simultaneously a great many of the details of the process.
Figure 32 shows a simple sketch of the cloud chamber. The upper section contains air, saturated with water vapour. It is covered with a glass plate at the top to permit observation; below there is a movable piston, covered by a damp layer of gelatine, so that the air above it is kept saturated with water vapour. The light necessary for observing the cloud tracks is admitted through an aperture in the side. The piston is suddenly moved downward, so that the air expands adiabatically and cools. The result is that the water vapour becomes supersaturated and the ionization produced by a particle entering the chamber causes condensation along its path—the well-known cloud tracks.
Figure 32.—A simplified sketch of Wilson’s cloud chamber.
Many phenomena which one would like to observe in the cloud chamber, in particular the phenomena of cosmic radiation, are extremely rare, and the observer may have to wait for several hours before they eventually take place. The chances of encountering just such a reaction when observing an expansion, are very small indeed. Therefore, if we had to rely on chance alone for the eventual observation of such rare occurrences, studies of this kind would require a very great deal of time. However, this handicap is eliminated by skilfully connecting the cloud chamber with a counter which acts, so to speak, as a sentry at the gate of the cloud chamber. The counter is adjusted so that it will react to the specific phenomenon which it is desired to observe in the cloud chamber. If such a phenomenon actually takes place, the counter immediately effects the expansion through an amplifier. This takes place so fast that the ions formed in the chamber have not yet diffused away from the paths of the particles, and they are therefore visible as cloud tracks. This is the method which has yielded the most important data known concerning the nature of cosmic radiation, during the past decade.
Finally, the photographic plate, too, can be used as a detector of charged particles. An example has been shown already in Figure 25.
These methods enable us to count or detect every radiation with which an electric charge is associated (alpha and beta radiation, as well as all other types of nuclear debris which carry an electric charge), and also gamma-ray photons, so that the question of the detection of neutrons remains the only one still to be explained. Since neutrons carry no electric charge, they do not themselves produce ionization, and therefore we must rely on the observation of a secondary effect in order to detect their presence. The simplest device for this purpose is a boron counter. The inside of the wall of this counter is lined with boron or some boron compound, and the tube is used in the proportional region, so that it will count alpha particles only. As the neutrons strike the boron layer, they produce there the following nuclear reaction:
5B10 + 0n1 → 3Li7 + 2He4
This reaction produces fast helium nuclei, i.e., artificial alpha particles—one alpha particle per neutron. Every neutron evoking a nuclear reaction causes the counter to react with an impulse, in which the lithium nucleus will also participate. Not every neutron, by any means, striking the counter causes a nuclear reaction; many of them traverse the tube wit
hout any effect whatsoever. Nevertheless, the counter registers a number of neutrons proportional to the actual total number of neutrons. The constant factor of proportionality being as yet unknown.
Another frequently employed method consists in placing a tracer at the point where neutrons are suspected to be present. A tracer is a substance—for example, a piece of silver foil—made artificially radioactive by a neutron-induced nuclear reaction. The following two reactions will then take place in the silver, in this order:
(1) 47Ag107 + 0n1 → 47Ag108
(2) 47Ag108 → 48Cd108 + -1e0
In other words, the silver isotope of mass number 107 first changes into another silver isotope, which has the mass number 108. The latter is unstable, has a half-life of 22 seconds, and emits an electron and thus changes into a nuclear isobar, the cadmium atom 48Cd108. The 47Ag108 atom must be unstable, because its nucleus has 61 neutrons and 47 protons, in other words, it is a ‘doubly odd’ nucleus.
Since, as we have already seen, slow neutrons are usually more prone to capture by a nucleus than are fast neutrons, a boron counter will register a greater number of slow neutrons than fast ones. If a counter of this type is brought into the vicinity of a source of fast neutrons, a loudspeaker can be used, which is capable of making the individual impulses audible at a certain average rate, say, one per second. As we already know, neutrons can be slowed down by being made to pass through a hydrogen-containing substance, such as paraffin. If the counter is surrounded by paraffin, the impulses will multiply very considerably and a crackling sound will be audible. Thus, contrary to the naive assumption that the application of paraffin is bound to reduce the effect, a quite considerable intensification of the latter is the result. This method for the increase of the output in nuclear transmutations by means of slowing down neutrons, is employed very frequently in nuclear physics.
