by W Heisenberg
The radiation of radioactive preparations is used also for the testing of industrial materials for internal, structural imperfections, to which use x-rays are customarily put. The gamma radiation is the one utilized for this purpose. This procedure is applied especially to pieces too thick to be penetrated by x-rays, but which will still let gamma radiation pass through. This method of testing industrial materials has the great advantage over other procedures that it does not injure the material itself.
Let us now discuss the production and utilization of radioactive substances in a little more detail.
In order to produce artificial radioactive substances, a suitable substance is irradiated by neutrons in a uranium reactor, or by protons or deuterons in a high-voltage generator or cyclotron. A certain practical difficulty is encountered here, owing to the fact that the substance which it is desired to produce is often present in the original material in the form of minute, imponderable impurities. This substance may be chemically different from the original substance or it may be identical with it, that is to say one of its own unstable isotopes. When the chemical properties of the two substances are different, and the substance is present in ponderable quantities, the two can always be separated by chemical means without the slightest difficulty. But the situation is different when dealing with imponderable quantities, as in the cases under consideration here. In these cases, adsorption phenomena frequently occur and prevent the application of the customary chemical processes of separation. The problem can often be solved by adding in advance a considerable amount of a non-radioactive isotope of the substance to be produced, to the original substance. The adsorption phenomena will then play just a negligible part, and the radioactive substance precipitates together with the stable isotopes.
The chemical processes which can be applied when handling short-lived radioactive isotopes, have been developed and perfected to a high degree by O. Hahn and his associates, in particular.
One of the most important artificial radioactive substances is radioactive phosphorus. For instance, carbon bisulphide (CS2) is bombarded by neutrons. The following reaction will occur in the sulphur atoms:
16S32 + 0n1 → 15P32 + 1H1
The sulphur atom of mass number 32, which constitutes approximately 32 per cent. of ordinary sulphur, and the neutron produce a radioactive phosphorus atom of the same mass number, plus a proton. The half-life of this radioactive phosphorus is relatively long, 14·5 days, which is obviously an advantageous feature as regards its practical utilization; it emits an electron and changes back into the original sulphur atom—viz.:
15P32 → 16S32 + -1e0
According to a suggestion by Erbacher, radioactive phosphorus is obtained simply by diluting the irradiated carbon bisulphide with water; the radioactive phosphorus is dissolved, in the form of ions, in the water which is then separated from the carbon bisulphide by one method or another.
The situation is more difficult where the radioactive substance is chemically identical with the parent substance. In such a case, it would be logical to expect a separation to be impossible. Nevertheless, it is feasible under certain circumstances; namely, when it is simply a case of the absorption of a neutron, the excitation energy is emitted in the form of a photon, as gamma radiation. But the latter produces a recoil in the nucleus which may cause the atom subsequently to become electrically charged, or to be torn out of its chemical bond. In this case, skilful chemical operations will effect the separation of the radioactive atoms from the other atoms. Such methods have been developed by Szilard and Chalmers, and others.
V. ARTIFICIAL RADIOACTIVE SUBSTANCES AS TRACERS
We have mentioned a few ways of utilizing artificial radioactive substances. But there is still another application of these substances which has been widely utilized during recent years, and which may be said to be the most important one at the present time. This application consists in the use of radioactive atoms as tracers. This is what is meant: Formerly, the identification, at some later instant, of individual atoms of a particular element was quite impossible. The reason for this was that the path followed by these atoms in biological or chemical processes could not be followed in detail because of the presence of other atoms of the same kind in the substance or organism being studied. Now, however, it is possible to attach a label to any element—as a ring is fastened to a leg of a carrier pigeon or migratory bird. This label is radioactivity which enables the path of the element to be followed in all its details.
We may illustrate this method by a simple example: Let us assume that we wish to study the diffusion of the atoms of a solid substance within the substance itself—for example, the diffusion of lead atoms in lead. Prior to the discovery of radioactivity, this would have been impossible, for one would never have been able to recognize an individual atom or to distinguish it from other lead atoms. But today, if a piece of lead containing radioactive atoms is brought into close contact with another piece which does not contain any, the lead atoms will be reciprocally interchanged between the two pieces by diffusion, and gradually increasing numbers of radioactive atoms will be discovered in places where originally there was no radioactivity at all. In this way we can obtain information concerning the velocity with which lead atoms diffuse in solid lead.
Let us take another example, the enormous practical value of which will perhaps be even more illuminating: In testing the filter of a gas mask, the main object is to determine to what degree it absorbs the poisonous substances, against which it is intended to serve as a protection. This can be accomplished quite simply in the following manner: Radioactive atoms of one of the elements contained in the poisonous substances are added to the latter, and the substances are then sent through the filter. These radioactive atoms undergo the same chemical reactions as the stable ones. After the poisonous substances have passed through the filter, one merely has to determine whether or not radioactivity appears at the other end of the filter, and if so, the intensity of this radioactivity will indicate what percentage of the poisonous substance has passed through the filter. The individual parts of the filter can also be checked, since, after the poisonous substance has passed through, the intensity of the radioactivity acquired by the parts in question by adsorption of the poisonous substance can be determined. This procedure will furnish information concerning the efficiency of the individual parts of the filter. Similarly, it is possible to check whether or not the rubber covering of the gas mask is actually hermetically sealed against the poison gas, by bringing the latter in contact with one side of the rubber covering and observing whether any radioactivity can be detected on the other side. If so, the rubber covering is proved to be unsuitable for a gas mask. Such testing procedures have been described by Born and Zimmer, and are actually being used.
VI. ARTIFICIAL RADIOACTIVE SUBSTANCES IN CHEMISTRY
In chemistry, artificial radioactive substances are used as tracers on an ever increasing scale. Let us give first an example from the field of quantitative analysis: Erbacher and Philipp attempted a quantitative analysis of a mixture of gold, iridium and platinum. After a reduction by hydrogen peroxide, the gold was precipitated in its metallic form, and was then weighed in order to determine whether its quantity was sufficiently close to the original quantity of gold introduced into the mixture—in other words, whether the gold has actually been separated quantitatively. This actually seemed to be the case—the separation appeared to have been completely successful. As a countercheck, a small amount of radioactive gold was added to the gold. It was found that the radioactivity of the separated quantity was perceptibly less than that of the original quantity. This proved that the gold had not been separated quantitatively, and that the seemingly perfect result had been due to the fact that a quantity of platinum and iridium had accompanied the precipitated gold, and the quantity of this platinum and iridium happened to be exactly equal to the missing quantity of gold.
This example reveals what the important factor is in the utilization of radioact
ive substances in quantitative analysis. A small quantity of a radioactive isotope of a substance is added, as a tracer—as a label or tag, as it were—to the substance itself. Then, the radioactivity of the tracer enables us to follow the progress of the substance throughout all its reactions, and all we need know is the half-life of that radioactive isotope to determine the quantity of it present at any given moment. The measurement of the intensity of radioactivity gives a result no less true than that which we should get by weighing the substance itself—in fact, as the above example indicates, an even more reliable result in many an instance, since it will show infallibly whether or not the substance in question is actually the one sought.
Secondly, in chemistry it may be necessary at times to investigate exchange processes, which used to be inaccessible to every method of investigation, particularly where it is a matter of an interchange of elements having the same properties between substances. The question to be answered may be, for instance, whether the sulphur atoms in sulphuric acid ions and sulphurous acid ions are interchanged between these two substances when they combine. Up to now, the difficulty consisted in the fact that it was impossible to distinguish the atoms of one of these two substances from the atoms of the other. But today, radioactivity affords the possibility of labelling at least some—but in any case a sufficient number—of the atoms of one of the two substances. If after the subsequent separation of the two substances, the atoms thus labelled are found to be present in the other substance, this will prove that atoms have been interchanged. Such experiments have shown that an interchange of sulphur atoms actually occurs between the SO”4 and SO”3 ions. (The two apostrophes after the symbol indicate the double negative charge of these ions.)
There are some further chemical applications I wish to mention here very briefly. The following table gives some idea of the various possibilities of application. Let us begin to consider inorganic chemistry and start with the study of new chemical elements.
There are certain elements, the places of which in the table of the periodic system were vacant till quite recently. These are elements which—to use the customary mode of expression—had not yet been discovered but were assumed to occur in nature. The best known among these are the elements of the atomic numbers (in other words, nuclear charges) 43 and 61. It was long believed that element 43 had been discovered in nature, and it was named masurium. But today we have every reascn to believe that this was an error, and that no stable element of this kind can exist. For in the meantime every isotope of this element of appreciable stability has been produced artificially, and every one of them has been proved to be radioactive. The mass number of the longest-lived isotope is 99; its half-life is approximately four million years. Since this is a short period of time relative to the age of the earth, element 43 cannot possibly occur in nature in any measurable quantity. Since this element can be produced artificially in the uranium reactor, it has recently been renamed technetium (symbol: Tc). The above mentioned most stable isotope is therefore designated by 43Tc99. The state of affairs is similar for element 61. This element, too, was believed to have been discovered in certain minerals, and it was named illinium. But, again, the discovery could not be confirmed. It is practically certain that neither of these elements exists in stable form.
Nuclear Transmutations and their Application in Chemistry
But since it is possible to produce these elements, in radioactive varieties at least, it is possible also to produce chemical reactions with them. Radioactivity does not interfere in the least with chemical reactions. Thus, the chemical properties of element 43 have been investigated in a whole series of experiments. The investigation of element 61 is more difficult, because it is one of the rare earth elements, and its chemical properties are almost identical with those of the other members of that group of elements.
The other two missing elements, those of atomic numbers 85 and 87, have also been produced artificially. Element 85 was produced by Corson, McKenzie and Segré from bismuth, by bombardment with alpha rays of high energy (32 Mev.),and was named astatine (symbol: At). It is formed by the following reaction:
83Bi209 + 2He4 = 85At211 + 0n1 + 0n1
Since that time, traces of 85At218 have been detected, by Karlik and Bernert, among the natural radioactive substances, as a product of the decay of 84RaA218 (84Po218).
Element 87 is a product of the decay of a neptunium isotope. Traces of it have also been detected by Perrey in the radioactive decay of natural actinium. It has been named francium.
Finally, the periodic system of elements, which used to end with uranium, has been extended artificially to atomic number 96. We have already mentioned neptunium and plutonium. These two elements are produced principally in the uranium reactor, in the way indicated in the following formulae:
But other isotopes of these elements have also been produced artificially.
Elements 95 (americium—Am) and 96 (curium—Cm) have been obtained as follows:
The production of new chemical elements is therefore no longer a dream of the future, but an important part of modern nucleonics.
Now we come to synthetic chemistry, the science of the building of new chemical compounds. Bismuth hydrate will be a good example. It had been concluded by chemical analogies that the production of such a compound was bound to be a distinct possibility. But owing to the extreme difficulty of detecting this gas, all attempts to produce it seem to have failed. However, this project has been crowned by success, thanks to the use of radioactive bismuth as a tracer.
It may be mentioned too that in colloid chemistry room for further applications has been found in connection with the detection of colloidal and crystalloid solutions, as well with the ageing of sols and gels.
VII. ARTIFICIAL RADIOACTIVE SUBSTANCES IN BIOLOGY AND BIOCHEMISTRY
Artificially radioactivated substances have also been used, with great advantage, as tracers in biology. In a living organism, changes are often far slower than in a test-tube. This fact calls primarily for the use of radioactive substances with longer half-lives.
One of the most important applications in this field has been the study of metabolism, carried out by Hevesy and others. Formerly, it was only possible to determine the overall picture of a metabolism by verifying what quantities of a certain substance, introduced into the organism for this specific purpose, were still present in its various individual parts after a certain length of time. However, it was impossible to distinguish this particular substance from the chemically identical one previously present in the organism. This circumstance resulted in considerable uncertainty, particularly as regards the speed of the distribution of the substances introduced among the various organs. But the technique of labelling atoms by their radioactivity quickly eliminated this difficulty. This tracer technique enables us to distinguish the atoms which have been deliberately introduced into the organism from those which were there previously.
For instance, Born, Schramm and Zimmer grew tobacco plants in a nutrient soil containing a substance enriched with radioactive phosphorus, which was absorbed by the plants, since phosphorus is one of substances essential for the maintenance of organic life. The progress and paths of the phosphorus in the plants could be observed, and the points of its strongest concentration could be recognized. The major part of the radioactive phosphorus passed into the uppermost, youngest still growing leaves, while less found its way into the lower leaves, and still less into the fully developed leaves. One leaf, in which the circulation of sap had altogether ceased, absorbed no phosphorus at all. Another study of a similar nature determined the speed of travel of the phosphorus in the plant; it was found to be about 10 cm. per second.
Artificially produced radioactive substances, and, again, radioactive phosphorus in particular, have been used to study animal metabolism. In these cases, the phosphorus was mixed with the food of the animals or injected into their bodies (Hevesy). This technique makes it possible to determine, after a while, in what parts of the org
anism the phosphorus has a tendency to settle, and also the speed of its elimination, so that information concerning metabolism can be obtained not only numerically and as to proportion, but also as to details. Among other things, it has been ascertained that after a certain length of time, the phosphorus settles principally in the bones and in the liver, and after another period of time, in the teeth. A great many important biological data can be obtained in this manner.
The quantities of radioactive substance required for such experiments are so minute that they cannot result in any harm to the organism.
Another important problem which could be tackled by the tracer method was that of the assimilation by plants of carbon dioxide. It is a well-known fact that green plants utilize the effect of sunlight (in other words, a photochemical process) to assimilate carbon dioxide from the air and convert it into hydrocarbons. This is how they store up solar energy. But very little was known about the details of this process, and various theories were formulated about its mechanism. It was known for a certainty that every plant must absorb approximately four photons of light so as eventually, with the aid of their energy, to accomplish the chemical reaction. In order to explain this problem, the American scientists Ruben, Hassid and Kamen used carbon dioxide containing radioactive carbon of mass number 11, which has a half-life of 20 minutes. They found that in order to prepare the way for an assimilation, first a reaction in darkness takes place, in the course of which both the carbon and the oxygen of the carbon dioxide (CO2) are combined with the addition of hydrogen in the form of the carboxyl group (of the residue COOH) in a giant organic molecule. Later on, glucose (grape sugar, C6H12O6) is formed from the carboxyl group, possibly by the roundabout way of larger sugar molecules. As a result, the important fact was established that the process of assimilation takes place in several stages. However, we shall not enlarge on further details of this subject here.
