Industrial Applications of Radioelements

J. Irvine*)

In the continuously expanding arsenal of experimental technique, radioelements have made their contribution in the form of two important tools of research. As has often happened in the past, these tools of research quickly passed from “pure” scientific laboratories into the sphere of applied research work and at the present time have already begun to move into the field of technology.

The first and, perhaps, most important tool is the possibility of labeling atoms and molecules. Such indication provides a means of analysis that has no equal in respect to the sensitivity and definiteness of the results obtained. The second tool is a source of high-energy radiation, characterized by small size, portability, and no need for an external source of energy.

Indicator Applications

A closer acquaintance with the possibilities of using radioelements shows that two categories of their use as labeled atoms can be distinguished, namely: chemical and physical indication. In chemical work one has to deal with radioisotopes of the element whose chemical properties are being studied. The element containing the isotope is introduced into the corresponding ion or molecule, or else is used in pure form or as one of the components of an alloy. This material is then added to the system under study, and all physical and chemical changes of the labeled substance are traced on the assumption that the behavior of the radioisotope accurately reflects the behavior of the labeled element.

As an example of this type of indication, we shall cite a study in which the distribution of phosphorus in slag and steel was investigated.

*) Analytical Chemistry 21, 364 (1949). Abridged translation by L. Belya.

Calcium phosphide containing \(P^{32}\) was added to the slag. The rate of appearance of \(P^{32}\) in the steel characterizes the rate of establishment of phosphorus equilibrium in both phases (Fig. 1). By determining chemically the total amount of phosphorus, one can find the equilibrium constant. The factors influencing the equilibrium and the rate of its establishment can be investigated by appropriately changing the temperature, the relative calcium and phosphorus content, etc.\(^{24}\).

Physical indication, i.e. the study of mass transfer of material, can be carried out by introducing a radioisotope

Fig. 1. Distribution of phosphorus between slag and steel\(^{34}\). It is easy to determine the rate at which equilibrium distribution between the two phases is established. The effect of temperature on equilibrium is also shown.

of any of the elements composing the system. In this case, obviously, the possibilities for choosing a radioelement are considerably broader. If no chemical reactions or phase changes occur in the components under study, then any radioactive element possessing suitable properties can be introduced into the system. Owing to the great sensitivity of these analytical methods, the concentration of foreign material can be made so small that it does not appreciably affect the properties of the system.

An example of physical indication is the use of radiomanganese to determine the distribution of DDT over the surface of the ground when sprayed from low-flying airplanes. A soluble-in-oil manganese compound (\(Mn^{52}\) was chosen because of the suitable properties of its rays and its convenient half-life) was mixed into the standard DDT composition. In this way it was possible easily to measure amounts of DDT not exceeding 25 g

per hectare. Dyes or other additives may also be used for the same purposes, but the radioactive method is preferable because of its high sensitivity and convenience. Because of the volatility of the oil solvent, it is impossible to study its behavior during spraying. In general, separation of the indicator from the substance under study should not occur in investigations of this kind.

APPLICATIONS OF RADIOELEMENTS AS SOURCES OF RADIATION

In work with labeled atoms, the main emphasis is placed on the substance emitting the rays; the radiation itself is used only for carrying out the measurements. When radioelements are used as sources of radiation, the situation is reversed. Now the nature of the substance itself is relatively immaterial, and the principal interest lies in the radiations and the effects they produce.

The use of such radiation sources may proceed along two lines. In one case, what is important is the action of the radiation on the substance. As is known, radiation from radioactive elements produces ionization in any substance through which it passes. This property has been used, for example, to remove electrostatic charges that form on moving insulators; a similar case occurs in the manufacture of paper and fabrics². The removal of charges occurs as a result of their leakage into the ionized air in contact with the moving materials. This prevents sparking and thereby eliminates the danger of explosions of dust or vapors.

Another method of using radioactive sources is based on the action of the substance on the radiation (as distinct from the action of the radiation on the substance). The basis of this method is the absorption of rays in the material placed between the source and the detector. Since the decrease in the intensity of the beam of rays depends on the absorber, the degree of this decrease can serve as a measure of the thickness of the latter. On this principle a thickness gauge for plastic films has been made⁵. Since the material being measured is not subjected to any mechanical action, it is possible to measure very soft plastics without danger of deforming them during the measurement process itself (Fig. 2). This method can also be used to measure the thickness of moving films, and it is therefore suitable for monitoring various technological operations.

Before considering the numerous examples of the industrial application of radioelements, it will be useful to define more precisely what is meant by the expression “industrial application.” In this article, by this term we shall understand the use of radioelements in investigations

or investigations whose immediate purpose consists in creating, improving, or controlling production processes or industrial products. It is obvious that any research leading to a better understanding of a technological process should also be included here, since a deeper understanding ultimately leads to practical consequences.

Labels in the figure:
Investigated film
Flat source of $\beta$-rays
Collecting electrode of an ionization chamber or fluorescent screen in combination with a photocell

Fig. 2. Schematic representation of a thickness gauge for thin films. Depending on the nature of the film being measured, a source emitting $\alpha$-, $\beta$-, or $\gamma$-rays may be used.

EXAMPLES OF INDUSTRIAL APPLICATIONS

Indicator Applications

The examples given below of the industrial application of radioelements are intended to illustrate various cases of investigation of a technological process under factory and laboratory conditions, as well as cases of control of a technological process and of finished products. Much has been written about possible practical applications of radioelements; something is known about work in progress, but very little has been reported about fully completed work. To avoid doubtful data, the discussion below will deal only with the last two categories of work.

One of the chief sources of the undesirable impurity of sulfur in steel is the coke used in the blast furnace. Sulfur occurs in coal in two different forms—pyritic and organic—and the ratio of these two forms depends on the grade of coal. It was therefore desirable to establish exactly which type of sulfur is transferred from coal into coke. If it turned out that some single type ...

of the sulfur forms is transferred to the coke to a lesser extent than the other, then the possibility would thereby arise of controlling the composition of the final product by a sensible choice of the starting material. Since it is not easy to create in the laboratory conditions similar to those encountered in a coke oven, experiments had to be carried out under plant conditions[^7],[^16].

Pyrite containing \(S^{35}\) was synthesized in the laboratory and then thoroughly mixed with 12 tons of coal. This amount of coal constitutes exactly one charge of a coke oven.

Fig. 3. Schematic representation of the behavior of sulfur during coking. Pyritic sulfur (\(S^*\)) is labeled with the indicator \(S^{35}\).

During coking, the evolution of gases was carefully measured, and at appropriate time intervals the total and radioactive sulfur content in the gas was investigated. After completion of coking, analyses were also carried out for the total and radioactive sulfur content in typical samples of the product. These experiments showed that the ratio of radioactive (pyritic) sulfur to its total amount was the same in the gas and in the coke and was equal to the ratio of pyritic sulfur to the total amount of sulfur in the initial coal. This result was interpreted in the sense that, during coking, both types of sulfur in the coal are equivalent and their ratio in the coal does not affect the sulfur content in the finished coke (Fig. 3).

A large series of laboratory investigations is also being carried out in the petroleum industry. Naturally, the chief interest is the mechanism of the reactions occurring during petroleum processing[^10]. Catalytic cracking and synthesis are being studied from various sides. Investigation of the passage of a three-phase system—oil—water—gas—through porous media should lead to a better understanding of certain problems connected with petroleum production[^1].

The mechanism of friction and the role of lubricating materials are also being studied.^{4, 12, 20, 21} It is known that, when two metallic surfaces rub against one another, some quantity of metal is transferred from each to the other surface. If one of the surfaces is made radioactive, it is possible to measure the quantity of transferred material even when the surfaces consist of the same metal, even a very hard one, as in the case of chromium-plated or nitrided steel (Fig. 4). When the rubbing surfaces are lubricated, metal transfer decreases, but

Fig. 4. Transfer of beryllium-copper alloy to a steel ball as a function of the hardness of the beryllium-copper alloy.^{20}

does not cease. Data have also been obtained on the role of additive elements in lubricating materials.

Lubricants are also used for textile materials in order to improve spinning and weaving processes. In one case it was necessary to determine the distribution of a lubricant on viscose yarn. Difficulties that arose during spinning and dyeing indicated a possible nonuniformity in the distribution of the lubricant, and as a result an attempt was made to measure this distribution. The lubricant contained sodium oleate; therefore radiosodium was introduced into the lubricant in the form of this compound. The yarn was treated on a laboratory scale, and the radioactive sodium was measured on samples which in some cases had a length of not more than one centimeter. Since the yarn was 150 denier (150 grams per 9000 meters), such samples weighed only on the order of 0.2 mg, and each sample contained only about 0.04 μg of sodium (Fig. 5). Laboratory tests indicated a high uniformity of sodium distribution, but since the latter served as a ...

“physical indicator,” no conclusion could be drawn from this as to the uniform distribution of the other components of the lubricating material. These tests, however, illustrate the extremely high sensitivity of such an analytical method.

A similar investigation was carried out on the amount of printing ink deposited during printing3. A small quantity of the varnish used in the ink was phosphorized with phosphorus pentachloride containing \(P^{32}\), and was then admixed with the ink. The ink thus obtained was then used for printing. Direct measurement of the \(\beta\)-radiation gave a measure of the amount of deposited ink. Such measurements make it possible to relate the quality of printing to the amount of ink used. In addition, the influence of the quality of the paper and of the printing temperature can also be studied.

In the literature there have appeared brief reports on other applications of radioelements for the investigation of technological processes, but without presentation of results. Thus, at the present time radiocerium is used to study the mechanism of rubber vulcanization and the action of modifiers in the production of synthetic rubber. A radiophosphorus is used to study the rate of diffusion of plasticizers in plastics, and also to determine the amount and location of phosphorus in steel1. Radiocarbon and a number of radioactive metals are used in a number of metallurgical investigations14, 15, 23.

Fig. 5. Weight of sodium in viscose yarn as a function of the concentration of the finishing solution. Radiosodium is added to the solution in the form of sodium oleate.

Radioelements also play a noticeable role in the search for new food products.

In one case, for example, it was necessary to test the action of various mineral compounds added to salt intended for animals17. In this connection, an optimum was sought between the solubility of the compound in water and the degree to which it was assimilated by the animal. Additives that were too soluble would be washed out, while sparingly soluble compounds could not be assimilated with full effectiveness. Radioactive iodine, copper, and iron in various forms were introduced into the animals’ food. After this, at definite times,

At intervals, samples of urine, excrement, and blood were analyzed for radioactivity, and the rate of metabolism was also determined. By this method it was possible to establish that diiodotyrosine is a suitable compound for introducing iodine, whereas iron phosphate, for example, was poorly assimilated by the animals because of its low solubility. As a result of these tests, a suitable compound was selected for each of the elements added to the salt.

In biology and medicine there are many such examples of the creation of new products. Often such work yields results of considerably more fundamental importance than a simple improvement in the quality of products^{13, 19}.

Radioactive indicators have also been used to monitor technological processes in order to obtain occupational-safety data.

As an example, let us cite a case of poisoning that occurred at a fluorescent-lamp factory. The injured worker soldered the lamps before they were evacuated. The lamp contained a small amount of mercury, and it was necessary to determine the amount of mercury inhaled during desoldering. With the aid of radioactive mercury it was found that the average concentration of mercury vapor in the desoldering apparatus was \(0.01\) mg per cubic meter, which is less than \(10\%\) of the maximum permissible value. In these measurements the amount of mercury sometimes did not exceed \(10^{-8}\) g.

Radioelements have also been used to study the currents of polluted rivers^{10}.

Other examples of indicator applications could also be given, but this field is so new that the number of published works is still very small.

APPLICATIONS AS SOURCES OF RADIATION

The use of radioelements as sources of radiation is steadily expanding. Before the appearance of radioelements produced in a pile, radium was of the greatest importance in this field, and at the present time it is still widely used. Applications based on ionization produced by radiation were mentioned above in connection with the problem of eliminating static charges. Luminous compositions for clock dials or the scales of measuring instruments are based on ionization produced in luminescent compositions. Less well known is the use of radioelements as continuously acting sources of ions in gas-discharge tubes. Some radar instruments require that the ignition delay in the corresponding gas-discharge tubes be less than a fraction of a microsecond. The absence of ions in the gas

tube can lead to an ignition delay exceeding this value by thousands of times. The introduction of small quantities of radium

Fig. 6. Schematic representation of an apparatus for measuring pipe thickness[^8]. \(S\) is a source of \(\gamma\)-rays (for example, Ra). \(SH\) is a lead shield for protecting the ionization chamber \(D\) from direct incidence of rays from the source.

or \(Co^{60}\) into the tube ensures the constant presence of ions and thereby the rapid and reliable operation of the tube. Several attempts have been made to use this method for starters of luminescent lamps.

Another common application of radiation sources is based on the dependence of the amount of radiation entering a detector on the distance between the latter and its source, or on the material located between them. This includes radiography of large metal castings. Radium, radon, and \(Co^{60}\)[^18] have found wide application in this field. Defects in castings are determined from especially dark areas of the photographic film, caused by lower absorption of the rays at the sites of defects. The thickness meter mentioned above also operates on this principle of differential absorption of rays. In the latter case, however, an electronic method of recording is used instead of a photographic one.

Fig. 7. Float-type liquid-level meter[^23]. A source of \(\gamma\)-rays, for example Ra or \(Co^{60}\), is placed in a hollow float in the reservoir. The rays are detected by means of a counter or an ionization chamber placed above the reservoir. The current from the detector actuates a valve regulating the height of the liquid level.

Another type of thickness gauge is based on scattering of radiation rather than on its transmission. This instrument is manufactured commercially and is used for measuring the thickness of pipes and plates in cases where only one side of the object is accessible.^8 For each given material the instrument can be calibrated directly in units of thickness. This instrument makes it possible to measure, with an accuracy of up to 5%, the wall thickness of pipes up to 1.25 cm (Fig. 6). It can also be used to measure thickness by transmitted rays, but for thin layers the sensitivity will be

Fig. 8. Diagram of an instrument for measuring area.

less than when measuring by scattering. This instrument has found wide application in the petroleum industry.

By measuring scattered radiation it is possible to determine the concentration of solutions moving through pipes or stored in closed cylinders. In addition, one can determine the position of the interface between two liquids, or between a liquid and a gas. Since the instrument operates electrically, it can be adapted for controlling a technological process by actuating the corresponding valves that regulate the flow rate or the concentration of the solution.

Another liquid-level gauge is based on the inverse-square dependence of the radiation intensity of a small source on the distance from the latter to the detector (Fig. 7). By placing the source on a float and the detector above the reservoir, it is not difficult to measure the height of the liquid in the reservoir.^22 Any source of $\gamma$-rays with a long half-life is suitable for these purposes.

One can measure the area of small plates of arbitrary shape by placing them over a source uniformly distributed over a plane and measuring the corresponding decrease in ionization² (Fig. 8). In this case polonium is usually used as the radiation source, since its \(\alpha\)-rays are completely absorbed by very thin layers (of the order of 0.025 mm).

Fig. 9. Diagram of radioactive semimicrobalances⁹.

Recently an ingenious method was proposed for increasing the sensitivity of semimicrobalances, making it possible to weigh samples of the order of a microgram. The source of \(\alpha\)-rays is placed at the end of the beam so that the rays enter a double ionization chamber. Displacement from the mean position is detected from the readings of the ionization chamber⁹ (Fig. 9).

CITED LITERATURE

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8. Engineering Laboratories Inc., Tulsa Okla., “Penetron”.
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10. A. R. Gardner, Modern Industry, Feb. 15 (1948).
11. C. Goodman, J. Irvine and C. Horan, J. Ind. Hyg. Toxicol. 25, 275 (1943).

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12. J. Stanley, Ibid. 1, 70 (1947).
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