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Some Cosmical Aspects of Radioactivity

The Distribution of Radioactive Matter

MY subject quite naturally divides itself into two parts, one dealing with the properties of the radioactive bodies themselves, and the other with the distribution of radioactive matter throughout the surface of the earth, and throughout the volume of our atmosphere. It is the latter division of the subject that I wish more particularly to discuss, but before doing so it may be advisable to mention briefly some of the more important properties of the radioactive bodies themselves, so that we may be able to judge of the methods employed to detect and measure the minute quantities of radioactive matter scattered throughout the earth and atmosphere. I shall not speak here of the early history of radioactivity, which is no cloubt well known to most of my audience. As a result of a large amount of detailed investigation, a considerable number of distinct radioactive substances have been isolated. Among the best known of these are uranium, thorium, actinium, radium and polonium, and the numerous substances which arise from their transformation. Of all the substances separated from uranium or thorium minerals, radium has occupied the most important position. This is partly due to the comparative • ease with which it can be chemically isolated and purified, but mainly to its great activity or radiating power, which is about two million times as intense as that of uranium. The distinguishing feature of radioactive bodies in general, is their power of spontaneously and continuously emitting special types of radiation, of giving heat, and also in some cases of giving out light. These properties in the case of radioactive substances like uranium and thorium, continue, if not indefinitely, at any rate for periods of time measured by millions of years. In the case of radium, the duration of the activity is shorter, but is still measured by thousands of years. This emission of radiations is. spontaneous and is completely independent of control, whether by physical or chemical agencies. I shall take radium as an example of a typical radio- element, but it must be borne in mind that most of the other active substances possess similar radioactive properties. Radium emits three types of rays, called the a, p, and 7 rays. The a rays are distinguished by their slight power of penetration of matter. A thin sheet of note paper suffices to cut off the a radiation completely. These rays have been shown to consist of heavy atoms of matter carrying a positive charge of electricity, expelled from the active substances at a speed of about 20,000 miles per second. It seems probable that the a particle consists either of a charged hydrogen or helium atom, and most probably the latter. The rays are more penetrating than the a rays and carry a negative charge of electricity and are projected from the active substance at a speed much greater than that of the a particle. Some of the fi particles from radium escape with a speed nearly equal to that of light, or 186,000 miles per second. Their apparent mass is only about one-thousandth of that of the a particle, and in fact they are identical with the electrons produced in the cathode ray discharge of a vacuum tube. .Jornial of the Royal Astronomical Society of Caunad. The 7 rays possess extraordinary penetrating power, passing readily through several inches of iron. It is now fairly certain that the 7 rays are ethereal waves similar in character to the well-known Rontgen rays, only of a more penetrating kind. All of these types of radiation possess in common the properties of acting on a photographic plate and of producing phosphorescence in a certain class of substances, but from the point of view of measurement, their most important property is their power of causing the discharge of electricity from electrified bodies. This property is by far the most delicate test of radioactive matter, and we shall consequently consider it in some detail. If we take an ordinary well- insulated gold-leaf electroscope and charge it so that the gold leaves diverge widely, it is well known that under ordinary conditions with good insulation, the leaves collapse extremely slowly, and over a few minutes' interval the leaves will appear to be almost stationary. Now bring some radioactive matter near the exposed plate of the electroscope. The leaves at once commence to collapse rapidly. This is due to the loss of charge from the electrified system, and takes place with equal rapidity whether the charge is positive or negative. The mechanism by which this discharge is produced has been most carefully studied, and it is known that the effect is due to the property possessed by these radiations of making the volume of the air surrounding the electroscope a partial conductor of electricity. The radiations in passing'through a gas produce a number of positively and negatively charged carriers or “ions.” These ions move in an electric field. If, for example, the electroscope is charged positively, the negative ions are drawn toward the charged system. The discharging effect is thus due to the drawing in of a great number of negatively charged ions to the positively charged conductor, or vice versa. The moment the radioactive substance is removed, the rapid movement of the leaves at once ceases. This property of the radiations of ionizing the air or other gas is an extraordinarily delicate test of the presence of radioactive matter. I bring up near the electroscope a watch glass on which has been evaporated a solution, containing only one millionth of a gramme of radium bromide. The leaves collapse in a few seconds. If I place the watch glass on the plate attached to the electroscope, a charge given to the electroscope' is almost instantly dissipated. The discharging effect in this case is due mostly to the a rays. This can be shown by placing a sheet of ordinary paper, which absorbs the a rays, over the watch glass—the rate of collapse of the leaves becomes now much slower. The residual discharging effect is then due to the 0 and 7 rays from the small quantity of radium. From the point of view of measurement, a millionth of a gramme of radium produces far too large an effect In practice it is found that a quantity of radium measured by one thousand millionth (10-9) of a gramme produces an effect of magnitude suitable for accurate measurement. With care, in a suitably designed electroscope, it is possible to measure the presence of one hundredth of this latter amount and in some cases one thousandth. The electroscope is thus capable of detecting by its increased rate of discharge a quantity of radium measuring 10-12 of a gramme. As an agent for detecting minutest quantities of radioactive matter, the electroscope is far more sensitive than the spectroscope. Such measurements do not of themselves throw any light upon the type of radioactive matter which produces the discharge. It would be difficult, for example, to be sure whether the radioactive matter present was radium, actinium or thorium. But there is another property of these substances which allows us to distinguish readily between them. Each of the substances, radium, thorium, and actinium, gives off steadily a radioactive “emanation” or gas which has very intense radioactive properties. If a current of air, for example, is passed over a thorium or actinium compound and then carried into an electroscope, a rapid collapse of the leaves is observed. This is due to the radiation from the emanation, which produces, a large number of ions in the air with which it is mixed. In the case of radium, the emanation does not escape from a solid compound, but is readily released by heat or by solution. I have here a solution of radium in a closed bottle. The emanation has collected in the air space above the solution, and on passing a slow stream of air through the solution into the electroscope, some of this emanation is carried with it, and as you see, causes an extremely rapid discharge of the electroscope. If the emanation were left in the electroscope, it would preserve its discharging power for several weeks. The discharging effect would not be constant but would decrease to half. value in four days, to one-quarter value in eight days, and so on; and would still be appreciable after a month's interval. In fact, the radium emanation is an unstable substance which breaks up with the emission of a particles. On an average, half of it breaks up in four days. The emanations of thorium and actinium are chemically quite distinct from that of radium and can be at once distinguished from it by the rapid rate at which their activity dies away with time. The emanation of thorium falls to half value in 54 seconds, and that, of actinium in 3.9 seconds. The production of the radioactive emanation by radium offers an extremely simple and reliable method of determining not only whether radium is present, hut also of measuring accurately the quantity present. Suppose, for example, that we wish to determine the amount of radium present in a given specimen of rock. This is dissolved and placed in an air-tight vessel and left for about one month. During this time the emanation collects in the solution and the air space above it and reaches a steady equilibrium value where the rate of production of new emanation compensates for the disappearance of emanation due to its further transformation. The solution is then boiled, and the air mixed with emanation is passed into a suitable electroscope and the rate of movement of the gold leaves noted. If the rate of discharge decreases to half its initial value after about four days, it is certain evidence that the radium emanation is present in the electroscope. The amount of radium in the rock is determined by treating in a similar manner a solution containing a known quantity of radium and observing the rate of discharge of the electroscope, produced by the emanation from it. By this method, we are not only able to detect the presence of radium in a substance, but also to determine the amount present with considerable accuracy.. In this way, a quantity of radium in a solution of only 10-u gramme can be readily measured. The emanation from radium is an unstable substance and breaks up into another substance which behaves as a solid and has quite distinct radioactive properties. The inside surface of a vessel containing the radium emanation becomes coated with an invisible deposit of radioactive matter. If the emanation is rapidly blown out by a current of air, this “active deposit,” as it is called, remains behind. The activity of this deposit is not permanent, but decays rapidly with the time. After several hours, the activity decreases in a geometrical progression, falling to half value in about 28 minutes. For thorium, the active deposit loses half of its activity in 11 hours, for actinium in 34 minutes. Tlife production of this characteristic active deposit from each emanation offers another very useful method of distinguishing whether thorium, radium, or actinium is present. An interesting property of the active deposit, which we shall see has played a notable part in the analysis of the radioactive state of the atmosphere, is its concentration on a negatively charged conductor in an electric field. The carriers of the active deposit become in some way positively charged and are drawn into the negative electrode and adhere, to it. RADIOACTIVE STATE OF THE ATMOSPHERE. I have now passed rapidly over some of the more important properties of radioactive bodies, which have proved of great utility in the attack on the question-..of the distribution of radioactive matter in the earth and atmosphere. The pioneers in this work were Profs. Elster and Geitel, teachers in the Gymnasium of Wolfenbuttel, Germany, and to these investigators we owe a large amount of the information now collected in this important and rapidly growing branch of radioactivity. Geitel had observed in 1900 that the open air possessed the property of causing a slow discharge of an electroscope, and showed that this effect was due to the production of positive and negative ions in the air. In seeking for a possible cause of this effect, it occurred to Elster and Geitel that it might be due to the presence of some radioactive matter in the atmosphere. They then tried an extremely bold experiment. I have shown you that the emanations from, the radioactive bodies produce an active deposit, which can be concentrated on a negatively charged wire. If any radioactive emanation were present in the atmosphere, the active deposit should collect on a negatively charged wire exposed in it. An insulated wire 20 or 30 meters long (Fig. 1) was strung outside a laboratory window and kept charged negatively to a potential of several thousand volts by means of a battery or electrical machine. After several hours, the wire was rapidly removed, coiled round a frame attached to the electroscope, similar to that shown in Fig. 2, and the rate of discharge of the electroscope observed. The gold leaf was found to collapse rapidly, indicating that some radioactive matter was present upon the wire. The magnitude of this effect was independent of the material of the wire and was conclusively shown to be due entirely to the presence of radioactive matter on its surface. We must now consider how to determine the kind of radioactive matter concentrated upon the wire. In the first place, it is improbable that particles of the solid radioactive bodies like radium, uranium or thorium should be present in the atmosphere.. But the emanations from radium, thorium and actinium are gaseous, and if the earth contains these substances, we might reasonably expect that some of the emanation released from them might escape into the atmosphere. We have at once a means of definitely settling this question. If the radium emanation alone is present in the atmosphere, the active deposit collected on the wire should decay at the rate characteristic of radium, i. e., after some hours, the activity should decay exponentially, decreasing to half value every 28 minutes. The rate of decay of the active deposit from the air has been shown to be very similar to this, so we conclude that the radium emanation is present in the atmosphere. The experiment of Elster and Geitel in Germany was repeated by Rutherford and Allan in Montreal and by Prof. McLennan in Toronto, and the results showed that the air of Canada is as radioactive as that of Germany. Falling rain or snow is also radioactive, and its activity decays at the characteristic rate to be expected if the falling rain or snow collects upon it the active deposit of radium distributed throughout the atmosphere. Preliminary experiments showed that most of the radioactive effect of the atmosphere was due to the presence of the radium emanation. Bumstead, in New Haven, and Blanc, in Italy, have, however, clearly shown that in these localities some thorium emanation is also present. This can be tested in a very simple way. A wire is made active by exposure as negative electrode to the open air for several days. On removal, part of the activity due to the active deposit of radium decays rapidly. After five or six hours, an activity still remains which decays much more slowly, decreasing to half of its value every eleven hours. This is the rate of decay characteristic of the active deposit of thorium and shows that the thorium emanation must also be present in the atmosphere. In some parts of Italy, Blanc has recently shown that fully 70 per cent of the activity on the wire must be ascribed to thorium, but apparently the effect due to radium predominates in most other localities. We may, consequently, conclude that the atmosphere over the surface Of the earth contains everywhere small quantities of the radium and thorium emanation and also the active substances arising from their transformations in situ in the atmosphere. The amount of ionization thus produced is small, but can be accurately measured by apparatus specially designed by Ebert for the purpose. By means of a fan driven by a spring motor, a steady current of air is drawn between two concentric cylinders. The inner insulated cylinder is charged and connected with an electroscope. As the ionized air passes between the cylinders, the ions are removed by the strong electric field between them. The rate of discharge of the electroscope serves as a measure of the number of ions per cubic centimeter of the volume of outside air. In the open air there are usually about 1,500 ions per cubic centimeter of the air; the number is not constant but fluctuates considerably. This number of ions present in the atmosphere is extraordinarily small compared with the number of un-ionized molecules. Each cubic centimeter of air contains about 4 X 1019 molecules, so that on an average only an infinitesimal proportion of the molecules are ionized at one time. While this ionization appears very insignificant, needing a delicate electroscope to detect and measure it, yet it has a very important bearing on the electrical state of the atmosphere. We must bear in mind that all of us are continuously inhaling the radium and thorium emanations and their products, and ionized air. In addition we are continuously undergoing a type of mild X-ray treatment, for the j3 and y rays from the earth and atmosphere continuously pass into and through our bodies. We are, in fact, subjected to a continuous bombardment by the radiations from active matter and are fortunately quite unaware of it. Some have considered that possibly the presence of radioactive matter and ionized air in the atmosphere may play some part in physiological processes, but this is a question quite outside the scope of my address. An examination of the electrical state of the atmosphere shows that the upper atmosphere is nearly always positively charged, so that the earth must be negatively electrified. There is- consequently an electric field normal to the earth which causes a steady movement of the positive ions in the air toward the earth and a corresponding movement of negative ions upward. The latter must tend to dissipate rapidly the positive electricity in the upper atmosphere. There are many theories as to the origin and maintenance of the positive charge in the upper atmosphere One very plausible hypothesis supposes that' the electrical state of the atmosphere largely results from the radioactive matter distributed throughout it. The appearance of positive electricity in the upper atmosphere is in this view due to the effect of falling rain carrying the negative charge to earth; for it is found that rain is always negatively electrified. The presence of an ionized layer of gas between the upper atmosphere and the earth tends continually to dissipate the positive charge left behind, and this ordinarily prevents the increase of positive electricity to the danger limit, when lightning discharges would pass to the earth. It thus appears probable that the insignificant amount of radioactive matter in the atmosphere plays an important r61e in causing and controlling the electrical state of the atmosphere. AMOUNT OF RADIUM EMANATION IN THE ATMOSPHERE. The amount of radium emanation in the air per cubic centimeter of its volume is extremely small, but can be measured by several methods* This problem has been attacked by A. S. Eve of Montreal, who found that a cubic kiilometer of the atmosphere contains an amount of emanation equivalent to that liberated from about half a gramme of radium bromide in radioactive equilibrium. Assuming that the amount of emanation in the air over the land surface of the globe is about the same as in the neighborhood of Montreal, and that on an average this distribution is uniform over a height of ten kilometers of the atmosphere, he concluded that the total amount of radium emanation in the atmosphere was equal to that liberated from about 200 tons of radium bromide in equilibrium. It is probable that the amount of radium emanation over the sea is less than that over the land, and is mainly conveyed there by winds from the land, but there is still some uncertainty on that important question. As I mentioned before, it is extremely improbable that much solid radium in the form of fine particles exists in the air, so that for the supply of emanation to the atmosphere we must look to some external source. We shall now discuss the evidence which leads us to believe that the supply of emanation to the atmosphere is kept up by its steady escape from the surface crust of the earth.

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