CHAPTER VII. Radioactivity

RADIOACTIVITY

Geological application—Röntgen rays—Becquerel rays—Mme. Curie and the discovery of Radium—The gold-leaf electroscope—The nature of α-, β- and γ-rays—Helium and its production from radium—The heat output of radium—Transformation of the radium atom—Disintegration theory—The half-life period of radium and its generation from uranium—The uranium, thorium and actinium families—Direct measurement of the heat output—Radioactivity independent of external influences—Pleochroic haloes—Measurement of radium in rocks and minerals.

In the limited space at our disposal, only a brief outline of the salient facts of the new science of radioactivity can be given. It is necessary to give some account of these recent discoveries because of their immediate application to the study of the earth itself. In a science like geology, the function of which is to study the consequences of energy transformations in the earth’s crust during the long ages of her gyrations through space, the recognition of sources of energy previously undreamed of is a stimulant to research which must profoundly affect the interpretations of the phenomena we observe. For our present purpose it will be necessary to review only those facts which have a bearing on the question of the earth’s age, and of its thermal history.

In 1895 Röntgen gave his first account of A New Kind of Radiation. This extraordinary radiation, now familiar to us as the X, or Röntgen rays, revealed itself by its remarkable property of passing through objects which are entirely opaque to ordinary light. It was found, for example, that a photographic plate was not protected from its influence by being wrapped in black paper, but was affected just as though it had been exposed.

This discovery awakened the deepest interest amongst all scientific workers, and the idea arose that phosphorescence and the Röntgen rays might in some way be vitally connected. Certain substances, after being exposed to sunlight, are able to shine in the dark with a phosphorescent light, and it was this radiation which was examined from the new point of view. Working on these lines, with uranium salts, Becquerel found, almost by accident, that they gave out rays which were capable of penetrating black paper and making themselves evident by their effect upon a photographic plate wrapped within it. This observation in 1896 marks the commencement of the harvest of wonderful discoveries which have revolutionised our conception of the structure of matter.

Taken in the dark by its own rays.]

Taken in ordinary daylight.

PHOTOGRAPHS OF PITCHBLENDE.]

The emission of rays was found to be an inherent property of uranium and entirely independent of the phosphorescence exhibited by certain of its compounds. Physical and chemical changes made no difference; the radioactive properties evidently belonged to the atom itself, for they could be detected under whatever conditions uranium happened to be present. Mme. Curie at once began an exhaustive examination of all the other known elements, and came to the conclusion that thorium alone possessed radioactive properties similar to those of uranium. In the course of her researches, she found that uranium-bearing minerals such as pitchblende (uraninite) were far more active in their emission of rays than could be accounted for by the uranium present. Systematically following up so significant a fact, she traced the activity to the bismuth and barium separated from these minerals. In 1898 two new elements were announced. One, associated with bismuth, was called Polonium; the other, found in minute quantities with the barium, was appropriately named Radium.

The spontaneous radiations of these elements, collectively known as the Becquerel rays, were soon distinguished from the Röntgen rays by their greater complexity. Their analysis is largely due to the scientific genius of Rutherford, who has classed them into three well-marked types, the Alpha or α-, Beta or β-, and Gamma or γ-rays. The instrument chiefly employed in their detection is the gold-leaf electroscope, which not only is simple in construction, but is capable of extreme sensitiveness. Under ordinary conditions the air is a good insulator, and a charged electroscope loses its charge only very slowly. However, if radium is brought near the instrument, the discharge proceeds much more rapidly, the effect of the Becquerel rays being to ionise the air and so render it conductive to electricity. In the electroscope this effect can be readily observed and accurately measured, for the leaf falls at a regular rate, which depends on the intensity of the radiation. The sensitiveness of this method so far surpasses that of the most delicately adjusted balance that by its means a quantity of radium can be detected which would need to be multiplied thousands of millions of times before the balance would turn to its weight.

It is easy—by interposing between the electroscope and the radium a gradually increasing thickness of suitable material such as thin sheet aluminium—to establish the existence of three kinds of rays which differ greatly in their power of penetrating matter. The γ-rays are, on the average, about a hundred times more penetrating than the β-rays, and these in turn are equally the superior of the α-rays. In each case the penetration in different materials depends approximately on their density. The three types can also be distinguished by their behaviour in a strong magnetic field. Giesel showed that the β-rays were deflected in such a way as to indicate that they consisted of particles carrying a negative charge of electricity. The α-rays were much more difficult to deflect, but Rutherford successfully turned them from their path by applying a very intense field, and from the direction of their deflection he concluded that they consisted of positively charged particles. The γ-rays have, however, resisted all attempts made to alter their course. Their real nature is not yet beyond dispute, and from the great similarity between their properties and those of the Röntgen rays they are generally considered to be, like the latter, either electro-magnetic pulses in the ether, or uncharged corpuscles of a kind not yet understood.

When an electric discharge is passed through a high-vacuum tube, rays are emitted from the cathode which, on reaching the walls of the tube, produce a yellowish-green phosphorescence. The cathode rays are identical with β-rays in every essential respect, and both are proved to be tiny, negatively charged particles, called electrons. The velocities of both cathode and β-particles are inconceivably high; in the latter case even approaching that of light. Now Röntgen rays, whatever may be their true nature, are set up by the sudden stoppage of electrons when they impinge upon the anti-cathode of a vacuum tube. In the same way it is supposed that the β-rays, by their sudden expulsion, give rise at the same moment to γ-rays. It is a significant fact in this connection that the β- and γ-rays are always associated together, whereas the α-ray may be quite independent.

The story of the brilliant researches which culminated in the measurement of the mass of an electron, and of the charge it carries, is one of the most fascinating in the annals of science. But here we must be content with the bare statement that the electron has only ¹/₁₇₀₀ of the mass of a hydrogen atom. The application of similar methods to the α-particle has demonstrated that it is of atomic dimensions, and consists either of a hydrogen atom with a single ionic charge, or of a helium atom with twice that charge. Other evidence decides conclusively in favour of the second alternative.

Of all the elements, helium has probably the most interesting history. It was first discovered in the sun during the eclipse of 1868, but it was not until 1895 that it was detected in terrestrial materials. In that year Sir W. Ramsay identified it in uranium- and thorium-bearing minerals by means of its highly characteristic spectrum. Even then, before radioactivity had been recognised, the association of helium with these elements had forced itself upon his notice. In 1902 the genetic relationship between helium and certain of the radioactive elements was predicted by Rutherford and Soddy. During the following year Ramsay and Soddy working in conjunction sought for, and actually observed, its generation from radium. Using a radium preparation from which helium was at first entirely absent they were able to prove, with the aid of the spectroscope, that in the course of a few months sufficient of the gas had been generated to be identified. This important discovery has received abundant confirmation during the last decade, and to-day we even know the number of helium atoms which a gram of radium emits in a given time. Moreover, the production of helium has been demonstrated from uranium, thorium and actinium, and from most of the other radioactive elements which are characterised by the emission of α-rays. The rate of generation of helium will be considered in more detail in Chapter X, where it finds an important application.

An observation of fundamental importance was made by Curie and Laborde in 1903 when they discovered that radium is able to maintain a temperature above that of its environment. The source of this emission of heat lies in the kinetic energy of the Becquerel rays. Projected from radium, they are sooner or later completely absorbed by the matter through which they penetrate. Simultaneously, their energy is given up to the molecules with which they collide, and it therefore reappears as heat. By far the greater proportion—over 85%—of the energy is carried by the relatively heavy α-particles which move with velocities nearly one-tenth that of light. Knowing their masses and velocities, and the number liberated per hour from a gram of radium, it is a simple dynamical problem to calculate their total kinetic energy. Expressing this quantity in heat units, it is found to be equivalent to an hourly production of 113 calories. (Ap. A, p. 177.) As we shall see later, this figure is in close agreement with the results of direct measurements.

The most remarkable feature about these phenomena is that they appear to continue unceasingly. Year after year the spontaneous production of helium goes on, accompanied by a steady evolution of heat. But what of the atoms that remain? They evidently cannot continue to be the same element after having given up part of their energy and expelled from themselves the material particles of the α- and β-rays. A close examination reveals the genesis of a new element—a gaseous substance known as radium emanation. Its atom represents the residual fraction of the radium atom after the separation of a single α-particle. The emanation is highly unstable; so rapidly does it give up energy and liberate helium that its original intense activity decreases by half every four days. In its place arises another product, radium A, of still more transitory existence. A long succession of similar transmutations has been traced, each accompanied by a setting free of energy.

In 1902 Rutherford and Soddy advanced the theory of the disintegration of the radioactive atoms. The radium atom is essentially unstable and ultimately breaks up, explosively emitting α- and β-particles. Each atom has a certain expectation of life which distinguishes it from the atoms of other radioactive substances; but such is the distribution of instability that a constant proportion of the total number of atoms present is always breaking up. Out of a million atoms of radium a definite number disintegrates each second, and the proportion has been proved to be invariable in every case it has been possible to test. From element to element it is this proportion, and therefore the average promise of life, which varies. An equilibrium between formation and transformation is gradually attained, and once it is established a given quantity of radium produces as many atoms of emanation as the emanation produces of radium A, and so on. When in equilibrium the longer-lived radio-elements are present in greater quantity than those of shorter lives in order to compensate for their comparatively slow rate of decay, and so to enable them to keep pace with their more rapidly changing associates.

We may now calculate the average life period of radium. The volume of emanation in equilibrium with one gram of radium has been measured and found to be O·58 cubic millimetres—less than the volume of a pin’s head. The rate of change into radium A also lends itself to direct measurement, and from these two quantities it can be shown (see Ap. A, p. 179) that in one year ¹/₂₆₀₀ of the radium originally in existence must undergo transformation. The same result can be arrived at more directly. We know the number of atoms in a gram of radium and also the number of helium atoms expelled in the course of a year. The proportion which has suffered disruption follows at once, and is found to be ¹/₂₅₀₀.

From evidence of this kind it is concluded that after about 1850 years only half of any given quantity of radium then maintains its identity as that element. Now it is clear that radium must have a progenitor by which it is formed as fast as it decays, or otherwise there could now, after the lapse of millions of years, be none in existence.

That progenitor is undoubtedly uranium—an element which disintegrates so slowly that its half-life period is three million times as long as that of radium. This association is revealed by the study of radioactive minerals. For every gram of uranium there exists in equilibrium with it, 3·4 × 10⁻⁷ grams of radium. The constancy of this proportion points to a genetic connection and admits of no other interpretation. The generation of radium has been experimentally verified by Soddy, who observed it after a number of years in a uranium preparation originally quite free from it. In practice this work is complicated, and the desired effects are retarded, by the intermediate production of another radio-element, ionium, which has an average life many times longer than that of radium. The time taken to establish equilibrium is therefore very long. The primary minerals from which the equilibrium ratio of radium to uranium is deduced, have existed for hundreds of millions of years and so far as they have remained unaltered by percolating waters they afford a safe guide. But in secondary minerals, like autunite, the age of which is often to be reckoned only in thousands of years, the equilibrium stage has not always been reached, and the ratio between the two elements is consequently somewhat lower than that found in the older minerals.

In the accompanying diagram the complete family of uranium is summarised together with the half-life period of each member and the rays it emits. Another series of elements related in the same way is derived from thorium which is a parent of even longer life than uranium. As to the final products of transformation, which, being stable, ought to survive and in the course of time to accumulate, helium is the only one which has been directly observed. That lead is the end product of the uranium series is most probable, as we shall see in a later chapter. Of the stable element to which the thorium series ultimately leads we are still in ignorance. Neither lead nor bismuth, which suggest themselves by their appropriate atomic weights, satisfy the conditions, for their presence in a mineral bears no relation to the quantity of thorium (see p. 190).

The actinium group of radio-elements calls for little notice here. Boltwood finds a fixed ratio between actinium and uranium in minerals, and the former would therefore appear to be a descendant of uranium just as radium is. Actinium, however, does not fall in the main line of descent. There is some probability that it marks the beginning of a collateral series, also springing from uranium. The proportion of uranium which disintegrates in this direction rather than towards radium can only be very small. It is possible that uranium atoms, or those of one of its products, may be of two different types, according to their internal structure, and that on disruption each may give rise to an independent chain of elements.

The heat output of radium and its relation to the energy of the α-particle has already been mentioned. The first direct measurement of the heat generated was made by Curie and Laborde. Using suitably constructed calorimeters, they were able to demonstrate a regular evolution of about 100 calories in an hour from a gram of radium—heat enough to raise the temperature of an equal mass of water from freezing to boiling-point. The radium in these experiments was in radioactive equilibrium with the emanation and radium A, B and C, so that altogether four α-particles were disengaged for each atom of radium which transformed. The approximate correctness of this first estimate will be seen from the following table, in which the best results up to date are quoted:

1903. Curie and Laborde 100 } 1904. Rutherford and Barnes 110 } gram calories 1904. Runge and Precht 105 } emitted per 1908. v. Schweidler and Hess 118 } hour per 1909. Duane 120 } gram 1911. Duane 117 } of radium 1912. St. Meyer and Hess 132 }

The correspondence of these results with the theoretical figure, 113 calories, which represents about 85% of the energy, is eminently satisfactory (see Ap. A, p. 177).

The disintegration theory affords a simple explanation of the origin of this energy. From the new point of view the atom is no longer indivisible, no longer the ultimate foundation of all matter; rather is it regarded as a tiny universe of electrons, a whirling assemblage of charged particles. These minute entities move with very high velocities within the atom itself. In general the energy is locked up, but in the radioactive elements the atomic structure is unstable and from time to time some of the energy can escape. The intrinsic energy of the atom itself is the real source of the heat liberated by radium, and the apparent permanence of its output is simply an expression of the immense supplies of energy which an atom can store within itself.

The essential features of radioactivity are two: the spontaneous emission of Becquerel rays, and the atomic character of the change. The radiation from a radioactive substance is proportional to the quantity of the substance and to nothing else. Temperature, being a function of molecular movement, is without influence upon atomic transformation. Experiments made at temperatures ranging between that of liquid air and 1600° C. show that the same law of disintegration holds throughout. Similarly, the effect of very high pressures indicates that these inter-atomic processes are quite independent of any external influence. Whether in solution or in chemical combination, whether taking part in energetic chemical reactions or suffering bombardment by its own radiations, the atom continues to obey the law which determines its life period, unmoved by any experiences through which we can oblige it to pass. The radioactive properties are simply superimposed upon the chemical and other properties of the substance, and as far as laboratory conditions are concerned, they are entirely spontaneous and can in no way be affected.

Whether or not radioactivity is a general property of atomic matter cannot yet be announced. Potassium and rubidium are feebly radioactive—a thousand times less than uranium—and emit β-rays. It has been thought that some other metals may share in a minor degree the same attributes, but up to the present no reliable results have been forthcoming.

In its geological applications radioactivity possesses a great interest. The most important bearing of all lies in the part it plays as a source of terrestrial heat. Radium is widespread amongst the surface materials of the earth’s crust. Rocks and soils, and natural waters all contain radium as a constituent, widely diffused but always present. In the following two chapters the influence of radio-thermal action in the earth and sun, and on speculations as to their ages, will be dealt with. Before passing on to these cosmic aspects of the subject, a brief account may be given of the way in which minute quantities of radium can be detected and accurately measured—quantities which in rocks average little more than a billionth of a gram in each gram of the rock.

Radium itself is not directly dealt with; it is the emanation in equilibrium with it which is actually measured. The emanation is separated from a suitable amount of rock, mixed with air, and introduced into an electroscope. After about three hours, the emanation has generated radium A, B and C in their equilibrium amounts, and the emission of three sets of α-particles then steadily proceeds. In the electroscope, the rate of fall of the changed leaf depends directly on the conductivity of the air or on the number of ions which are utilised as carriers of electricity. Now each α-particle, in virtue of its enormous energy, is capable of producing in air about 100,000 ions, and the total number present is therefore determined almost wholly by the number of α-particles. The β- and γ-rays certainly have an ionising effect, but so small that in this connection it may be ignored. To be quite sure that all the ions are taking an active part in the discharge and collapse of the leaf, it is only necessary to observe the latter before its divergence from the central support falls to less than, say, 10°. The rate of collapse can easily be gauged by observing the leaf through a microscope with a divided scale attached to the eyepiece, the time during which the leaf passes over a given number of divisions being taken with a stop watch. The rate of fall is proportional to the conductivity, which depends in turn on the number of α-particles expelled, and therefore on the quantity of emanation present in the electroscope; this quantity, provided that it was initially in equilibrium with the radium in the original rock, determines the amount of that element which was present. A constant can be determined once for all by experiment with known materials, which directly connects the rate of fall in scale divisions per hour with the equivalent amount of radium or uranium in grams. Some idea of the delicacy of the method may be gained from a particular example. In the apparatus devised by Prof. Strutt and used both in his own investigations and those of the present writer, a leakage of one scale division per hour corresponds to 9 × 10⁻¹³ grams of radium.

To use this apparatus it is necessary to bring the rock into solution before the emanation can be extracted. The solution is stored up for a few weeks until the equilibrium amount has been generated and the flask A containing it is then attached to the water condenser B (Fig. 14). The emanation can be expelled by vigorous boiling, and greatly diluted with air, it passes out into the gas-holder C. The stream condenses in B and drops back. At the end of an hour the cooling water is run out of B and the steam then drives all the air charged with emanation into C. In order that none should be allowed to escape back, the connection at D is closed. Meanwhile the electroscope F has been exhausted and the emanation is made to pass into it through the tap at E, the gas-holder being slowly filled with water. After three hours the measurements may then be made.

Apparatus for estimating Radium by its Emanation.]

Joly has recently varied this method in order to save the labour attending the process of getting a rock into clear solution. The mixture of finely-powdered rock and fusion mixture is heated in an electric tube-furnace, and the expelled gases containing the emanation are drawn straight from the furnace. Carbon-dioxide is absorbed by soda-lime, and the remaining gas is collected and finally passed into the electroscope as before. The absence or presence of thorium makes no appreciable difference in these measurements, for the life of its emanation is many thousands of times less than that of radium emanation. Joly has utilised this fact in his solution-method for determining minute quantities of thorium in rocks and minerals—a method similar in principle to that described above for radium.

The presence of radioactive elements in rocks sometimes reveals itself in a most beautiful way. In mica, cordierite, hornblende, chlorite, tourmaline, and a few other minerals, small circular spots known as pleochroic haloes are sometimes seen. A tiny crystal can generally be detected in the centre, usually of zircon, but sometimes of apatite, epidote, rutile, or sphene. Until quite lately the nature and origin of these intensely pleochroic spots was entirely unknown. Joly showed in 1907 that they are due to the radioactivity of tiny inclusions around which they spread spherically outwards. The α-rays discharged from the minute central crystals are able to ionise the biotite (or other mineral) and this effect, to which the colouring action is due, spreads just as far as the α-particle can penetrate. The range of the different α-particles from the uranium family is given overleaf, both for air and for biotite.

-----------------------+----------------------- | RANGE IN MILLIMETRES. +---------+------------- SOURCE OF Α-PARTICLE. | | | IN AIR. | IN BIOTITE. -----------------------+---------+------------- Radium C | 70·6 | 0·033 Radium A | 48·3 | 0·023 Ra. Emanation | 42·3 | 0·020 Radium F | 38·6 | 0·018 Radium | 35·4 | 0·017 Ionium | 28·0 | 0·013 Uranium 1 and 2 | 27·0 | 0·013 -----------------------+---------+-------------

Now Bragg has shown that the ionising effect of an α-particle is greatest just before it comes to rest. We should therefore expect to find a number of intensely coloured spherical shells existing around the central crystal and corresponding in each case to one or other of the range limits here given. In section, and magnified 700 diameters, the appearance of the shells would be as in the following diagram.

Pleochroic Haloes in Biotite due to Uranium and its transformation products. Magnified 700 diameters.]

The dimensions of the actual haloes are exactly in agreement with the distances to which the different α-particles can travel, and occasionally the successive spheres of ionisation and colouring are beautifully developed. In the biotite of Co. Carlow granite, Joly has found several very perfect examples, and by his courtesy four of his photographs are reproduced in the Frontispiece. Not only is the proof that the haloes are due to α-rays conclusive beyond question, but the correctness of Bragg’s laws from which the range in biotite is calculated are established in an unexpected way. Thorium haloes are also found (as in Fig. 2, upper left-hand part of the field), and these again have diameters in perfect accord with the demands of theory.

A point of interest in the history of igneous rocks is that the haloes disappear when the rock is heated. The presence of well-formed haloes, therefore, implies the maintenance for very long periods of fairly uniform conditions. So slowly do these haloes form that it may become possible, when they have been further investigated, to give a rough minimum estimate of the age of the minerals in which they occur. The radioactivity of a zircon is much more intense than that of the rock enclosing it, but the smallness of the quantities involved is such that two or three weeks may elapse between the expulsion of successive α-particles or helium atoms. By the accumulative effects of millions of these atoms a spherical halo, faint in youth, but assuming a deeper tint with age, is gradually produced.