Popular Science Monthly/Volume 71/December 1907/Radioactivity of Ordinary Substances

by W. W. Strong

Layout 4

RADIOACTIVITY OF ORDINARY SUBSTANCES

By W. W. STRONG

JOHNS HOPKINS UNIVERSITY

DURING the latter part of the nineteenth century a great deal of work was done upon electrical discharges in rarefied gases. In 1895 Röntgen made the epoch-making discovery that such a discharge was the source of very penetrating radiations. These radiations he called X-rays on account of their unknown nature, and he found that they possessed the power of making a gas a conductor of electricity by producing in it a great number of positively and negatively charged carriers or ions. Besides ionizing a gas, the X-rays were found to affect a photographic plate just as light rays do and to be able to penetrate thin sheets of the metals and many other bodies which are opaque to light. It was found in the course of experimentation that these X-rays were closely related to the stoppage of the cathode particles or corpuscles, and the phosphorescence on the walls of the vacuum tube which these corpuscles excite. In 1897 J. J. Thomson found that these cathode particles or corpuscles were small negatively charged particles of an apparent mass only one seven-hundredth that of the hydrogen atom and that in a "high vacuum" tube in a strong electric field they acquired a velocity approximating that of light. All the properties of the corpuscles were found to be the same, no matter what kind of gas or electrodes were in the discharge tube. Their mass was found to vary with their velocity in such a way that the whole mass of the corpuscle could be ascribed to the electric charge which it carried. From this most important discovery it was concluded that all the common substances were partly made up of corpuscles, and this conclusion has been strengthened by all later discoveries. After Thomson's discovery, Stokes showed that the sudden stoppage of the corpuscles by the walls of the discharge tube caused intense electromagnetic disturbances to travel out from the point of impact. These disturbances are the X-rays and travel with the velocity of light.

 

Discovery or Radioactivity

When Röntgen announced his discovery, it created a great impetus in the study of everything related to electrical discharges. Now it had been known for a long time that some bodies like the uranium salts phosphoresce when exposed to sunlight, and it occurred to H. Becquerel that such a phosphorescing body might emit X-rays, this emission being analogous to the origin of X-rays in the phosphorescing glass walls of a vacuum tube. In accordance with this view in 1896 he exposed a photographic plate to uranium sulphate which was covered with copper and aluminium foil and found that the plate was acted upon. Accidentally he found that this action took place, no matter whether the uranium nitrate was phosphorescing or not, and he found that uranium which had never been exposed to sunlight possessed the same property. He found that these radiations from uranium were similar to the X-rays in their penetrating power. This was the first discovery of the possession of radioactivity by a body, i. e., the power of a body to ionize a gas, to affect a photographic plate or to produce phosphorescence.

Madame Curie then took up the problem of finding whether other substances possessed the properties of uranium and found that thorium did. She made a detailed investigation then of all the elements and found that none, with the exception of uranium and thorium, possessed these properties even to the order of the hundredth part of that of uranium. She, however, found that some minerals possessed a greater radioactivity than uranium or thorium, and concluded that these must contain elements more highly radioactive than either of these. After much tedious, but brilliant, work she was able to separate out the very radioactive element radium. As a result of the work of the Curies, and many others, it was found that thorium, uranium, radium and actinium were radioactive, the latter two being intensely so. None of the other elements were found to possess any radioactivity to within the limits of the experimental errors of the method of observation.

The study of these radioactive elements has been the source of very important discoveries in physics. These elements are found to emit spontaneously a continuous flight of material particles, projected with great velocity, and also to be the source of radiations similar to X-rays and called $\gamma$ rays. We will now describe the material particles, which are of two kinds, the $\alpha$ and $\beta$ rays.

The a rays consist of positively charged particles shot out by the radioactive body with a velocity approaching that of light. They are readily absorbed by thin sheets of metal foil or by a few centimeters of air. The $\beta$ rays are far more penetrating in character than the a rays and consist of negatively charged bodies projected with velocities of the same order of magnitude as that of light. As far as known, they are identical with the corpuscles. Of the three kinds of rays, the a rays produce the greatest amount of ionization and the $\gamma$ rays the least. With a thin layer of unscreened radioactive matter spread on the lower of two plates, say 5 cm. apart, it will be found that the relative order of ionization due to the $\alpha ,\beta$ and $\gamma$ rays is as 10,000 to 100 to 1, whereas the average penetrating power is inversely proportional to the relative ionization. The photographic action is due almost entirely to $\beta$ rays.

 

The Disintegration Theory

The radioactivity of the radio-elements is not a molecular, but an atomic, property, and the rate of emission of the radiations depends on the amount of the element present and is unaffected by the application of any known physical or chemical forces. In order to explain the emission of positively and negatively charged particles, Rutherford and others consider the radio-elements as undergoing spontaneous changes and that the energy of projection of the α and β rays had previously been stored up in the atom as rapid oscillatory or orbital motion. This breaking up of the atoms is considered to be accompanied by the production of a series of new substances which have distinct physical and chemical properties. For instance, thorium produces an intensely radioactive substance, thorium X, which is soluble in ammonia. Thorium gives rise also to a gaseous product, the thorium emanation, and this is the source of another substance, which is deposited on the surface of bodies in the neighborhood of the thorium, and which is known as the "excited activity" or the "active deposit." If a negatively charged wire is brought into close proximity with thorium salts, the "active deposit" will form upon it. The "active deposit" itself decays into a succession of products.

Following will be given some of the products of the various radioactive elements and some of their properties.

 

Table I.

Transformation Products. The Thorium Group

Product			Time to be Half Transformed	Radiations	Range of α Rays	Physical and Chemical Properties
		Thorium	2(10)9 yrs.	Rayless(?)		Insoluble in ammonia.
		Radiothorium	?	α rays	3.9 cm.
		Thorium X	3.6 days	α rays„	5.7 cm.	Soluble in ammonia
		Emanation	54 secs	α rays„	5.5 cm.	Inert gas condensing at -120°C.
Active Deposit	$\scriptstyle {\left\{{\begin{matrix}\ \\\\\ \ \end{matrix}}\right.}$	Thorium A	11 hrs.	Rayless		The active deposit is concentrated on the cathode in an electric field.
		Thorium B	1 hr.	α rays	5.0 cm.
		Thorium C	Very short	α,β,γ rays	8.6 cm.
The Uranium Group
		Uranium	(10)9 yrs.	α rays	3.5 cm.	Soluble in an excess of ammonium carbonate.
		Uranium X	22 days	β & γ rays		Insoluble in ammonium carbonate.

The Actinium Group

 

The Radium Group

In connection with these tables it may be well to consider the a particles a little more. It has been found that the $\alpha$ particles of any one product are emitted with the same velocity. It is found that after passing through a definite distance of gas, they then cease to ionize it. If the gas is air under normal conditions of pressure and temperature, this distance will be the range of the $\alpha$ particle. All experiments up to the present indicate that the α particles of the different products differ only in the speed of projection, this speed determining the range of ionization. Rutherford has found an empirical relation between the range of the $\alpha$ particle and its velocity at any point in its path. If r is the remaining range after passing through a screen, its velocity is $V=.348\ V_{0}\ {\sqrt {r}}+1.25$, where $V_{0}$ is the initial velocity of the $\alpha$ particles emitted from radium C, and is $2.06\ 10^{9}$ cm. per sec. The initial velocity of expulsion of an $\alpha$ particle from a certain product will then be a constant. The value of e/m for all $\alpha$ rays measured has been found to be the same and to be about $5.07\ 10^{3}$ electro-magnetic units. It thus follows that all the radio-elements possess the common constituents, the $\alpha$ and $\beta$ particles. It has been found that after a certain critical velocity has been reached, the $\alpha$ particles all at once cease to produce any ionization, phosphorescence or photographic action. If a substance emits particles with a velocity less than this critical velocity, we should have no method at present available for detecting them. As to the enormous amount of ionization produced by radium, one can partly grasp it when one considers that an a particle produces about 80,000 ions and one grm. of radium emits about $6\ 10^{10}$ a particles per second. The $\gamma$ rays also differ in penetrating power. The $\gamma$ rays from radium and thorium are very much stronger than those from uranium and actinium.

 

The Relationship between Various Elements

By actual experiment in the laboratory it is possible to watch the gradual formation of helium and actinium. The rate of formation of helium from radium is known roughly, so that if in any rock the helium formed from radium has not been allowed to escape, a quite accurate estimation of the age of the rock can be made. Radium has also been found by Boltwood and Rutherford to grow in actinium solutions. But by investigating elements which appear together in the rocks it is possible to learn much more. In fact it was from the occurrence of helium in radioactive minerals that the brilliant prediction of the production of helium was made. Boltwood, Strutt and McCoy have shown that the amount of radium present in radioactive minerals always bears a constant ratio to the amount of uranium present. For every gram of uranium there is present $3.8\ 10^{-7}$ grms. of radium. From this coexistence in a constant ratio, one is justified in assuming that radium is a product of uranium. If this is true it is easy to explain the existence of radium in rocks that contain uranium. Otherwise, on account of the short period of disintegration of radium, it would be difficult to account for its distribution through the rocks. It has also been found that minerals of the same age contain uranium and lead in the same ratio, so that it seems quite certain that lead is a disintegrated product resulting from radium. Recent experiments by Rutherford seem to indicate, however, that actinium is not a direct product of uranium, as radium is considered to be. The existence together of various other elements has been used as an argument for their relationship. At present, however, the evidences are very meager and often conflicting. This field of experiment is one that promises very important results, however.

 

The Distribution of Radium

Considerable work has recently been done by the Hon. E. J. Strutt upon the amount of radium contained in various kinds of rocks widely distributed over the earth. As this kind of work will probably have considerable geological importance, a few of Strutt's results will be given. A solution of a definite amount of the rock was stored until the equilibrium amount of radium emanation had accumulated. Now, as we have seen that uranium and radium occur in a constant proportion, it is possible to use a mineral of known uranium content and to compare the amount of radium emanation emitted by this and that emitted by the given rock. Following are some of Strutt's results, using $3.8\ (10)^{-7}$ grms. of radium as accompanying 1 gram of uranium in uranium minerals.

 

Table II.

Radium Content of Igneous Rocks

 

Radium Content of Some Sedimentary Rocks

These two tables give but a few of the analyses of Strutt. They show the very wide distribution of radium both in the igneous and in the sedimentary rocks. The average radium content of the rocks examined by Strutt is high, whereas that of sea salt is quite small. Strutt finds that the radium content necessary to maintain the earth at a constant temperature is about $1.75\ (10)^{-13}$ grms. of radium per cubic centimeter of the earth. This is very much less than the lowest radium content of any of the rocks. For this reason he believes that radium is to be found only in an outer crust of the earth, at least if the earth is becoming cooler. In making these calculations, the effect of thorium and uranium and the possible radioactivity of ordinary materials is not taken into account. If the heating effect of ordinary materials is of the same order of magnitude as is to be expected from the ionization they produce, the earth's temperature gradient would be many times larger than that observed. Strutt believes this to be an argument that ordinary matter possesses no genuine radioactivity of its own. C. B. Thwing claims, however, that he has been able to find a temperature gradient in small cylinders of the various metals and rocks. At present nothing very definite can be said as to the heating effect of the radioactivity of substances upon the temperature of the earth. Having considered the radioactivity of the various rocks, we will now take up the atmosphere.

 

Radioactivity of the Atmosphere

It was found after considerable work had been done on ionization that the free air is very considerably ionized. Now in the table of the transformation products of the radio-elements, it will be noticed that several products have the property of condensing on a highly negatively-charged wire. Elster and Geitel and others tried exposing such a wire in the open air and found that there was an active deposit of radium and thorium formed on the wire. The amount of active deposit was found to depend upon the locality and the weather conditions. If the air had been undisturbed for some time as the air in caves and cellars, it was found that the active deposit formed was much greater. Air sucked through the pores of the ground was found to be very active. From these results, Elster and Geitel concluded that the radium and thorium emanations (which behave like gases) ooze up through the ground and percolating waters and have their origin in the radium and thorium in the soil. The emanation then breaks up into the various products as given in Table I. The emanation in this course gives rise to positively charged carriers, which are driven to a negatively charged wire by the electric field. It is to the emanation and its products that the ionization of the air is attributed. Thorium C and radium C give off $\gamma$ rays, and, as these are very penetrating, they would be the source of a very penetrating radiation, and this latter has been discovered several years ago. The ionization in a closed electroscope is measured, and thick lead screens are then placed around the electroscope, and the ionization is again measured. The ionization in the latter case is found to be very considerably decreased, the penetrating radiation having been largely cut off. Whether all the penetrating radiation can be explained as due to radium C and thorium C will be taken up later.

As the potential of the earth is negative compared with that of the air, the active deposit is dragged down to the surface of the ground and upon the leaves and branches of plants and trees. A hill or mountain top concentrates the earth's field and so receives a greater amount of the active deposit. In this way Elster and Geitel explain the greater ionization on hills and mountains. Experiments show that the active deposit tends to collect on dust particles. These dust particles serve as nuclei for the condensation of raindrops and snowflakes. The deposit resulting from evaporating rain and snow should be very radioactive. This was found to be true by Wilson and Allen. Again, a big rain or snow should carry down most of the active deposit, and as the emanation does not emit $\gamma$ rays, the amount of $\gamma$ radiation from the radioactive matter in the air should be very much decreased. The penetrating radiation, if it consists mainly of $\gamma$ rays, should then become very small. This has been found to be borne out by experiments made by the writer. It must be remembered that the emanation is insoluble in water and as this does not seem to be carried down by water or snow, the products radium C and thorium C would soon be in equilibrium again after the rain or snow.

The effect of the presence of radioactive matter in the atmosphere upon ordinary phenomena is perhaps very great, though at present little is known. It has been found that deep wells and hot springs contain considerable radium. From this Elster and Geitel suggest that the curative effect of thermal springs and the physiological action of the air at high levels may be related to the large amount of radioactive matter present. The presence of radioactive matter, and therefore of ionization, in the air probably plays a very important role in the growth of plants. It has been found that vegetables grown in an atmosphere electrified positively are much above those grown in normal fields both in quantity and in quality. The ionization and nucleation produced by radioactive matter in the air are very essential for the condensation of rain and hail, and serve to explain the enormous accumulation of static electricity during thunderstorms. Simpson and others have measured the activity of the air which has blown over the sea and have found it small. Now if most of the radium and thorium emanations come from the pores of the soil and underground cavities, the results obtained by the above investigators would be expected, for, as will be seen from Table II., the radium content of ocean water is very small. Eve has recently measured the ionization over the ocean and has found it to be the same as the ionization over the land, a rather unexpected result. In this state the matter rests at present. A crucial test would be to expose negatively charged wires far out in the ocean and find whether there was any active deposit and to test for the presence of a penetrating radiation. According to J. Joly, the distinguished geologist, Eve's results can easily be explained. Geologists have for some time made an approximate estimate of the age of the oceans by making determinations of the amount of salt which they contain. By analyzing the waters of rivers flowing into the ocean for the salt which they contain and determining the total annual outflow of all the rivers into the ocean, and supposing these constants to have been practically constant during the past, it is easy to make an estimate of the approximate age of the oceans. Now if radium and uranium always exist in a constant proportion, the present radium content of the ocean would have been supplied by the rivers in a comparatively short length of time. For this and other reasons Joly believes that uranium and radium are not always to be found associated together. Now we know that radium has a short period of decay, so that it must constantly be supplied from somewhere. Joly believes that the source is at least partly outside of the earth. This radium is gradually being brought down to the surface. This would account for the ionization over the ocean and the wide distribution of radium over the earth. Elster and Geitel's theory of the escape of the emanation from the upper layers of the soil would still hold true. If radium exists outside the earth, it would be expected that the upper layers of the earth's atmosphere would be highly ionized by the $\gamma$ rays. This highly ionizing radiation would serve to explain some of the phenomena of atmospheric electricity. According to C. T. R. Wilson, the positive potential of the atmosphere is largely to be attributed to the carrying down of negative charges by raindrops and snowflakes. The upper layers of the atmosphere, being highly ionized and quite good conductors, would conduct the remaining positive charge to places of lower potential and would thus always aid in equalizing the potential of wet and dry regions.

 

Radioactivity of the Metals

After the discovery that several of the elements were radioactive, it was natural to ask if radioactivity was a universal property of all the elements. Madame Curie's work showed that if the ordinary elements are radioactive at all, they must possess this property to but a very slight degree. In order to detect any possible radioactivity, it was necessary to have very sensitive instruments. It was found by Wilson and Geitel that there is a leakage of electricity through a gas in a closed vessel and that this leak could be measured very accurately by means of an electroscope. Now either the ions are produced spontaneously in the gas, by a radiation which is capable of penetrating the sides of the electroscope or by radiations from the walls of the electroscope itself. Rutherford, Cooke and McClennan have shown that some thirty or forty per cent, is due to a very penetrating radiation supposed to be the $\gamma$ rays emitted by the radium and thorium products in the air and ground. By using lead screens around the electroscope, they were able to decrease the rate of leak to a certain limiting value beyond which they were unable to go, no matter how much lead was used. Strutt and others then found that for electroscopes of the same dimensions, the amount of ionization depended on the material forming the walls. For vessels of the same shape and size, lead walls gave the greatest amount of ionization, tin and iron considerably less, aluminium and glass the least of all. Strutt found that different specimens of the same metal gave a different ionization and he therefore concluded that the radioactivity of the metals was probably due to a common impurity.

Patterson then tried using different gases and found that the ionization was proportional to the density. This fact is strong evidence that the ionization is not spontaneous within the gas, but is due to a radiation from the walls of the vessel. Patterson also found for the given vessel which he used (30 cm. in diameter and 20 cm. long) that the current through the gas was independent of the pressure above 300 mm. of mercury and varied directly as the pressure below 80 mm. The ionization was found independent of the temperature up to 450° C. That the ionization was related to the pressure as stated above would indicate that above 300 mm. of mercury all the radiation was absorbed, whereas below 80 mm. it was not all absorbed.

The most complete work on the radiations from the metals and their salts has been done by Campbell. In experiments on the radiations from the metals, Campbell used an aluminium-lined box. Inside this was a wire gauze cage containing a gauze electrode. The cage would allow the admission of radiations, but not of ions. Then by placing two sheets of metal so as to radiate into the cage, one sheet being arranged to slide back and forth, it was possible to measure the ionization produced at different distances of this sliding sheet from the cage. The curve which was plotted from the values of the ionization and the distances gave the values of various constants from which it was possible to determine the values given in a table which is shortly to follow. Before considering this table it is needful to say that the curves indicated (when the external penetrating radiation was cut off): (1) an easily absorbable radiation from the sheets of metal placed aside of the cage; (2) a more penetrating radiation from the same; and (3) the radiation from the gauze cage. When the external penetrating radiation was not screened off, the curves showed in addition an ionization due to (4) the external pentrating radiation; (5) to the penetrating radiation excited by it; and (6) to the easily absorbable radiation also produced. In the table, $a$ is Bragg's constant for the intrinsic absorbable radiations, a constant which corresponds to the range of the $\alpha$ particles of the radioactive elements; $s$ is the number of ions produced per second by the intrinsic absorbable radiation from one square centimeter of the surface of the metal, when totally absorbed in air; $\lambda$ is the coefficient of absorption of the easily absorbable secondary radiation; $s^{'}$ is the number of ions produced per cubic centimeter by the easily absorbable secondary radiation from one square centimeter of the metal under the circumstances of the experiment; $V$ is the number of ions produced in 1 c.c. by the intrinsic penetrating radiation from the whole box and lead screen; $V^{'}$ is the number of ions produced in 1 c.c. by the external radiation and the penetrating radiation excited by it.

 

Table III.

$\lambda$ is the coefficient of absorption of the easily absorbable secondary radiation.

 

By using a strong electrostatic field, an attempt was made to determine whether the ionizing agents for the intrinsic absorbable radiation were charged. These radiations were found certainly not to be of the $\beta$ type and very probably to have a nature similar to that of the $\alpha$ rays. No radium emanation was able to be detected from the lead used.

From the constancy of the value of $s$ for the different specimens of the same metal and on account of the variation in the values of $\alpha$ for the different metals, Campbell rightly concludes that there seems to be no doubt but that the ordinary metals are feebly radioactive. In some cases the experimental and theoretical curves agree so well that it would seem that the radiations are homogeneous.

Campbell has also investigated the radioactivity of the metals and their salts in a similar way. He finds that the emission of radiations is an atomic property and that salts prepared by totally different methods and from materials derived from different sources, produce the same ionizing effect. It is only the metal that produces any measurable ionizing effect. The following are some of the results:

 

The use of numerals after the name of the substance is to indicate that the substance was made by a distinct method of analysis.

It will be noticed that the ionization from potassium and rubidium is very large compared with that from the other metals. It was found that the penetrating power of the potassium and rubidium radiations was also quite large. A given sample of a potassium salt gave the following results:

	Number of Sheets of Foil		Ionization		Decrease
	0		467
	1		361		106
	2		299		62
	3		265		34
	4		240		25

It will be seen that the rays are very heterogeneous and vary in penetration from that of the very penetrating β rays of uranium downward. Sodium, lithium and ammonium salts showed no more activity than zinc. The rays from rubidium were found less penetrating than those from potassium. The activity of uranium is about a thousand times that of potassium. Photographs were also taken by making use of the rays from potassium and rubidium.

Campbell's results are in consonance with the experiments made some time ago by J. J. Thomson. Thomson showed that rubidium and potassium emit negatively charged particles which were deflected by an electrostatic field in the same way as the ordinary corpuscles.

 

Conclusion

In summing up we find that:

1. Some of the elements, as radium and thorium, are intensely radioactive.

2. Radium is very widely distributed through the rocks of the earth, and in radioactive minerals is found to exist in a constant proportion with uranium.

3. Radium and its products are also to be found in the air and play an important rôle in atmospheric phenomena.

4. The ordinary metals are slightly radioactive, emitting radiations that seem very much like the α radiation from the radio-elements.

5. Potassium and rubidium emit radiations similar to the β rays.