A BRIEF ACCOUNT OF
RADIO-ACTIVITY

BY

FRANCIS P. VENABLE, Ph.D., D.Sc., LL.D.

PROFESSOR OF CHEMISTRY, UNIVERSITY OF NORTH CAROLINA
AUTHOR OF
"A SHORT HISTORY OF CHEMISTRY,"
"PERIODIC LAW," ETC.

D. C. HEATH & CO., PUBLISHERS
BOSTON     NEW YORK     CHICAGO

Copyright, 1917,
By D. C. Heath & Co.

IA7

PREFACE

I have gathered the material for this little book because I have found it a necessary filling out of the course for my class in general chemistry. Such a course dealing with the composition and structure of matter is left unfinished and in the air, as it were, unless the marvellous facts and deductions from the study of radio-activity are presented and discussed. The usual page or two given in the present text-books are too condensed in their treatment to afford any intelligent grasp of the subject, so I have put in book form the lectures which I have hitherto felt forced to give.

Perhaps the book may prove useful also to busy men in other branches of science who wish to know something of radio-activity and have scant leisure in which to read the larger treatises.

It is needless to say that there is nothing original in the book unless it be in part the grouping of facts and order of their treatment. I have made free use of the writings of Rutherford, Soddy, and J. J. Thomson, and would here express my debt to them—just a part of that indebtedness which we all feel to these masters. I wish also to acknowledge my obligations to Professor Bertram B. Boltwood for his helpful suggestions in connection with this work.

CONTENTS

A BRIEF ACCOUNT OF
RADIO-ACTIVITY

A BRIEF ACCOUNT OF
RADIO-ACTIVITY

CHAPTER I

In 1895, Röntgen modestly announced his discovery of the X rays. This attracted immediate and intense interest. Among those who undertook to follow up these phenomena was Becquerel, who, because of the apparent connection with phosphorescence, tried the action of a number of phosphorescent substances upon the photographic plate, the most striking characteristic of the X rays being their effect upon such sensitive plates. In these experiments he obtained no results until he tried salts of uranium, recalling previous observations of his as to their phosphorescence. Distinct action was noted. Furthermore, he proved that this had no connection with the phenomenon of phosphorescence, as both uranic and uranous salts were active and the latter show no phosphorescence. Becquerel announced his discoveries in 1896 and this was the beginning of the new science of radio-activity.

The rays given off by uranium and its salts were found to differ from the X rays. They showed no appreciable variation in intensity, no previous exposure of the substance to light was necessary, and neither changes of temperature nor any other physical or chemical agency affected them.

At first uranium and its compounds were the only known source of these new radiations, but many other substances were examined and two years later thorium and its compounds were added to the list. In general the discharging action seemed about the same. Other elements and ordinary substances show a minute activity. Only potassium and rubidium have a greater activity than this, and theirs is only about one-thousandth that of uranium.

In the examination of uranium and thorium compounds it was found that the activity was determined by the uranium and thorium present; it was proportto the amount ofional these elements present and independent of the nature of the other elements composing the compound. The conclusion was, therefore, that the activity was an inherent property of the atoms of uranium and thorium, that is, an atomic property. This was a long step forward and introduced into science the conception of a new property of matter, or at least of certain forms of matter.

In examining a large number of minerals containing uranium and thorium, Mme. Curie made the important observation that many of these were more active than the elements themselves. In measuring the activity she made use of the electrical method which will be described later. In the following table giving her results for uranium minerals the numbers under i give the maximum current in amperes. They serve simply for comparison.

The last three are pure uranium and compounds of that element given for comparison with the first five, which are naturally occurring minerals. The last compound has the same composition as chalcolite and is simply the artificially prepared mineral. It has the activity which would be calculated from the proportion of uranium present, the copper and phosphoric acid contributing no activity.

Since the activity is not dependent upon the composition but upon the amount of uranium present, the activity in all of the minerals should be less than that of uranium. On the contrary, it is several times greater. Natural and artificial chalcolite also show a marked difference in favor of the former. The supposition was a natural one, therefore, that these minerals contained small quantities of an element, or elements, undetected by ordinary analysis and having a much greater activity than uranium. Similar results were obtained in the examination of thorium minerals and thorium salts.

Following up this supposition, M. and Mme. Curie set themselves the task of separating this unknown substance. Starting with pitchblende, a systematic chemical examination was made. This is an exceedingly complex mineral, containing many elements. The processes were laborious and demanded much time and minute care. They need not be described here. It is sufficient to say that along with bismuth a very active substance was separated, to which Mme. Curie gave the name of polonium for Poland, her native land. Its complete isolation is very difficult and sufficient quantities of the pure substance have not been obtained to determine its atomic weight and other properties, but some of the lines of its spectrum have been determined. Chemically it is very closely analogous to bismuth.

In a similar manner a barium precipitate was obtained from pitchblende which contained a highly active substance. The pure chloride of this body and barium can be prepared together and then separated by fractional crystallization. To the new body thus found the name of radium was given. It is similar in chemical properties to barium. Its atomic weight has been determined by several careful investigators and is accepted as 226. Its spectrum has been mapped and its general properties are known. It is a silvery white, oxidizable metal. In one ton of pitchblende about 0.2 gram of radium is present; this is about 5000 times greater than the amount of polonium present. The activity of the products was depended upon as the guide in these separations. The radium found is relatively enormously more active than the pitchblende or uranium.

In the above separations use was made of relationships to bismuth and barium. Similarly, by taking advantage of chemical relationship to the iron group of elements, another body was partially separated by Debierne, to which he gave the name actinium. Boltwood discovered in uranium minerals the presence of a body which he named ionium, and which is so similar to thorium that it cannot be separated from it. It, however, far exceeds thorium in activity.

The lead which is present in uranium and thorium minerals—apparently in fairly definite ratio to the amount of uranium and thorium—is found, on separation and purification, to possess radio-active properties. This activity is due to the presence of a very small proportion of an active constituent called radio-lead, which has chemical properties identical with those of ordinary lead. The bulk of the lead obtained from radio-active minerals differs in atomic weight from ordinary lead and appears also to be different according to whether its source is a thorium or a uranium mineral.

A large number of other radio-active substances have been separated and some of their properties determined, but these were found by different means and will be noted in their proper place. They number in all more than thirty. The sources or parents of these are the original uranium or thorium, and the products form regular series with distinctive properties for each member.

CHAPTER II

When this apparatus is set up a minute current will be observed without the introduction of any radio-active matter. This, as Rutherford says, has been found due mainly to a slight natural radio-activity of the matter composing the plates. If radio-active matter is spread on plate A, which is connected with one pole of a grounded battery, and if plate B is connected with an electrometer which is also connected with the earth, a current is caused which increases rapidly with the difference of potential between the plates, then more slowly until a value is reached that changes only slightly with a larger increase in the voltage.

According to the theory of ionization, the radiation produces ions at a constant rate. The ions carrying a positive charge are attracted to plate B, while those negatively charged are attracted to plate A, thus causing a current. These ions will recombine and neutralize their charges if the opportunity is given. The number, therefore, increases to a point at which the ions produced balance the number recombining.

When an electric field is produced between the plates, the velocity of the ions between the plates is increased in proportion to the strength of the electric field. In a weak field the ions travel so slowly that most of them recombine on the way and consequently the observed current is very small. On increasing the voltage the speed of the ions is increased, fewer recombine, the current increases, and, when the condition for recombination is practically removed, it will have a maximum value. This maximum current is called the saturation current and the value of the potential difference required to give this maximum current is called the saturation P.D. or saturation voltage.

The picture, then, is this. The radiations separate the components of the gas into ions, or carriers of electricity, half of which are charged negatively and half positively. In the electric field those negatively charged seek the positive plate and those positively charged seek the negative plate. If time is given, these ions meet and recombine, their charges are neutralized, and there is no current.

This theory of the ionization of gases has been most interestingly confirmed by direct experiment. For instance, the ions may form nuclei for the condensation of water, and in this way the existence of the separate ions in the gas may be shown and the number present actually counted.

When air saturated with water vapor is allowed to expand suddenly, the water present forms a mist of small globules. There are always small dust particles in air and around these as nuclei the drops are formed. These drops will settle and thus by repeated small expansions all dust nuclei may be removed and no mist or cloud will be formed by further expansions.

If now the radiation from a radio-active body be introduced into the condensation vessel, a new cloud is produced in which the water drops are finer and more numerous according to the intensity of the rays. On passing a strong beam of light through the condensation chamber, the drops can readily be seen. These drops form on the ions produced by the radiation.

If the condensation chamber has two parallel plates for the application of an electric field like that already described, the ions will be carried at once to the electrodes and disappear. The rapidity of this action depends upon the strength of the electric field and experiment shows that the stronger the field the smaller the number of condensation drops formed. If there is no electric field, a cloud can be produced some time after the shutting off of the source of radiation, showing that time is required for the recombination of the ions.

If the drops are counted (there being special methods for this) and the total current carried accurately measured, then the charge carried by each ion may be calculated. This has been determined. The mass of an ion compared with the mass of the molecules of gas in which it was produced can also be approximately estimated. In the study of these ions the view has been held that the charged ion attracted to itself a cluster of molecules which surrounded the charged nucleus and traveled with it. It is roughly estimated that about thirty molecules of the gas cluster around each charged ion.

Utilizing the fact that these ions with their clusters of molecules form nuclei for the condensation of water vapor, C. T. R. Wilson has by instantaneous photography been able to photograph the track of an ionizing ray through air. The number of the ions produced, and hence the number of drops, is so great that the trail is shown as a continuous line. In the copy of this photograph it will be seen that at some distance from its source the straight trail is slightly but abruptly bent. Near the end of its course there is another abrupt and much sharper bend. These bends show where the ionizing ray, in this case an alpha particle, has been deflected by more or less direct collision with an atom. These collisions and the final disappearance of the ray will be discussed later.

Taking up now other means of examining these radiations, it is well to consider their action upon a photographic or sensitive plate. It will be recalled that this was the method by which their existence was originally detected. To illustrate the method, the following account of how one such photograph was taken may be given.

The plate was wrapped in two thicknesses of black paper. The objects were placed upon this and the radio-active ore, separated by a board one inch thick, was placed above. The exposure lasted five days. The action is much less rapid and the result not so clearly defined as in the case of photographs taken by X rays. Of course, the removal of the board and the use of more concentrated preparations of radium would give quicker and better results. The method, however, on account of time consumed and lack of definition is ill adapted to accurate work.

The radiations from radio-active bodies can discharge both positively and negatively electrified bodies by making the air surrounding them a conductor of electricity. To demonstrate this, use is made of an electroscope. If the hinged leaf of such an instrument be electrically charged and a radio-active body be brought into its neighborhood, the electricity will be discharged and the leaf return to its original position. The rapidity of this discharge is used to measure the degree of activity of the body giving off the radiation.

It was found by Crookes that a screen covered with phosphorescent zinc sulphide was brightly lighted up when exposed to the radiations. This is due to the bombardment of the zinc sulphide by a type of ray called the alpha ray. Under a magnifying glass this light is seen to be made up of a number of scintillating points of light and is not continuous, each scintillation being of very short duration. By proper subdivision of the field under the lens, the number of scintillations can be counted with close accuracy.

A simple form of apparatus called the spinthariscope has been devised to show these scintillations. A zinc sulphide screen is fixed in one end of a small tube and a plate carrying a trace of radium is placed very close to it. The scintillations can be observed through an adjustable lens at the other end of the tube. Outer light should be cut off, as in a dark room. The screen then appears to be covered with brilliant flashes of light. Other phosphorescent substances, such as barium platino-cyanide, may be substituted for the zinc sulphide, but they do not answer so well.

By penetrating power is meant the power exhibited by the rays of passing through solids of different thicknesses and gases of various depths. This power varies with different radiations and with the nature of the solid or gas. For instance, a sheet of metallic foil may be used and the effect of aluminum will differ from that of gold and the different rays vary in penetrating power. In the case of gases air will differ from hydrogen, and it is noticed that certain rays disappear after penetrating a short distance, while others can penetrate further before being lost.

If the radiations are subjected to the action of a strong magnetic field, it is found that part of them are much deflected in the magnetic field and describe circular orbits, part are only slightly deflected and in the opposite direction from the first, and the remaining rays are entirely unaffected.

By the use of these methods of investigation it is learned that the radiations consist of three types of rays. These have been named the alpha, beta, and gamma rays, respectively. Some radio-active bodies emit all three types, some two, and some only one. The distinguishing characteristic of these types of rays may be summed up as follows:

The alpha rays have a positive electrical charge and a comparatively low penetrating power. They are slightly deflected in strong magnetic and electric fields. They have a great ionizing power and a velocity about one-fifteenth that of light.

The beta rays are negatively charged and have a greater penetrating power than the alpha rays. They show a strong deflection in magnetic and electric fields, have less ionizing power than the alpha rays, and a velocity of the same order as light.

The gamma rays are very penetrating and are not deflected in the magnetic or electric fields. They have the least ionizing power and a very great velocity.

The penetrating power of each type is complex and varies with the source, so the statements given are but generalizations. The alpha rays are projected particles which lose energy in penetrating matter. As to the power of ionizing gases, if that for the α rays is taken as 10,000, then the β rays would be approximately 100 and the γ rays 1.

The rays are examined and measured in several ways: 1. By their action on the sensitive photographic plates. The use of this method is laborious, consumes time, and for comparative measurements of intensity is uncertain as to effect.

2. By electrical methods, using electroscopes, quadrant electrometers, etc. These are the methods most used.

3. By exposure to magnetic and electric fields, noting extent and direction of deflection.

4. By their relative absorption by solids and gases.

5. By the scintillations on a zinc sulphide screen.

The alpha rays have been identified as similar to the so-called canal rays. These were first observed in the study of the X rays. When an electrical discharge is passed through a vacuum tube with a cathode having holes in it, luminous streams pass through the holes toward the side away from the anode and the general direction of the stream. They travel in straight lines and render certain substances phosphorescent. These rays are slightly deflected by a magnetic field and in an opposite direction from that taken by the cathode rays in their deflection. The rays seem to be positive ions with masses never less than that of the hydrogen atom. Their source is uncertain, but they may be derived from the electrodes.

The beta rays are identical in type with the cathode rays and are negative electrons.

The gamma rays are analogous to the X rays and are of the order of light. They are in general considerably more penetrating than X rays. For example, the gamma rays sent out by 30 milligrams of radium can be detected by an electroscope after passing through 30 centimeters of iron, a much greater thickness than can be penetrated by the ordinary X rays.

CHAPTER III

In 1900 Crookes precipitated a solution of an active uranium salt with ammonium carbonate. The precipitate was dissolved so far as possible in an excess of the reagent, leaving an insoluble residue. This residue was many hundred times more active, weight for weight, than the original salt, and the solution containing the salt was practically inactive. At the end of a year the uranium salt had regained its activity while the residue had become inactive.

Another method of obtaining the same result is to dissolve crystallized uranium nitrate in ether. Two layers of solution are formed, one ether and the other water coming from the water of crystallization. The aqueous layer is active, while the water layer is inactive. Similarly, by adding barium chloride solution to a solution of a salt of uranium and then precipitating the barium as sulphate, the activity is transferred to this precipitate. These experiments give proof of the formation and separation of a radio-active body by ordinary chemical operations.

So, too, in the case of thorium salts a substance can be obtained by means of ammonium hydroxide which is several thousand times more active than an equal weight of the original salt. After standing a month, the separated material has lost its activity and the thorium salt has regained it. Here, again, there is the formation, separation, and loss of a radio-active body.

Now, these are ordinary chemical processes for the separation of distinct chemical individuals. The results, therefore, lead naturally to the conclusions: (1) it would seem that uranium and thorium are themselves inactive and the activity is due to some other substance formed by these elements; (2) this active substance is produced by some transformation in those elements, for on standing the activity is regained. This latter conclusion is startling, for it indicates a change in the atom which, up to the time of this discovery, was deemed unchangeable under the influence of such physical and chemical changes as were known to us.

The search for new radio-active bodies and the study of their characteristics has been systematically and successfully carried on. The bodies obtained in the above experiments were named uranium X and thorium X, respectively. Further, it became clear from the investigation of uranium minerals that radium, polonium, actinium, and ionium originated from uranium. From thorium minerals a body was separated called mesothorium, which was analogous to radium. Both thorium and radium were found to give off a radio-active gas. The first lost half of its activity in less than one minute. The second was more stable and lost half of its activity in about four days. The name radium emanation was given to the latter and it was found chemically and physically to belong to the class of monatomic or noble gases, such as helium, argon, neon, etc., which had been discovered by Ramsay. In some cases the chemical action was determined and these new bodies were found analogous to well-known elements, as radium to barium, polonium to bismuth. The physical properties were investigated and, where possible, spectra were mapped and atomic weights determined.

It is clear, therefore, that these bodies are elemental in character and as such are made up of distinct, similar atoms, just as the commonly recognized elements are believed to be. In this way more than thirty new elements have been added to the list. These new elements are called radio-active elements, but it is an open question whether all atoms do not possess this property in greater or less degree. Certainly, it is possessed in varying degree by four of the old elements widely separated in the Periodic System, namely, uranium, thorium, rubidium, and potassium. The last two, while feebly active themselves, do not form any secondary radio-active substance so far as is known. Only two of the elements, then, can definitely be said to go through these transformations. It is just possible that radio-activity may be found to be a common property of all atoms and of all matter.

It is important to know how these new bodies were discovered and distinguished from one another. Two properties are relied upon. One is the nature of the rays emitted and the other is the duration of the activity. Of course, knowledge of the physical and chemical properties is also of great importance whenever obtainable.

The nature of the radiation is a distinguishing characteristic, though similarity here does not prove identity of substances. Some emit α rays only, some emit β rays, some emit two of the possible rays, as for instance, β and γ, and some emit all three. The rays may also differ in the velocity with which they are emitted by different radio-active substances. Thus, in the case of one substance the α rays may have a slightly greater or less penetrating power than those emitted by some other substance, and this may be true also of the other rays.

The duration of the activity is called the life period. This is absolutely fixed for each body and furnishes the most important mode of differentiating among them. It measures the relative stability and is the time which must elapse before their activity is lost and they, changing into something else, entirely disappear. The measure usually adopted is the half-value period. Two hypotheses are made use of:

1. That there is a constant production of fresh radio-active matter by the radio-active body.

2. That the activity of the matter so formed decreases according to an exponential law with the time from the moment of its formation.

These hypotheses agree with the experimental results. The decrease and rise of activity, for example, of uranium and uranium X, and also of thorium and thorium X, have been measured, plotted, and the equations worked out.

Manifestly, a state of equilibrium will be reached when the rate of loss of activity of the matter already produced is balanced by the activity of the new matter produced. This equilibrium and the knowledge of the rate of decrease in general will have little value if this rate, like chemical changes, is subject to the influence of chemical and physical conditions. The rate of decrease has been found to be unaltered by any known chemical or physical agency. For instance, neither the highest temperatures applicable nor the cold of liquid air have any appreciable effect.

In order to measure the disintegration of a radio-active body in units of time so that the rate may be comparable with that of other radio-active bodies, the relation between the amounts under consideration must be a definite one. For this purpose equal weights of the bodies are not taken, but use is made of the amounts which are in equilibrium with a fixed amount of the parent substance.

One gram of radium has been settled upon as the standard for that series and a unit known as the "curie" has been adopted to express the equilibrium quantity of radium emanation. Thus, a curie of radium emanation (or niton) is the weight (or, as this is a gas, the volume at standard pressure and temperature) of the emanation in equilibrium with one gram of radium. This, by calculation and experiment, is found to be 0.63 cubic millimeter. When this amount has been produced by one gram of radium, the formation and decay will exactly balance one another. This is, therefore, one curie of emanation.

The measurement of the rate of decay is difficult but can be carried out with great accuracy, even down to seconds, in the case of certain short-lived bodies. Errors crept in at first from the failure to completely separate the substances produced in the series, and sometimes because of the simultaneous production of two substances.

As stated, the decay follows an exponential law. The time required for the decay of activity to half-value does not mean, therefore, that there will be total decay in twice that time. Thus the half-value period for uranium X is about 22 days. The period for complete decay is about 160 days. This half-value period corresponds to the half-value recovery period of uranium, which is also 22 days.

These were the earlier figures obtained for uranium X and they illustrate some of the difficulties surrounding such determinations. It was found later that the body examined as uranium X was really a constant mixture and of course the decay and recovery periods were also composite. It required later and very skilful work to separate them into the bodies indicated in the disintegration series.

The half-value period for thorium X is much shorter, namely, a little over four days, and this is also the recovery period for thorium X. The plotted decay and recovery curves will intersect at this point.

The consecutive disintegration series, with the half-value periods, for the uranium and thorium series as given by Soddy are seen in the following tables. They are probably subject to some changes on further and more accurate determination. The nature of the rays emitted is also given.