Radio-activity

Radio-activity is a book written in 1904 (a 2nd edition was published in 1905) by Ernest Rutherford. This was the year prior to Albert Eistein's paper on the special theory of relativity would be published. The following quotes are from the 1st edition of Radio-activity, unless otherwise noted. Rutherford's theory of radioactivity was initially opposed by Pierre Curie, who believed, due to conservation of energy concerns, that radioactive substances causing the energy to be produced from the environment, outside of the radioactive material. Marie Curie had initially suspected that the energy source might be the radioactive material itself, possibly due to disintegration in the atom. She was, however, convinced by Pierre, that this was a ridiculous assumption.

Preface

 * I have found the theory that the atoms of the radio-active bodies are undergoing spontaneous disintegration extremely serviceable not only in correlating the known phenomena but also in suggesting new lines of research.
 * p.vii


 * The rapid advance of our knowledge of radio-activity has been dependent on the information already gained by research into the electric properties of gases.
 * p.vii

Ch. I: Radio-active Substances

 * The close of the old and the beginning of the new century have been marked by a very rapid increase of our knowledge of that most important but comparatively little known subject-the connection between electricity and matter.
 * p.1


 * The study of the radio-active substances and of the discharge of electricity through gases has supplied very strong experimental evidence in support of the fundamental ideas of the existing atomic theory. It has also indicated that the atom itself is not the smallest unit of matter but is a complicated structure made up of a number of smaller bodies.
 * p.1


 * A great impetus to the study of this subject was initially given by the experiments of Lenard on the cathode rays and by Rontgen's discovery of the X rays.
 * p.1


 * An examination of the conductivity imparted to a gas by the X rays led to a clear view of the mechanism of the transport of electricity through gases by means of charged ions.
 * p.1


 * This ionization theory of gases has been shown to afford a satisfactory explanation not only of the passage of electricity through flames and vapors, but also of the complicated phenomena observed when a discharge of electricity passes through a vacuum tube.
 * p.1


 * Study of the cathode rays showed that they consisted of a stream of material particles, projected with great velocity, and possessing an apparent mass small compared with that of the hydrogen atom. The connection between the cathode [rays] and Röntgen rays [i.e., X rays] and the nature of the latter were also elucidated. Much of this admirable experimental work on the nature of the electric discharge has been done by Professor J.J. Thomson and his students in the Cavendish Laboratory Cambridge.
 * p.2


 * An examination of natural substance, in order to see if they gave out dark radiations similar to X rays, led to the discovery of the radio active bodies which possess the property of spontaneously emitting radiations, invisible to the eye, but readily detected by their action on photographic plates, and their power of discharging electrified bodies.
 * p.2


 * In order to explain the phenomena of radio-activity, a theory has been put forward which regards the atoms of the radio-active elements as suffering spontaneous disintegration, and giving rise to a series of radio-active substances which differ in chemical properties from the parent elements. The radiations accompany the breaking-up of the atoms, and afford a comparative measure of the rate at which the disintegration takes place. This theory is found to account in a satisfactory way for all the known facts of radio-activity, and welds a mass of disconnected facts into one homogeneous whole.
 * p.2


 * On this view, the continuous emission of energy from the active bodies is derived from the internal energy inherent in the atom, and does not in any way contradict the law of the conservation of energy. At the same time, however, it indicates that an enormous store of latent energy is resident in the radio-atoms themselves. This store of energy has previously not been observed, on account of the impossibility of breaking up into simpler forms the atoms of the elements by the action of the chemical or physical forces at our command.
 * p.2


 * On this theory we are witnessing in the radio-active bodies a veritable transformation of matter. This process of disintegration was investigated, not by direct chemical methods, but by means of the property possessed by the radio-active bodies of giving out specific types of radiation.
 * p.3


 * Except in the case of a very active element like radium, the process of disintegration takes place so slowly, that hundreds if not thousands of years would be required before the amount transformed would come within the range of detection of the balance or the spectroscope. In radium, however, the process of disintegration takes place at such a rate that it should be possible within a limited space of time to obtain definite chemical evidence on this question.
 * p.3


 * The recent discovery that helium can be obtained from radium adds strong confirmation to the theory; for helium was indicated as a probable disintegration product of the radio-active elements before this experimental evidence was forthcoming. If the production of helium by radium is completely substantiated, the further study of radio-active bodies promises to open up new and important fields of chemical inquiry.
 * p.3


 * In this book the experimental facts of radio-activity and the connection between them are interpreted on the disintegration [of atoms] theory. Many of the phenomena observed can be investigated in a quantitative manner... the agreement of any theory with the facts, which it attempts to explain, must ultimately depend upon the results of accurate measurement.
 * p.3


 * The value of any working theory depends upon the number of experimental facts it serves to correlate, and upon its power of suggesting new lines of work. In these respects the disintegration theory, whether or not it may ultimately be proved to be correct, has already been justified by its results.
 * p.3


 * The term "radio-active" is now generally applied to a class of substances, such as uranium, thorium, radium, and their compounds, which possess the property of spontaneously emitting radiations capable of passing through plates of metal and other substances opaque to ordinary light.
 * p.3


 * A strongly radio-active body like radium is able to cause marked phosphorescence and fluorescence on some substances placed near it.
 * p.4


 * The most remarkable property of the radio-active bodies is their power of spontaneously and continuously radiating energy at a constant rate, without, as far as is known, the action upon them of any external exciting cause. The phenomena at first sight appear to be in direct contradiction to the law of conservation of energy, since no obvious change with time occurs in the radiating material. The phenomena appear still more remarkable when it is considered that the radio active bodies must have been steadily radiating energy since the time of their formation in the earth's crust.
 * p.4


 * Immediately after Röntgen's discovery of the production of X rays, several scientists were led to examine if any natural bodies possessed the property of giving out radiations which could penetrate metals and other substances opaque to light. As the production of X rays seemed to be in some way connected with cathode rays, which cause strong fluorescent and phosphorescent effects on various bodies, the substances first examined were those that were phosphorescent when exposed to light. ...there seems to be little doubt that the effects are due to short ultra-violet light waves, capable of passing through certain substances opaque to ordinary light. These effects, while interesting in themselves, are of quite a distinct character from those shown by the radio-active bodies which will now be considered.
 * p.4


 * The first important discovery in the subject of radio-activity was made in February, 1896, by M. Henri Becquerel, who found that a uranium salt [potassium uranyl sulfate], the double sulphate of uranium and potassium, emitted some rays which gave an impression on a photographic plate enveloped in black paper. These rays were also able to pass through thin plates of metals and other substances opaque to light. ...It was at first natural to suppose that the emission of these rays was in some way connected with the power of phosphorescence, but later observations showed that there was no connection whatever between them. ...The amount of action on the photographic plate does not depend on the particular compound of uranium employed, but only on the amount of uranium present in the compound. The non-phosphorescent are equally active with the phosphorescent compounds. The amount of radiation given out is unaltered if the active body is kept continuously in darkness. The rays are given out by solutions, and by crystals which have been deposited from solutions in the dark and never exposed to light. This shows that the radiation cannot be due in any way to the gradual emission of energy stored up in the crystal in consequence of exposure to a source of light.
 * p.5


 * These radiations from uranium are persistent, and as far as observations have yet gone, are unchanged either in intensity or character, with lapse of time. Observations to test the constancy of the radiations for long periods of time have been made by Becquerel. ...No perceptible weakening of the radiation has been observed over a period of four years. Mme Curie has made determinations of the activity of uranium over a space of five years... but found no appreciable variation during that period.
 * p.6


 * Since the uranium is thus continuously radiating energy from itself, without any known source of excitation, the question arises whether any known agent is able to affect the rate of its emission. No alteration was observed when the body was exposed to ultra-violet light or to ultra-red light or to X rays. ..The intensity of the uranium radiation is not affected by a variation of temperature between 200 &#186;C and the temperature of liquid air.
 * p.6


 * The radiations from uranium are analogous, as regards their photographic and electrical actions, to [[w:X-ray|Röntgen rays], but compared with the rays from an ordinary X ray tube, these actions are extremely feeble. While with Röntgen rays a strong impression is produced on a photographic plate in a few minutes or even seconds, several days' exposure to the uranium rays is required to produce a well-marked action, even though the uranium compound, enveloped in black paper, is placed close to the plate. The discharging action, while very easily measurable by suitable methods, is also small compared with that produced by X rays from an ordinary tube.
 * p.7


 * The absence of reflection, refraction, or polarization in the penetrating rays from uranium necessarily follows in the light of our present knowledge of the rays. It is now known that the uranium rays, mainly responsible for the photographic action, are deviable by a magnetic field, and are similar in all respects to cathode rays, i.e. the rays are composed of small particles projected at great velocities. The absence of the ordinary properties of transverse light waves is thus to be expected.
 * p.7


 * The rays from uranium are complex in character, and in addition to the penetrating deviable rays, there is also given off a radiation very readily absorbed by passing through thin layers of metal foil, or by traversing a few centimeters of air. The photographic action due to these rays is very feeble in comparison with that of the penetrating rays, although the discharge of electrified bodies is mainly caused by them. Besides these two types of rays, some rays are emitted which are of an extremely penetrating character and are non-deviable by a magnetic field. These rays are difficult to detect photographically, but can be readily examined by the electric method.
 * p.8


 * The question naturally arose whether the property of spontaneously giving out penetrating radiations was confined to uranium and its compounds, or whether it was exhibited to any appreciable extent by other substances. By the electrical method, with an electrometer of ordinary sensitiveness, any body which possesses an activity of the order of 1/100 of that of uranium can be detected. With an electroscope of special construction, such as has been designed by C.T.R. Wilson for his experiments on the natural ionization of air, a substance of activity 1/10000 and probably 1/100000 of that of uranium can be detected.
 * p.8


 * Mme Curie made a detailed examination by the electrical method of the great majority of known substances, including the very rare elements, to see if they possessed any activity. In cases when it was possible, several compounds of the elements were examined. With the exception of thorium and phosphorus, none of the other substances possessed an activity even of the order of 1/100 of uranium.
 * p.8


 * The ionization of the gas by phosphorus does not, however, seem to be due to a penetrating radiation like that found in the case of uranium, but rather to a chemical action taking place at its surface. The compounds of phosphorus do not show any activity, and in this respect differ from uranium and the other active bodies.
 * p.9


 * Le Bon has also observed that quinine sulphate, if heated and then allowed to cool, possesses for a short time the property of discharging both positively and negatively electrified bodies. It is necessary, however, to draw a sharp line of distinction between phenomena of this kind and those exhibited by the naturally radio-active bodies. While both, under special conditions, possess the property of ionizing the gas, the laws controlling the phenomena are quite distinct in the two cases. ...the activity of the naturally radio active bodies is spontaneous and permanent. It is exhibited by all compounds and is not as far as is yet known altered by change in the chemical or physical conditions.
 * p.9


 * The discharging and photographic action alone cannot be taken as a criterion as to whether a substance is radio-active or not. It is necessary in addition to examine the radiations, and to test whether the actions take place through appreciable thicknesses of all kinds of matter opaque to ordinary light. For example, a body giving out short waves of ultra-violet light can be made to behave in many respects like a radio-active body. As Lenard has shown, short waves of ultra-violet light will ionize the gas in their path, and will be rapidly absorbed in the gas. They will produce strong photographic action, and may pass through some substances opaque to ordinary light. The similarity to a radio-active body is thus fairly complete as regards these properties. On the other hand, the emission of these light waves, unlike that of the radiations from an active body, will depend largely on the molecular state of the compound, or on temperature and other physical conditions.
 * p.9


 * But the great point of distinction lies in the nature of the radiations from the bodies in question. In one case the radiations behave as transverse waves, obeying the usual laws of light waves, while in the case of a naturally active body, they consist for the most part of a continuous flight of material particles projected from the substance with great velocity. Before any substance can be called "radio-active" in the sense in which the term is used to describe the properties of the natural radio-active elements, it is thus necessary to make a close examination of its radiations; for it is inadvisable to extend the use of the term "radio-active" to substances which do not possess the characteristic radiating properties of the radio-active elements which we have described, and the active products which can be obtained from them. Some of the pseudo active-bodies will however be considered later.
 * p.10


 * Thorium compounds gives out a material emanation made up of very small particles themselves radio-active. The emanation behaves like a radio-active gas; it diffuses rapidly through porous substances like paper, and is carried away by a current of air. ...In addition to giving out an emanation, thorium behaves like uranium in emitting three types of radiation, each of which is similar in properties to the corresponding radiation from uranium.
 * p.11


 * It seemed probable that the large activity of some of these minerals, compared with uranium and thorium, was due to the presence of small quantities of some very active substance, which was different from the known bodies thorium and uranium. This supposition was completely verified by the work of M. and Mme Curie, who were able to separate from pitchblende by purely chemical methods two active bodies, one of which in the pure state is over a million times more active than the metal uranium. This important discovery was due entirely to the property of radio-activity possessed by the new bodies. The only guide in their separation was the activity of the products obtained. ...The activity of the specimens thus served as a basis of rough qualitative and quantitative analysis, analogous in some respects to the indication of the spectroscope.
 * p.12


 * The chief difficulty lay in the fact that pitchblende is a very complex mineral and contains in varying quantities nearly all the known metals. ...The analysis of pitchblende by chemical methods, using the procedure sketched above, led to the discovery of two very active bodies, polonium and radium. The name polonium was given to the first substance discovered by Mme Curie in honor of the country of her birth. The name radium was a very happy inspiration of the discoverers, for this substance in the pure state possesses the property of radio-activity to an astonishing degree.
 * p.13


 * Radium is extracted from pitchblende by the same process necessary to separate barium, to which it is very closely allied in chemical properties. ...The chloride of radium is less soluble than that of barium and can be separated from it by the method of fractional crystallization.
 * p.13


 * Both polonium and radium exist in infinitesimal quantities in pitchblende. In order to obtain a few decigrammes of very active radium, it is necessary to use several tons of pitchblende, or the residues obtained from the treatment of uranium minerals. ...M. and Mme Curie were indebted for their first working material to the Austrian government, who generously presented them with a ton of the treated residue of uranium materials, from the State manufactory.
 * p.13


 * Mme Curie has utilized the coloration of the crystals of radiferous barium as a means of controlling the final process of purification. The crystals of salts of radium and barium deposited from acid solutions are indistinguishable. The crystals of radiferous barium are at first colorless, but in the course of a few hours, become yellow, passing to orange and sometimes to a beautiful rose color. The rapidity of this coloration depends on the amount of barium present. Pure radium crystals do not color, or at any rate not as rapidly as those containing barium. The coloration is a maximum for a definite proportion of radium, and this fact can be utilized as a means of testing the amount of barium present. When the crystals are dissolved in water the coloration disappears.
 * p.14


 * Giesel has observed that pure radium bromide gives a beautiful carmine color to the Bunsen flame. If barium is present in any quantity only the green color due to barium is observed and a spectroscopic examination shows only the barium lines. This carmine coloration of the Bunsen flame is a good indication of the purity of the radium.
 * p.15


 * Since the preliminary announcement of the discovery of radium, Giese has devoted a great deal of attention to the separation of radium, polonium, and other active bodies from pitchblende. ...Using the method of fractional crystallization of the bromide instead of the chloride he has been able to prepare considerable quantities of pure radium. By this means the labor of final purification of radium has been much reduced. He states that six or eight crystallizations with the bromide are sufficient to almost completely free the radium from the barium.
 * p.15


 * Mme Curie has made successive determinations of the atomic weight of the new element with specimens of steadily increasing purity. ...In these experiments about 0.l gram of pure radium chloride has been obtained by successive fractionations. The difficulty involved in preparing a quantity of pure radium chloride large enough to test the atomic weight may be gauged from the fact that only a few centigrams of fairly pure radium, or a few decigrams of less concentrated material, are obtained from treatment of about 2 tons of the mineral from which it is derived. ...Runge and Precht have examined the spectrum of radium in a magnetic field, and have shown the existence of series analogous to those observed for calcium, barium, and strontium. These series are connected with the atomic weights of the elements in question, and Runge and Precht have calculated by these means that the atomic weight of radium should be 258--a number considerably greater than the number 225 obtained by Mme Curie by means of chemical analysis. Marshall Watts, on the other hand, using another relation between the lines of the spectrum, deduced the value obtained by Mme Curie. Considering that the number found by Mme Curie agrees with that required by the periodic system, it is advisable in the present state of our knowledge to accept the experimental number rather than the one deduced by Runge and Precht from spectroscopic evidence.
 * p.17


 * There is no doubt that radium is a new element possessing remarkable physical properties. The detection and separation of this substance, existing in such minute proportions in pitchblende, has been due entirely to the characteristic property we are considering, and is the first notable triumph of the study of radio-activity.
 * p.18


 * On account of its enormous activity the radiations from radium are very intense: a screen of zinc sulphide, brought near a few centigrams of radium bromide, is lighted up quite brightly in a dark room, while brilliant fluorescence is produced on a screen of platino-barium cyanide. An electroscope brought near is almost instantly discharged, while a photographic plate is immediately affected. At a distance of one meter, a day's exposure to the radium rays would produce a strong impression. The radiations from radium are analogous to those of uranium, and consist of the three types of rays: easily absorbed, penetrating, and very penetrating. Radium also gives rise to an emanation similar to that of thorium, but with a very much slower rate of decay. The radium emanation retains its activity for several weeks, while that of thorium lasts only a few minutes. The emanation obtained from a few centigrams of radium illuminates a screen of zinc sulphide with great brilliancy. The very penetrating rays of radium are able to light up an X ray screen in a dark room, after passage through several centimeters of lead and several inches of iron.
 * p.18


 *  All the salts of radium are naturally phosphorescent. The phosphorescence of impure radium preparations is in some cases very marked. All the radium salts possess the property of causing rapid colorations of the glass vessel which contains them. For feebly active material the color is usually violet, for more active material a yellowish-brown and finally black.
 * p.19


 * Polonium was the first of the active substances obtained from pitchblende. It has been investigated in detail by its discoverer Mme Curie. ...This active substance... is so closely allied in chemical properties to bismuth that it has so far been found impossible to effect a complete separation. Partial separation of polonium can be made by successive fractionations.
 * p.19


 * The polonium prepared by Mme Curie differs from the other radio-active bodies in several particulars. In the first place the radiations include only very easily absorbable rays. The two penetrating types of radiation given out by uranium, thorium, and radium are absent. In the second place the activity does not remain constant, but diminishes continuously with the time. Mme Curie found that the polonium lost half its original activity in the course of eleven months.
 * p.20


 * The decay of the activity of polonium with time has led to the view that polonium is not a new active substance, but merely active bismuth, i.e. bismuth which in some way had been made active by admixture with radio-active bodies. The activity of any product is not necessarily a proof that a radio-element is present, for it has been shown that many inactive elements become active by association with active matter. The activity of these substances, when removed from the active element, is however only transient, and decays gradually with the time. This activity is not due to the presence of the radio-element itself. For example, barium separated from radium is strongly active, although the spectroscopic examination shows no trace of the radium lines.
 * p.20


 * The discussion of the nature of polonium was renewed by the discovery of Marckwald that a substance similar to polonium, of which the activity did not decay with time, could be separated from pitchblende. ...The radiations from Marckwald's substance are similar to of polonium, for no penetrating rays are present. The radiations are very intense. They have a marked photographic action, and cause many substances, like zinc oxide and the diamond, to phosphoresce brightly. The strong luminosity of the under these rays can be utilized to distinguish the diamond imitations, for glass is only slightly phosphorescent in comparison. ...Marckwald ...states that his preparations have preserved their activity unchanged while the polonium of the Curies undoubtedly loses its activity in the course of a few years.
 * p.21-22


 * Debierne has obtained from pitchblende a very active substance which he named actinium. This active substance is precipitated with the iron group, and appears to be very closely allied in chemical properties to thorium, though it is many thousand times more active. It is very difficult to separate from thorium and the rare earths. ...Debierne has obtained a substance comparable in activity with radium. The separation, which is difficult and laborious, has so far not been carried far enough to bring out any new lines in the spectrum. Actinium gives out easily absorbed and penetrating deviable rays like those of radium, and also a radio-active emanation, which is more allied to the emanation of thorium than to that of radium. The emanation has a distinctive rate of decay; it loses its activity in the course of a few seconds, while the thorium emanation loses half its activity in one minute. The distinctive character of the radiations and emanations, together with the permanence of the activity; make it very probable that actinium will prove to be a new element of very great activity.
 * p.22


 * Giesel also has obtained from pitchblende a radio-active substance which in many respects is similar to the actinium of Debierne. The active substance belongs to the group of cerium earths, and is precipitated with them. The method of preparation of this material is the same as that employed for the separation of the rare earths. This substance is similar in radio-active behavior to thorium, but intensely active in comparison. From the method of separation, thorium itself cannot be present except in minute quantity. ...If a piece of paper is placed in a small closed vessel containing the active material, in a short time the paper itself becomes powerfully active. This is especially the case if it is moistened with water.
 * p.23