The discussion between P . Curie and E . Rutherford ( 1900 – 1904 )

A serious discussion took place between P. Curie in Paris and E. Rutherford in Montreal from 1900 until 1904 through the medium of scientific publications. P. and M. Curie had observed that radium produces “induced radioactivity”, whereas, at the same time, Rutherford had observed that thorium emits a kind of “emanation”. P. Curie, performing different experiments with his collaborators, believed that radioactive atoms draw from the environment the energy that they will release. Rutherford, together with F. Soddy, in a series of delicate experiments, discovered that radioactivity is the spontaneous transformation of one radio-element into another. Finally, in March 1904, P. Curie “adopted the point of view of Mr Rutherford”. In October of 1899, several new radioactive elements had been discovered and the analysis of the emitted radiations was progressing. But nobody yet understood the cause of radioactivity: where did this spontaneously and continuously emitted energy come from? In 1900–1904 a remarkable discussion took place between two leading physicists, based on reliable experiments performed on both sides of the Atlantic. This discussion does not appear much (when it appears at all) in Anglo-Saxon textbooks or biographies of E. Rutherford, probably because the route followed by him comes out to be straight and self-consistent. Only M. Malley in her dissertation and in an article analyses this scientific debate, see reference [34]. In France this chapter of early research in radioactivity was generally not, or not much, mentioned in textbooks or biographies: Pierre Curie’s interpretation had been proved wrong. Contrary to P. Curie himself, his wife and the successive biographers were not eager to recall this unfortunate episode. Only A. Hurwic [31] and L. Barbo [32] wrote about this discussion/competition. At the very start however, the two conflicting interpretations put forward seemed to be equally extraordinary. In November of 1899 in Paris, Pierre and Marie Curie reported an astonishing observation [1]: any material, a sheet of aluminium, of zinc, of lead or even a sheet of paper, put in the vicinity of a strong source of radium, exhibited a radioactivity distinct from that of radium. They called this new property “induced radioactivity”. This radioactivity, which was independent of the material, did not disappear when the sheets were carefully washed in water. a e-mail: [email protected] 2 The European Physical Journal H At the same time, Ernest Rutherford, at McGill University in Montreal, with similar laboratory apparatuses, observed a strange property of thorium oxide and thorium salts [2]: these “thorium compounds continuously emit radio-active particles of some kind, which retain their radio-active powers for several minutes”. Rutherford called these particles “emanation”. From his experiments, he concluded that emanation is not due to dust but may be a vapour given off from thorium compounds. He also observed that the positive ions produced by emanation possess the remarkable property of giving rise to a temporary radioactivity in all solid substances on which they fall [3]. This he called “excited radioactivity”. He thought that it is due to the deposition of radioactive particles, because he was able to completely remove it by scouring the surface layer with sand or emery-paper for a long time or by dissolving it in sulphuric or hydrochloric acid. In a footnote added to the proofs of his second paper, Rutherford indicated that he had read the report by P. and M. Curie [1] and that the phenomena of excited and induced radioactivity are, in some respects, similar; however his interpretation was different. It should be noted that Rutherford had only low intensity sources at his disposal − no strong radium source was available to him − and that the publication delays (including mail from Canada to Europe) were longer for him than for the French physicists (e.g. his paper on emanation [2], published in the Philosophical Magazine of January 1900, is dated September 13th 1899). He was a young physicist, not yet recognised. In June of 1900 in Germany, E. Dorn [4], following closely the experimental method of Rutherford [2], observed that radium also emits an “emanation” (in 1923 it would be named radon). A month later, André Debierne observed that actinium also gives rise to an “induced” radioactivity [5]. In 1900, Pierre and Marie Curie divided their efforts. Pierre Curie devoted his time to the properties of radium, and Marie Curie tried to completely isolate pure radium salts. In March of 1901, Pierre Curie, together with Debierne, resumed the work on induced radioactivity [6, 7]. They found that induced radioactivity is stronger when the experiment is performed in a closed vessel. They thought that it is transmitted through the air. They took note of the experiments of Rutherford and Dorn, but they concluded: “The emanation theory of Mr Rutherford explains rather well these different results; but, as it is possible to imagine easily alternative satisfactory explanations, we think that it is premature to adopt any theory. New facts are necessary in order to clarify this question.” In their second March paper [7] they reported on experiments they performed in order to investigate the role of gases at different pressures, including the gases occluded in the radioactive material, on the propagation of induced radioactivity. After this new paper of Curie and Debierne, Rutherford believed that the French physicists had arrived at the same conclusions as himself [8]. The instruments used in Paris and in Montreal were quite similar: the methods of measurement were almost the same. P. Curie made more use of glass-blowing techniques. E. Rutherford employed insulated electrodes mounted in a wooden vessel or a brass tube; for almost two years he joined forces with a talented and clever chemist, Frederick Soddy. P. Curie and Debierne carried out further experiments [9,10] in order to understand induced radioactivity. In July of 1901, they commented [9]: “Here is a theory which allows to coordinate rather well these phenomena of radioactivity: one can assume that each radium atom works as a continuous and constant source of radioactive energy; it is not necessary to specify where this energy comes from. The radioactive energy accumulated by radium in a substance tends to dissipate in two different ways: 1) by radiation . . . 2) by conduction. . . (induced radioactivity)”. At this point they recall the different hypotheses which Marie Curie had put forward in 1899 in order to explain the continuous energy release corresponding to Pierre Radvanyi: Induced or excited radioactivity? 3 The radioactive substance is in a. The plates B, D and E are equally activated. Only the upper (exposed) face of A and the external face of the last plate of C are strongly activated. Fig. 1. Experimental set-up of Pierre Curie and André Debierne for the study of induced radioactivity, reference [6]. the spontaneous emission of radiation by radioactive bodies. These were [11]: “1. Radiation is a phosphorescence of very long duration produced by light. This hypothesis is very unlikely. . . 2. Radiation is an emission of matter, resulting in a loss of weight of the radioactive substances. 3. The energy available from radioactive substances continually decreases. . . 4. Radiation is a genuine secondary emission induced by rays analogous to X rays. These excitatory rays would exist constantly in space. . . 5. . . . Radiation is produced at the expense of heat in the environment, contrary to Carnot’s principle. . . ”. The true explanation, P. Curie argued, should necessarily correspond to one of these hypotheses. In 1900, however, in the conclusion of their report to the International physics conference, P. and M. Curie had written [14]: “The spontaneity of radiation is an enigma, a cause of deep astonishment. What is the origin of the energy of the Becquerel rays? Should one look for it in the radioactive bodies themselves or outside?”. In January of 1902, P. and M. Curie publish a methodological paper [15]. They state that in their investigations they prefer to set forth very general hypotheses and move forward step by step, rather than to make specific, bold and more detailed hypotheses. They recall that, since the beginning of their work, they have admitted that radioactivity is an atomic property, each atom of a radioactive substance acting as a constant source of energy. In a foot-note they add that polonium, the activity of which is found to decay with time, is probably an exception and is perhaps not a genuine element. As for the origin of the energy of radioactivity, they formulate “two very general hypotheses: either 1. Each radioactive atom possesses, in the form of potential energy, the energy which it will release . . . then one can imagine that the radioactive atoms are changing, but this has not been, as yet, experimentally confirmed; or 2. Each radioactive atom is a machine which, at each moment, draws from outside itself the energy which it will release . . . This energy could be drawn. . . from the heat of the environment. It could also be drawn from unknown sources, for instance from radiation unknown to us. It is indeed probable that we know very little about the medium which surrounds us, our knowledge being restricted to the phenomena which can directly or indirectly activate our senses”. 1 P. Curie had written [13] in a letter to Ch. E. Guillaume, on December 30, 1898, that these hypotheses were “all as unlikely as the first one”. 4 The European Physical Journal H For one and a half more years, Pierre Curie would adhere stubbornly to this second general hypothesis. In his mind, such a hypothesis allowing for an unknown source of energy was probably able to provide a general explanatory frame for different kinds of phenomena, including the Brownian movement, which were being investigated at that time. One should also note that, in those days, many scientists − including P. and M. Curie themselves − were interested in experiments in spiritualism; they were looking for a rational, scientific explanation of spiritualistic experiments [16]. In November of 1902, Pierre Curie measured the decrease of induced radioactivity when the original radium source has been removed [17]; in a closed vessel, it follows an exponential law with a half-life of about four days, much longer than when the observation is carried out in free air. He concluded that this decrease does not depend on the nature of the gas filling the vessel or on the material forming its wall, and that, in gases, energy is stored in a special form which disappears exponentially. He was misled by the very minute quantities of emanation. In January of 1903, P. Curie found that the half-life remains constant independent of the temperature of the vessel, between 450◦ and –180 ◦C. But he wrote [18]: “Mr. Rutherford seems to believe in the material nature of emanation, and, in one of his most recent reports, he considers it likely that it is a gas of the same kind as those of the argon group. I think that at present there are not enough reasons to concede the existence of an emanation of matter in its ordinary atomic form.” After criticising the conclusions of Rutherford, P. Curie wrote: “I also consider it unlikely that the effects which accompany the existence of emanation should have their origin in a chemical change.” He added: “The expression of emanation is convenient . . . I shall also use this expression, which for me indicates radioactive energy emitted by radioactive bodies in the special form in which this energy is stored in gases and in vacuum.” Summarising different possible theories of radioactivity, he concluded: “There remains a third hypothesis, which consists of assuming that emanation does not have ordinary matter as its support, and that there are energy condensation centers located between the gas molecules which can be carried along with it”. In February of 1903, together with Jacques Danne, P. Curie found [19] that solid bodies which have been exposed to the emanation of radium in a closed vessel lose their activity rather fast − with a half-life of 28 min. − once they have been taken out of the vessel. They stated: “Thus the radioactive energy disappears much faster when it is in the form in which it appears on an activated solid body than when it is in the form of emanation.” In March, P. Curie and Albert Laborde [20] discovered that one gram of radium releases a quantity of heat of about 100 calories per hour. This measurement would have important applications in geophysics. They concluded: “The continuous release of such a large quantity of heat cannot be explained by an ordinary chemical change. If one looks for the origin of the production of heat in an internal change, this change must be of a deeper nature and must be due to a modification of the radium atom itself . . . The hypothesis of a continuous change of the atom is not the only one which is compatible with the heat release by radium. This heat release can also be explained by assuming that radium is using an external source of energy of unknown nature.” The first of these hypotheses, as we shall see, is the one put forward by Rutherford and Soddy; the second one is the hypothesis which P. Curie has been defending until then. In the meantime Rutherford had been moving forward. His method was different. He was pragmatic, intuitive and realistic. In this very open new field, he searched experimentally for new facts − however minute − and formulated possible explanations; 2 Curie P. is clearly referring to the papers of Rutherford E. and F. Soddy of September and November 1902 [22,23] in which they published their main discoveries; their final conclusions were published in May 1903 [28]. Pierre Radvanyi: Induced or excited radioactivity? 5 further experiments were then performed in order to confirm or dismiss these explanations; and the work went on systematically. The generalisation came later; when finally, together with Soddy, he arrived at a general understanding, he drew wide and far-reaching conclusions about the properties they have discovered. He stated later the “complicated mass of facts was reduced to order by the application of the transformation theory”. Rutherford and H.T. Brooks observed that the emanations from thorium and radium, as well as the corresponding “excited” radioactivities decay at very different rates [21]. In 1901, Rutherford had asked the young chemist, F. Soddy, to join him in a physical and chemical study of the thorium emanation. They started working together in October of that year. To their astonishment, they found [22] that emanation is in fact produced by a non-thorium type of matter possessing distinct chemical properties, which they call Thorium X. The time taken by the chemically separated Th X to lose half its activity is equal to the time taken by the remaining thorium to recover one half of its lost activity! Agreeing completely with the views of M. and Mme Curie that radioactivity is an atomic phenomenon, they concluded that, since “radioactivity is at once an atomic phenomenon and accompanied by chemical changes in which new types of matter are produced, these changes must be occurring within the atom, and the radioactive elements must be undergoing spontaneous transformation . . . Radioactivity may therefore be considered as a manifestation of subatomic chemical change.” In their second paper [23], they reported that “emanation from thorium (and from radium) behaves in all respects like a temporarily radioactive gas, and diffuses rapidly through porous substances, as, for example, thick cardboard”. It also passes without loss through all the reagents employed.” The interpretation of the above experiments must therefore be that the emanation is a chemically inert gas analogous in nature to the members of the argon family . . . Just as a chemical change is proceeding in thorium whereby a non-thorium material is produced, so the latter. . . gives rise to a gaseous product which in the radioactive state constitutes the emanation.” In the January 1903 issue of the Philosophical Magazine [24], stimulated by the discussion, Rutherford became even more precise; in a footnote he stated: “The term excited has been used throughout these investigations rather than induced, which has found favour with many Physicists. I have avoided using the latter term, as to my mind, it conveys the idea that the effect is in some way due to an action across the medium; while the experiments in this paper show conclusively that excited radioactivity is transmitted by means of a convection of positively charged carriers.” Summarising the results he wrote: “Excited radioactivity produced by thorium and radium compounds is due to the deposit of radioactive matter, which is derived from the emanation given out by these bodies . . . The emanations, and the matter which gives rise to excited activity are the result of a succession of chemical changes occurring in radioactive matter. In thorium there is evidence for at least four distinct chemical changes.” At the end of February of 1903 Rutherford read the paper published by P. Curie in January (see above, [18]), and simultaneously a paper published in the same issue of the Comptes Rendus by Henri Becquerel comparing the photographic and electric detection methods, and got angry. He wrote a letter to the Philosophical Magazine, which was published in the April issue [25]; he quoted (in French!) whole paragraphs of both papers and refuted the arguments of the two French physicists. Concerning Pierre Curie, Rutherford recalls the different important results he has obtained, alone and together with Miss Brooks and with F. Soddy; in particular he writes: “M. Curie has, apparently, not observed a recent paper by us . . . in which it is shown that if the emanations of thorium or radium . . . are passed slowly through a spiral tube 6 The European Physical Journal H Uranium Thorium Radium ↓ ↓ ↓ Uranium X Thorium X Radium Emanation ↓ ↓ ↓ ? Thorium Emanation Radium-Excited-Activity I ↓ ↓ Thorium-Excited Activity I ditto II ↓ ↓ ditto II ditto III ↓ ↓ ? ? The three queries represent the three unknown ultimate products Fig. 2. First outline of the radioactive series by E. Rutherford and F. Soddy, reference [28]. immersed in liquid air, the emanations are condensed in the tube, and the issuing gas is completely free from activity. . . . it is very difficult to explain such phenomena except on a material hypothesis . . . the greater proportion of the radiation from the emanation is material in nature . . . In view of these results, which so strongly confirm the theory of the material nature of the emanation, the alternative theory proposed by M.P. Curie . . . appears to me unnecessary.” He adds that such results cannot be explained on the laws of ordinary chemical change, but the difficulty disappears on the view already put forward by F. Soddy and himself that radioactivity is a manifestation of sub-atomic chemical change. In May, Rutherford and Soddy explained how, in their experiments, the emanations could be condensed by passing through liquid air and then again volatilised [26,27]. Through their diffusion properties the emanations exhibit the behaviour of inert gases of fairly high molecular weight. The two scientists then published the summary of their work with their final conclusions [28]. They write: “These various new [radioactive] bodies differ from ordinary matter, therefore, only in one point, namely, that their quantity is far below the limit that can be reached by the ordinary methods of chemical and spectroscopic analysis. . . A body that is radioactive must ipso facto be changing. . . In the naturally occurring minerals containing the radio-elements these changes must have been proceeding steadily over very long periods, and, unless they succeed in escaping, the ultimate products should have accumulated in sufficient quantity to be detected, and therefore should appear in nature as the invariable companions of the radio-elements.” They explain: “There is every reason to suppose, not merely that the expulsion of a charged particle accompanies the change, but that this expulsion is the change. . . . The proportional amount of radioactive matter that changes in unit time is a constant. . . [This constant] may therefore be suitably called the “radioactive constant”. . . The energy liberated in radioactive processes does not disobey the law of conservation of energy. . .Mme Curie’s original position, that radioactivity is a specific property of the element, must be considered to be beyond question. . . In radioactive change the chemical atom must suffer disintegration”. Atoms of elements different from the original elements are produced. Rutherford and Soddy still maintained a distinction between the three longlived radio-elements (uranium, thorium and radium) and the numerous new unstable short-lived atoms which result in succession from their disintegration; the question whether polonium is a new radio-element is still left open. The two scientists calculated the energy radiated: “The energy of radioactive change must therefore be at least twenty-thousand times, and may be a million times, as great as the energy of Pierre Radvanyi: Induced or excited radioactivity? 7 any molecular change.” They state that in one gram of uranium or thorium “less than a milligram would change in a million years. In the case of radium, however, the same amount must be changing per gram per year. The “life” of the radium cannot be in consequence more than a few thousand years”. They conclude: “The existence of this [atomic] energy accounts for the stability of the chemical elements. . . It must be taken into account in cosmic physics. The maintenance of solar energy, for example, no longer presents any fundamental difficulty”. In June of 1903 [29], Pierre Curie and Jacques Danne report on experiments they performed in order to measure the diffusion of radium emanation in air. Their results are in agreement with those obtained by E. Rutherford and H.T. Brooks. P. Curie and Danne confirm that, under other circumstances as well, emanation is behaving as a gas. They also confirm the experimental results obtained in Montreal, that radium emanation condenses at the temperature of liquid air. Finally, in March of 1904 [30], P. Curie and J. Danne study in detail the radioactivity induced by radium on solid surfaces; they write: “One can theoretically interpretate these results by adopting the point of view of Mr. Rutherford. . . ”. The two French physicists were thus able to explain the decay curves, assuming the appearance of three radioactive substances on the solid surface, decaying one into the other with three distinct half-lifes. Pierre Curie adopted completely the transformation theory of E. Rutherford; but he still had some regrets and afterthoughts [31], p. 180, as it appears also in his Nobel lecture [33] given in 1905, after the Nobel prize in physics he received in 1903 jointly with Marie Curie and Henri Becquerel. The feelings of Marie Curie were stronger. However, in 1923, in her book on Pierre Curie [36], Marie Curie wrote: “In his scientific relations, he (Pierre Curie) had no bitterness and did not let himself be influenced by pride and personal feeling. Every great success gave him pleasure even in a domain where he expected to have priority”. The transformation theory opened the study of the radioactive series and led to numerous applications. If Pierre Curie had not stuck so long to his own very decided views, he would in all likelihood have participated more, with his outstanding skill, in the foundation and development of the new theory.