Understanding Nuclear Power, #3


[Figures available at www.wlym.com/~bruce/radioactive.zip]

The discovery of radioactivity and its properties in the period from 1896-1903 created a crisis in physical chemistry. The phenomena seemed to challenge several fundamental axioms of science. These were (1) Carnot’s principle describing the relationship of heat and work, and (2) the principle which had guided all chemical investigations since Lavoisier that no new element was created or destroyed in a chemical transformation–a principle sometimes known as the indestructibility of matter. In the usual textbook approach, these paradoxes are passed over quickly, and the problems “solved” by the modern theory of radioactive decay and nuclear transformation. It is much more fun to look at the real papers from the period, to puzzle over the mystery, and work through the process of hypothesis formation and experiment by which the paradoxes are resolved. That is the only way to get any real understanding of what nuclear science is about. Here we will try to summarize some of the basic material which is to be mastered.

In the French scientific journal {Comptes Rendus} of December 1898, a note co-authored by Pierre and Marie Curie, and G. Bemont describes the properties of a new and strongly radioactive substance extracted from the ore of pitchblende. The new substance possessed many analogous properties to barium, and the team had made considerable effort to be sure it was not some unique form of the element barium. They called this new substance {radium.} In an earlier note the same year, this team of collaborators had described another radioactive substance separated from the same ore, this one sharing similar properties with the metal bismuth. They called it {polonium.} [fn 1] “Radioactivity” had been discovered just two years earlier by Henri Becquerel. The curious emissions from uranium ore which he discovered, while looking for something else, were first called Becquerel rays. Marie Curie first used the term “radioactivity” in 1898 when she discovered that minerals containing the element thorium also showed these properties. Becquerel had been studying phosphorescence, a property of certain materials which glow in the dark after exposure to light. He had been curious if the phenomena of phosphorescence might in some way be related to the peculiar x-rays which had just been discovered in 1895. As these curious things come up again in our story we will pause here to briefly explain them. X-rays were first discovered in a simple apparatus called a Geissler or cathode ray tube. A tube of glass is formed with a metal electrode inserted into each end. The air in the tube is pumped out by a vaccum pump, until only a small amount remains inside; or, other gases are introduced in very small amounts. When a voltage is applied across the electrodes, the interior of the tube begins to glow, its color dependendent on the gas contained. The neon lights in signs are a familiar example of such a device. The behavior of gases in apparatus such as these had been under study since the 1840s by Auguste de la Rive, a collaborator of Ampere. Studies of the tubes were made in Germany in the 1850s, and they received the name Geissler tubes after the Bonn instrument maker Johann Geissler. The alternate name of cathode ray tube, came about after Eugen Goldstein discovered in 1876 that a faint ray could be seen propagating from the negative electrode (cathode) to the positive anode. With a high voltage, it was noticed that the glass of the tube also develops a glow. Experimenting with such devices in 1895, Wilhelm Conrad Roentgen observed something really unusual. A faint green light which developed at the wall of his tube was passing through nearby materials, including paper, a book, and some wood. As he tried putting other materials in front of the tube, he saw the bones of his hand projected on the wall! He described the phenomenon in a paper in 1896 calling them “Radiation X,” or X-rays. They are also known as Roentgen-rays.

– Radioactivity – Reports of this exciting discovery spread quickly, and Becquerel wondered if the phenomenon of phosphorescence he was investigating might be related to this radiation X. One of his experiments had been to place each of the mineral samples which showed phosphorescence over a photographic plate wrapped in black paper and left in the dark. All results were negative, until he tried minerals containing the element uranium. (The element uranium had been discovered by Martin Klaproth in 1789 in ores containing the mineral pitchblende; at the time it was primarily used as an additive in the glassmaking process for giving color to glass.) Becquerel’s uranium samples caused the photographic plate to darken. The darkening occurred even if the uranium had not been previously exposed to light, so it was clear the phenomenon was not due to phosphorescence. The radiation was passing through black paper and exposing the photographic plate. Perhaps it was the radiation X?

Pierre and Marie Curie soon began experiments with samples of uranium ore, most of them obtained from mines in Bohemia, then part of Austria. While still supposing that the effect might be due to the radiation X, their work led to the discovery of a very important anomaly. The work began with the creation of a device for measuring the activity of the sample more accurately than could be done with a photographic plate. It had been found that these substances had the property of making the air around them conductive. To measure how much, the sample was ground into a powder and placed on the lower of two parallel metal plates (B). (See Figure 1). This plate was attached to a set of batteries producing a potential usually around 50 or 100 volts. The upper plate (A) was attached through a switch to ground. A radioactive substance would cause the air to become conductive allowing a current to flow through plate A to ground, when the switch was closed. When the switch was opened, the upper plate developed a charge whose value could be determined by the electrometer (E in the figure). The quantity of charge produced was considered a measure of the radioactivity of the substance. A device developed by Pierre Curie from his studies of the piezoelectric properties of crystals, the quartz piezoelectric balance, greatly improved the accuracy of the electrometer. (See Denise Ham article in {21st Century,} Winter 2002.)

Being accomplished chemists, the Curies tried experiments to remove the uranium from the pitchblende ore. By subjecting samples of the ore to acid, they could cause much of the uranium to precipitate out as a salt. When these samples of ore with much of the uranium removed were placed in the measuring device a remarkable thing happened. They showed more radioactivity than the ore samples containing uranium. The Curies then isolated pure uranium metal from the ore and compared its activity. The ore samples they had from several Austrian mines showed a radioactivity three to four times greater than the pure uranium. They became convinced that a new element, many times more active than uranium, must be present in the ore. They began a process of chemical separatiom. Aided by theri precision device for measuring radioactivity, they were able to separate out the portions of the ore which showed greater radioactivity. By June 1898, they had separated a substance with 300 times the radioactivity of uranium. They supposed they had found a new element which they named {polonium,} after Marie Sklodowska Curie’s embattled Poland. There was still some doubt as to whether it was an element. It had not been isolated yet, but always appeared with the already known element bismuth. By December of 1898, the Curies had separated another product from the Bohemian ores which showed strong radioactive properties. This one appeared in combination with the known element barium, and behaved chemically much like barium. Again it had not been isolated in a pure form, and there was uncertainty as to whether it was a distinct element. Spectral analysis showed mostly the spectral lines characteristic of barium, but their friend, the skilled spectroscopist Demarcay, had detected a very faint indication of another line not seen before. [fn. 2] On the basis of the chemical and spectral evidence and the power of its radioactivity, the Curies supposed it to be a new element, which fit in the empty space in the second column (Group II) of Mendeleyev’s periodic table, below barium. They named it {radium.}

The Curies now dedicated themselves to obtaining pure samples of these new elements. It took four years of dedicated labor, working heroically under extremely difficult conditions to isolate the first sample of pure radium. Polonium proved more difficult. While they were engaged in this effort, research was under way in other locations, sparked by the earlier papers of Becquerel and the Curies announcement of two new radioactive elements.

One of the most important lines of development led to the discovery that there was more than one type of radiation coming from the radioactive substances. Becquerel had already reported from his early experiments with uranium that he suspected this to be the case. In 1898 Ernest Rutherford, a young New Zealander working at the Cavendish Laboratory in England, used an apparatus based on the Curie’s radiation detector to examine the radiation from uranium in a slightly different way. He placed powdered uranium compounds on the lower metallic plate of the Curie apparatus described above, and covered it with layers of aluminum or other metal foils. It was found that most of the radiation as measured by the charge collected on the upper plate was stopped by a single thin layer of foil. But some of it got through and was only stopped after a considerable number of layers had been added. The conclusion, already suggested by earlier work of Becquerel, was that there were at least two different types of radiation, to which Rutherford gave the name {alpha rays} for the less penetrating, and {beta rays} for those which were stoped only by more layers of foil.

In 1899, three different groups of experimenters (Becquerel in France, Stefan Meyer and E. von Sweidler, and Friedrich Giesel in Germany) found that the radioactive radiations could be deflected by a magnetic field. A sample of the substance was placed in a lead container with a narrow mouth, so that radiation could only escape in one direction. The container was placed between the poles of a powerful electromagnet, and it was found that the emerging radiation was curving in the same direction as had been observed with the cathode rays mentioned above (Figure 3). It had been recently demonstrated that these cathode rays were electrical particles of negative charge, to which G. Johnstone Stoney had given the name {electron.} Thus, it was supposed that radioactive susbtances were probably giving off electrons.

More careful experiments by Pierre and Marie Curie in 1900, showed that only a part of the radiation was deflected by the magnet. Marie Curie then showed that the undeflected part of the radiation had a lesser penetrating power. It was thus likely that the rays which behaved like electrons were what Rutherford had named beta radiation, and the other part the so-called alpha radiation. It was to take a few more years before these were identified. Under a stronger magnetic field, these more massive alpha particles could be deflected by a smaller amount in the opposite direction of the beta rays, indicating that they were more massive and positively charged.

A laboratory anecdote recounted by Marie Curie in her doctoral thesis provides a striking illustration of the identity of the radiation from radium with electricity. In preparation for opening a sealed glass vial containing a solution of radium salt, Pierre scored a circle around the glass vial with a glass cutter. He immediately recieved a considerable shock. The sharp edge made by the glass cutter had permitted the sudden discharging of the electrical charge accumulated on the container, according to a simple principle which readers of Benjamin Franklin’s writings on the lightning rod will recognize. [fn. 3]

– Induced Radioactivity and Transmutation – One other paradoxical phenomena first observed by the Curies is important to the next step in the understanding of radioactivity. In their work with radium, the Curies had noted that every substance which remains for a time in the vicinity of a radium salt (usually radium chloride) became radioactive. The radioactivity disappeared some time after the substance was removed from the presence of the radium. They called this new phenomenon {induced radioactivity.} Careful studies of the rate of decay of the radioactivity showed that it declined according to an asymptotic law. The effect was independent of the substance put in the vicinity of the radium; glass, paper and metals all acquired the same degree of induced radioactivity. The induced radioactivity was greater in closed spaces, and could even be communicated to a substance through narrow capillary tubes. The air or other gas surrounding the radium was found to be radioactive, and if captured and isolated it would remain active for some time.

Many things suggested that the induced radioactivity might be due to a new gaseous element. But the Curies carried out spectral analysis of the gas found around radium, and found no evidence of the presence of a new element. A peculiar experiment carried out in 1900 by a very peculiar English scientist, Sir William Crookes, set the stage for the next big step in the understanding of radioactivity. Crookes added ammounium carbonate to a solution of uranium nitrate in water, causing a precipitate to form and to redissolve leaving a small quantity of a residue which resembled a tuft of wool. He found the residue to be very radioactive, as determined by its effect on a photographic plate, while the remaining solution was virtually inactive. Crookes concluded that this new substance, which he gave the name uranium X, was the radioactive component of uranium, and that Becquerel and the Curies were mistaken in supposing that radiactivity was an inherent property of the element uranium.

Becquerel tried a similar experiment, precipitating barium sulfate from a solution of uranium. He found that the barium sulfate precipitate was radioactive, while the solution, which still contained all of the uranium was not. However, he could not accept Crookes’ conclusion, arguing that “the fact that the radioactivity of a given salt of uranium obtained commercially, is the same, irrespective of the source of the metal, or of the treatment it has previously undergone, makes the hypothesis not very probable. Since the radioactivty can be decreased it must be concluded that in time the salts of uranium recover their activity.” [fn 4]

To prove his supposition that the uranium would recover its activity, Becquerel set aside some of the inactive uranium solution and its radioactive barium sulfate precipitate for a period of 18 months. Late in 1901, he found that the uranium had completely regained its activity, whereas the barium sulfate precipitate had become completely inactive. Becquerel wrote: “The loss of activity … shows that the barium has not removed the essentially active and permanent part of the uranium. This fact constitutes, then, a strong presumption in favor of the existence of an activity peculiar to uranium, although it is not proved that the metal be not intimately united with another very active product.” [fn 5]

Relocated to McGill University in Montreal, Ernest Rutherford, working with the young Oxford chemist Frederick Soddy, took the next crucial step in resolving the paradox. Instead of uranium X, they created a radioactive residue from a precipitate of thorium which they called thorium X. Like Crookes’ uranium X, the residue showed all the radioacitivity, whereas the thorium which remained in solution appeared inactive. But the activity of the substances was such that after only a few days they observed what Becquerel had seen after 18 months. The thorium X lost some of its radioactivity, while the thorium from which it had been obtained, which was kept a considerable distance away, regained some its activity. A quantititave study of the rate of decay and recovery of the activity by the two substances showed that the rates of decay and recovery were the same, about one month. The famous chart depicting their relative activity is pictured in Figure 4. Rutherford and Soddy repeated the observations using uranium X, and found the same effect occurring over a longer time span, about six months. These observations were considered together with the anomalous phenomenon of induced radioactivity discovered by the Curies. Rutherford had carried out his own investigations and concluded in 1900 that the induced radioactivity was due to a radioactive gas, which he called an emanation. The work with thorium X showed evidence of an emanation, which we know today as the radioactive gas radon.

Rutherford and Soddy now drew a radical conclusion from these results. They posited that the atoms of the radioactive elements were undergoing a spontaneous disintegration. By the emission of an alpha or beta particle they were changing to form a new element, and they posited that this process continues in a series, at a different rate for each step. They summarized the viewpoint in the introduction to their first paper on the subject, in 1902:

“Radioactivity is shown to be accompanied by chemical changes in which new types of matter are being continuously produced. These reaction products are at first radioactive, the activity dinminishing regularly from the moment of formation. Their continuous production maintains the radioactivity of the matter producing them at a definite equilibrium-value. The conclusion is drawn that these chemical changes must be sub-atomic in character.” [fn 6]

As later developments were to show, Rutherford and Soddy were fully correct in their general statements, even if some of the details required further elaboration. It could be argued, as the Curies and Becquerel did, that there was not sufficient evidence to support the hypothesis with certainty when put forward in 1902. I am not sure at what point they became fully convinced. In 1903, when the Curies and Henri Becquerel gave their Nobel prize acceptance speeches, they were still cautious about the Rutherford-Soddy hypothesis. One reason for the caution was that chemistry since the time of Lavoisier had relied on the assumption of the stability of the elements. Transmutation was associated with the unscientific practices of alchemy. An assumption underlying all of Lavoisier’s experiments was that in the course of a chemical reaction, the weight and elemental identity of the products would not change. Mendeleyev underlined this point in the preface to the Seventh Russian edition of his textbook Principles of Chemistry, written in St. Petersburg in November 1902. By the dating, one suspects that Mendeleyev may have been adding his voice to the skepticism concerning the Rutherford-Soddy hypothesis. [fn 7]

Today it is well understood that the radioactive elements uranium, thorium, and plutonium pass through a decay series by which they are transformed successively down the periodic table until arriving at a stable form of lead (atomic number 82). There are four known decay series, that of uranium-238, uranium-235, throrium, and plutonium. Without any interference by man, all of the elements above lead are continuously undergoing such transmutation in the Earth. Elements such as radium, polonium and radon are steps on this path, appearing temporarily and then decaying to pass over on to other elements.

In 1903 Soddy with Willam Ramsay established the identity of the alpha particle with helium. Later the alpha particle was understood to be the ionized (positively charged) nucleus of helium with its two electrons stripped off. As we understand it today, when an element emits an alpha particle it is transformed two steps down the periodic table. But before this could be fully grasped, two important new concepts had to emerge: the notion of atomic number, which describes the number of positive charges or protons in the nucleus, and the existence of isotopes–nuclei of the same charge but different atomic weights. These conceptions, along with the picture of the atom as consisting of a compact, positively charged nucleus surrounded by distant electrons, emerged in the period about 1909-1913. With the addition of one more conception, the neutron, which was first proposed in the early 1920s by Robert J. Moon’s teacher, William Draper Harkins, and experimentally established in 1932 by Chadwick, it became posible to explain the radioactive decay series with precision. So, for example, when the abundant isotope of uranium, U-238, emits an alpha particle, it transmutes two atomic numbers down to become 90-thorium-234. Now, thorium-234 is a beta emitter. We view the beta emission as resulting from the decay of a neutron in the nucleus. Harkins first conceived the neutron as an electron condensed on a proton. (When it was detected experimentally, the neutron was found to be a neutral particle with a mass almost exactly equal to the sum of the masses of the electron and proton.) When it decays, the neutron throws off the very light electron and leaves the more massive proton behind, increasing the charge of the nucleus by plus one. Thus beta decay causes the atomic number to increase by one, without increasing the atomic weight. 90-thorium-234 becomes 91-protactinium-234. This is also a beta emitter which thus decays to 92-uranium-234. (Notice that we have gone two steps down and two steps back up, but we are at a much lighter isotope of uranium. From here the U-234 emits an alpha particle to become 90-thorium-230. This emits an alpha particle to become 88-radium-226, which emits an alpha particle to become 86-radon-222 (see Figures 5a,b). To add to the fun, each of these decay products has its own rate of decay which is measured as a half-life, the time it takes for one half the mass of the substance to disappear. For some substances in the decay chain, this is quite fast–3.82 days for radon-222, for example, and 0.00016 seconds for polonium-214. Others give off their radiation at a much slower rate–uranium-238, for example, takes 4.5 billion years to lose half its mass. When Becquerel, the Curies, and the other early experimenters were detecting the radioactivity of uranium, for example, most of the emissions they detected were not from the uranium, but rather from the decay products mixed in with the uranium. Crookes’s creation of uranium-X was thus actually the chemical separation of the decay product, thorium-234, from the uranium. As the half-life of thorium-234 is just 24.1 days, it was emitting radiation millions of times faster than the uranium. Actually the uranium itself was a mixture of the slow decaying U-238 (4.5 billion years), U-235 (half life = 713 million years), and the decay product, U-234 (half life = 248,000 years). This is why the uranium-X sample at first showed such a high activity, while the remaining uranium seemed inactive. Over time, the uranium-X lost its activity by decay, while the mixture of uranium isotopes slowly built back up their decay products, thus increasing the measurable activity of that portion. It was not the uranium emission that was increasing, but the emission from its faster decaying products. The radon gas which was also a part of the decay chain was what Rutherford had called the {emanation.} Part of the difficulty of detecting it was its short half-life. Rutherford’s thorium-X, was what is now known as radium-224. It decays with a half-life of 3.64 days, by alpha particle emission, to Radon-220, the emanation.

By extrapolating the rate of decay of natural uranium, we can determine that about 4.5 billion years ago there was twice the amount of uranium-238 in the Earth as today. Half of it has undergone a transmutation in that time span, which is thought to be about equal to the age of the Earth. Radium, polonium, radon gas, and the other elements above lead on the periodic table, are all temporary appearances on their way to becoming something else. It is not out of the question that all the 92 elements are undergoing natural transmutation, and that those we call stable are simply decaying on a time scale longer than we have been able to observe. In any case, by artificial means, such as collision with a charged particle from an accelerator, and with enough expenditure of energy, we can today transmute virtually any element into any other. The alchemists’ dream of transmuting base metals into gold is thus acheivable, and has been demonstrated in the laboratory. This however can be only be accomplished in very small amounts, and at a high cost, so that even with Weimar rates of hyperinflation, laboratory transmutatiion is not presently a viable means of producing the metals we need.

So we see, that even this non-living domain within the biosphere is not quite dead either. It is undergoing constant change of a very radical sort. Even the stable elements, whether or not they ever change their identity, are in a state of constant and very rapid internal motion and, as I believe, of continuous and very rapid re-creation on a nonlinear time scale.

– Notes –

1. {Comptes Rendus,} vol. 127, pp. 1215-1217 (1898) http://web.lemoyne.edu~GIUNTA/curiesra.html The earlier discovery of polonium is described in {Comptes Rendus,} vol. 127, pp. 175-178, http://web.lemoyne.edu~GIUNTA/curiespo.html

2. We shall have more to do with spectroscopy later. Upon heating, each chemical element shows a characteristic color. Most people have seen the green color produced in a flame by a copper-bottomed pot. If the light produced when the element is heated be passed through a prism, it is dispersed into a band of color, just as sunlight passing through a prism forms a rainbow. Within the colorful band, known as a spectrum, certain sharp and diffuse lines appear. Bunsen and Kirchoff began work in 1858 which established a means for identifying each element by its flame spectrum (Figure 2).

3. We mention in passing one other anomaly associated with the discovery of radium: its production of light and heat with no apparent source for the energy. We will have more to say on this in coming installments. In the 1898 paper cited above, Curie, Curie and Bemont noted:

“The rays emitted by the compounds of polonium and radium make barium platinocyande flouorescent. Their action from this point of view is analogous to that of Roentgen rays [x-rays], but considerably weaker. To make the experiment, one places on the active substance a very thin leaf of aluminum, upon which a thin film of barium platinocyanide is spread; in the dark the platinocyanide appears weakly luminous in front of the active substance.”

This property of the radioactive substances of producing light (and, it was later noted, considerable heat) without any apparent source of energy was quite paradoxical and caused the team to note at the end of the second paper of 1898: “Thus one constructs a source of light, a very weak one to tell the truth, but one that functions without a source of energy. There is a contradiction, or an apparent one at the very least, with Carnot’s principle.”

Later in her 1903 doctoral thesis, Curie noted that samples of radium are also much warmer than the surrounding air. Calorimetric measurements were able to quantify the heat produced.

Sadi Carnot’s principle, derived from his study of steam engines, stated that the work gained by use of steam depended upon the difference in the heat of the steam coming from the boiler, and the heat of the water vapor after it had done its work in expanding against a piston. Work could only be gained by transfer from a warmer to a colder body. This is the beautifully adduced principle of the operation of heat engines, which Rudolf Clausius attempted to make into a universal principle of amorality by arguing that all processes progress to a state of increasing disorder (“entropy strives toward a maximum.”) What was the source of power for the light and heat produced by these radioactive substances? In noting the apparent contradiction with Carnot’s principle, Marie Curie, the probable author of the jointly signed note, had put her finger on a new principle of power. It was to take another several decades, and the work of many teams of investigators to begin to unravel the puzzle. The answer in short, was the existence of a new domain within the microcosm, the atomic nucleus, in which processes of enormously greater raw power than could be observed on the macroscopic or chemical scale took place.

4. cited in Samuel Glasstone, {Sourcebook on Atomic Energy,} (Princeton: Van Nostrand, 1958) p. 121]

5. cited in Glasstone, op cit, p.121

6. Rutherford and Soddy, Philosophical Magazine, 4 (1902), 370-396, and web.lemoyne.edu/~giunta/ruthsod.html

7. I have examined the circumstances surrounding the Rutherford-Soddy paper with some care. The question on my mind was how, given the evident epistemological weakness of the British school, so much of the progress in atomic science during several decades beginning about 1900 could have taken place there. A subsidiary question was how Rutherford, who by the 1920s had become such an obstacle to new ideas in atomic theory, according to the testimony of Dr. Moon and his teacher Harkins, should have taken such a bold step in 1902. I found it useful to think of the question in two aspects, both of which are clarified by examining it in the historical context.

First, at the time of Rutherford’s discovery, the British were carrying out a buildup for world war, and feared the German pre-eminence in science. For a brief window of time, a general unleashing of scientific progress was permitted. Rutherford and Soddy were both outsiders in the British class system, the one a colonial, and the other the son of a shopkeeper, permitted to carry out their work in the outpost of Montreal. Later, by the 1920s and after the great war, Rutherford had become a part of the insider establishment, which was already asserting a kind of non-proliferation doctrine. H.G. Wells’s adoption of Soddy’s work, as in his popularization of an ultimate weapon to control populations by one-world government (“The World Set Free,” 1914), exemplifies this general aspect of the problem. The later achievement of nuclear fission put nuclear science even more tightly under the control of a military-industrial elite of Wellsian predilection well known to us.

Second is the unfortunate fact that the hegemony of British empiricism, dating approximately to the death of Leibniz, has meant that progress in science has been forced to proceed largely through the resolution of experimental paradox, without benefit of the superior method of metaphysics–as Leibniz called it. We know some very few but notable exceptions, among which Riemann stands out. Otherwise, the better scientists have developed a use of the creative method, as if by instinct, drawn from cultural traditions which are not necessarily evident to them. The general demoralization which followed the First World War tended to wipe out much of the epistemological advantage which had remained in some German and French scientific practice from the respective Kepler-Leibniz and Ecole Polytechnique traditions. A figure such as Dr. Moon represented a countercultural trend, in the good sense of the word, embodying in his deepest moral-philosophical outlook the better aspects of the American Leibnizian tradition, even where that might not be explicitly enunciated. Moon’s creative reaction to LaRouche and Kepler in his 1986 formulation of his nuclear space-time hypothesis conclusively demonstrate that point.