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Vol. 33 No. 1
January-February 2011
 
Physics and Radioactivity after the Discovery of Polonium and Radium

by Pierre Radvanyi

Physics and chemistry were quite interwoven in the early history of radioactivity. In fact, the man considered to be the father of nuclear chemistry, Ernest Rutherford, was a physicist by training and title. In 1908, he was awarded the Nobel Prize in Chemistry.

Ernest Rutherford (1871–1937).

The young Rutherford arrived in England from New Zealand in 1895 with a scholarship and began working with Joseph J. Thomson at Cambridge on the ionization of gases. After the discovery of polonium, but before the discovery of radium by the Curies, Rutherford studied the Becquerel rays, the radiation emitted by uranium. He found that this radiation was complex and consisted of “at least two distinct types . . . one which will be termed for convenience the α radiation, and the other . . . which will be termed the ß radiation.” In 1900, at the École Normale in Paris, Paul Villard discovered a third type of radiation that is very penetrating and analogous to X-rays, which will be later termed γ radiation.

At the end of 1898, Rutherford became a professor of physics at McGill University in Montreal, Canada, where he began studying the radioactivity of thorium compounds. He observed, in 1899, a strange phenomenon: the continuous production by thorium of what seemed to be a radioactive vapor or gas which he called “emanation.” This emanation left on all bodies with which it came in contact an “excited radioactivity,” later called the “active deposit.” (Rutherford 1900). In 1900, in Germany, Ernst Dorn observed a similar emanation from radium.

Perplexed by the nature of emanation, Rutherford asks Frédéric Soddy, a young chemist just arrived from Oxford, to work with him on the problem. To them it appears to be an inert gas. At the beginning of 1902, on the basis of new experiments, they reach the conclusion that there exists an intermediate substance, which they call thorium X (called today radium 224), formed continuously in thorium, and giving rise to the emanation (today radon 220). They generalize that radioactivity is thus the spontaneous transmutation of an element into another by the emission of radiation. At first, Pierre Curie does not believe in the “material existence” of emanation. However, when Rutherford and Soddy succeed in liquefying emanation passing through liquid air, Pierre Curie gives in and accepts the interpretation of Rutherford and Soddy. At the beginning of 1903, Pierre Curie and Albert Laborde observe that radium continuously gives out heat: in one hour radium is able to melt more than its own weight of ice.

In their leading paper of 1903 (Rutherford 1903), Rutherford and Soddy explain radioactive change, put forth the exponential law of radioactive decay, and define the radioactive constant. The two young scientists also provide the first tentative sketch of radioactive series; such a series should begin with a very long-lived radio element and end with a stable element. Measuring the kinetic energy of an alpha-particle and estimating the number of alpha-particles emitted, they compare the energy of radioactive change in one gram of radium to the energy liberated in a chemical reaction such as the union of hydrogen and oxygen to form one gram of water. They conclude that “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 any molecular change.” In addition, they state that “The maintenance of solar energy, for example, no longer presents any fundamental difficulty.”

Many of the symbols used in the three natural, or classical, series (i.e., the uranium, thorium, and actinium series) were assigned before the nature of the isotopes was understood and now are obsolete. For example, in the thorium series, thoron (Th) is now called radon–220, and thorium D (ThD) is now called lead–208 (1996, Encyclopedia Britannica, Inc.).

Their findings soon allow scientists to determine the age of rock and mineral samples. Between 1905–1907, the American physicist Bertram B. Boltwood, following Rutherford’s suggestions, makes the first significant measurements of the age of minerals by comparing their lead (ultimate product of the radioactive series) and uranium content: he finds ages on the order of billions of years. Boltwood also discovers ionium (thorium 230), the long-lived parent element of radium.

From this point on, many laboratories worldwide—Paris, Montreal, Manchester, Vienna, Berlin—endeavor to complete the radioactive series (e.g., U 235, the long-lived parent of the actinium series, is not identified until 1929 with the help of mass spectroscopy).

Alpha-Particles and the Discovery of the Nucleus
At that time of Rutherford’s early work on radiation, it was strongly suspected that alpha-particles were swift helium atoms. After becoming a professor of physics in Manchester in 1907, Rutherford spent much time obtaining decisive experimental proof that these particles carry two unit electric charges. To do so, he wished to count the alphas one by one. The scintillation method, developed by W. Crookes, J. Elster, and H. Geitel, allowed just that. However, Rutherford wanted to count them by an “electric” method and constructs, together with his young German co-worker Hans Geiger, the first particle counter in 1908. In order to ascertain the properties of the alpha-particles, he asks Geiger and an English-New Zealand student, E. Marsden, to study their scattering through thin metallic foils. In 1909, the two physicists observe that some alpha-particles are scattered backwards by thin platinum or gold foils (Geiger 1909).

Potential barrier around a uranium nucleus presented to an alpha particle. The central well is due to the average nuclear attraction of all the nucleons and the hill is due to the electric repulsion of the protons. Alpha particles with energy E trapped inside the nuclear well may still escape to become alpha rays, by quantum mechanically tunnelling through the barrier.

It takes Rutherford one and a half years to understand this result. In 1911, he concludes that the atom contains a very small “nucleus” where almost all its mass is concentrated; the nucleus should carry the positive charges he theorizes, whereas it is surrounded by negatively charged electrons (Rutherford 1911). The consequences of this discovery for physics are substantial. A Dutch amateur physicist, Antonius van den Broek, suggests that the Mendeleev serial number corresponds to the charge of the nucleus; so for each of these numbers there exists a distinct element. This is verified experimentally with the help of X-ray spectroscopy by Henry Moseley in 1913. On the basis of the Rutherford atom, using Planck’s quantification rules, the young Danish theoretician Niels Bohr calculates a new model of the atom (Bohr 1913). Radioactivity, he asserts, is a property of the nucleus.

The number of new radioelements, in the limited higher part of the Mendeleev table, become larger and larger, and some appear to be chemically identical (e.g., radium D and lead). To explain this phenomenon, Soddy proposes in 1911 the existence of “isotopes,” radioelements of the same chemical species that have different atomic weights. Such isotopes should then also exist for nonradioactive elements he proposes. The so-called “displacement laws” for α- and ß-decay are formulated in 1913, independently by K. Fajans, G. v. Hevesy, A.S. Russell, and Soddy.

Meanwhile, at the Radium Institute in Vienna, Victor Hess wishes to understand the background always present in radioactivity measurements. In the course of balloon ascents during 1911–1912, he discovers the existence of radiation from outer space, later called “cosmic radiation.” The first observation of a “nuclear reaction” is made by Rutherford, still in Manchester, in 1919, on nitrogen nuclei bombarded by alpha-particles; this reaction gives rise to the emission of protons. This is the beginning of nuclear physics. This same year, Rutherford becomes director of the Cavendish Laboratory at Cambridge.

Further Progress in the Study of Radioactivity
Rutherford and others have shown that the α-rays emitted by radioactive substances are monoenergetic. But what about the ß-rays? Between 1910 and 1912 in Berlin, Adolf von Baeyer, Otto Hahn, and Lise Meitner used a simple magnetic spectrometer followed by photographic plates to find that the beta-spectra consist of discrete lines, which they think are the primary ß-rays. However, in 1914, James Chadwick uses a magnet followed by counters to observe a continuous ß-spectrum under the discrete lines. Chadwick informs Rutherford, who reaches the conclusion that these spectra are actually the primary ß-decay rays.

Following World War I, Charles D. Ellis, who was a prisoner of war with Chadwick, joined Rutherford’s laboratory in Cambridge; he shows that the discrete electron lines are internal conversion electrons of γ-rays, and that these γ-rays correspond to different energy states of the nucleus. Ellis is the first to draw a nuclear level scheme (Rutherford 1930).

Enrico Fermi (1901–1954).

There remains a puzzle: Why do ß-rays form continuous spectra? A heated discussion takes place between Meitner, Chadwick, and Ellis. Finally, Ellis and W.A. Wooster show in 1927, in a careful calorimetry experiment, that the mean energy liberated in the ß-decay of radium E is only about one third of the maximum energy of its ß-spectrum. Physicists are abashed: where is the rest of the available energy going? Niels Bohr is ready to give-up on the idea of energy conservation in individual nuclear events. However, in 1930 in Zurich, Wolfgang Pauli comes up with an unexpected explanation: in ß-decay two particles are emitted and not just one. The electron is emitted together with a yet unknown particle, which is electrically neutral and a negligibly small mass. This new particle will be called a “neutrino.” However, the first direct experimental observation of neutrinos will not be made until 1953–1956.

Pauli’s proposal finds general acceptance. On the basis of this hypothesis, at the end of 1933 in Rome, Enrico Fermi formulates his theory of ß-decay: electrons and neutrinos (antineutrino) are not present inside the nucleus; they are emitted at the instant of their creation (Fermi 1934). A new type of interaction is postulated that will later be called “weak interaction.”

In 1928, a Russian-born young theoretician, George Gamow, travelled from Copenhagen to Cambridge to give a talk on his new results. With the newly developed quantum mechanics, he is able to explain and to calculate α-decay on the basis of a “tunnel effect” through the potential barrier surrounding the nucleus (Gamow 1928). This potential barrier arises from the opposed effects of the electromagnetic interaction and the forces providing the cohesion of the nucleus (later called “strong interaction”). Listening to this talk, J.D. Cockcroft, one of Rutherford’s associates, gets the idea that Gamow’s argument could be reversed: low-energy protons should be able to penetrate a light nucleus and split it. Rutherford agrees; Cockcroft and E.T.S. Walton construct a low-energy proton accelerator and, in 1932, succeed in observing the first artificial disintegrations of lithium 7 nuclei.

In 1932, following an experiment of Frédéric and Irène Joliot-Curie in Paris, James Chadwick at the Cavendish Laboratory discovers the existence in the nucleus of “neutrons,” neutral particles having about the same mass as the proton. The following year in Germany, Werner Heisenberg assumes that nuclei are formed by protons and neutrons put on the same footing; they will later be called “nucleons.”

Artificial Radioactivity
In 1932, in California, Carl David Anderson discovers, with the help of a cloud chamber, the positive electron (or positron) among the cosmic rays; it is the “antiparticle” of the ordinary negative electron.

At the Institut du Radium in Paris, directed by Marie Curie, in January 1934, Frédéric and Irène Joliot-Curie discover “artificial radioactivity” (I. Curie and Joliot 1934). They had observed positrons and neutrons, emitted by an aluminium foil bombarded by a strong source of alpha-particles. They now realize that the number of these positrons diminishes according to the exponential law characteristic of radioactive decay, when the α-source is removed. They had produced radioactive phosphorous 30, an isotope of the stable phosphorous 31, inside the aluminium foil, by the nuclear reaction: Al 27 + α P 30 + n. Radioactive P 30 decays into stable Si 30 by positron emission; this is the first case of ß+ radioactivity. In ß+ radioactivity a proton of the nucleus changes into a neutron, whereas in ß- radioactivity a neutron changes into a proton. Frédéric and Irène Joliot-Curie confirm their conclusions by the chemical separation of the radioactive phosphorous from the aluminium foil. They find two other cases of artificial radioactivity among light elements. This is a remarkable generalization of the natural radioactivity discovered by Becquerel and the Curies in 1896–1898. In a few months, Fermi and his team in Rome, making use of neutrons as projectiles in order to penetrate heavier nuclei, were then able to produce almost 50 new artificial radioelements.

Several important applications followed from this discovery. In 1935 in Copenhagen, George von Hevesy used radioactive isotopes of elements with great interest to biologists to develop his indicator method. In 1949 in Chicago, Willard F. Libby, having observed the continuous production of carbon 14 (the half-life of which is 5570 years) on atmospheric nitrogen by cosmic rays, invented his dating method (used for age determinations in archeology, geology, and geophysics).

In January 1934, Frédéric and Irène Joliot-Curie discovered “artificial radioactivity.”

Then, other types of radioactivity are discovered. Quantum mechanics predicts that an inner electron of an atom (mainly a K electron) has a finite probability to be found inside the nucleus; so radioactivity by electron capture can take place, in possible competition with ß+ decay, if permitted by energy balance. In 1937 in Berkeley, Luis W. Alvarez finds the first case of electron capture. In December 1938 in Berlin, Otto Hahn and Fritz Strassmann discover fission of uranium nuclei bombarded by neutrons. In 1940, the Russian physicists Goeorgy N. Flerov and K.A. Petrjak observe the spontaneous fission of uranium, which takes place by a tunnel effect analogous to what happens in α decay. In 1981 in Darmstadt, Germany, radioactivity by the emission of protons is observed.

In the 1920s, nuclear physics was considered to be part of the field of radioactivity; less than 20 years later, radioactivity was considered to be part of nuclear physics.

Pierre Radvanyi is honorary director of research at CNRS, a nuclear physicist, and a historian of science at Institut de Physique Nucléaire, Orsay, France.

See the References section for works cited in this article.


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