The Discovery of Radioactive Decay

Following Roentgens announcement of X-rays, the French physicist Antoine Henri Becquerel (1852-1908) began studying X-rays. Becquerel s experiments led him serendipitously to the discovery of radioactivity in samples of uranium salts in his possession. For this discovery, Becquerel shared the 1903 Nobel Prize in physics with his French colleagues physicist Pierre Curie (1859-1906) and Pierre s wife. Polish-bom chemist Marie Sklodovska Curie (1867-1934), who both had made significant contributions to an early understanding of what is involved when radioactive decay occurs. 

In the early days, their work was very difficult. 

It was especially hampered by the fact that the electron was not even discovered until 1897, the proton shortly thereafter, and the neutron not until 1932. Even the existence of the atomic nnclens was unknown until its discovery in England in 1911 by the New Zealand-born physicist Ernest Rutherford (1871-1937). Scientists also had not yet understood the concepts of atomic number or isotopes. 

Every elementary particle has an antiparticle-a particle that has the same mass as the ordinary particle but the opposite charge. 

Roentgen was not looking for X-rays and Becquerel was not looking for radioactivity. Both discoveries were completely serendipitous. 

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As early as 1907 Bertram Boltwood had used the discovery of radioactive decay laws by Ernest Rutherford and Frederick Soddy to ascribe an age of over two billion years to a uranium mineral. In 1947 Willard Libby at the University of Chicago used the decay of to measure the age of dead organic matter. The cosmogenic radionuclide, becomes part of all living matter through photosynthesis and the consumption of plant matter. 

By this time, the Periodic Table of elements was well developed, although it was considered a function of the atomic mass rather than atomic number. Before the discovery of radioactivity, it had been estabUshed that each natural element had a unique mass thus it was assumed that each element was made up of only one type of atom. Some of the radioactivities found in both the uranium and thorium decays had similar chemical properties, but because these had different half-Hves it was assumed that there were different elements. It became clear, however, that if all the different radioactivities from uranium and thorium were separate elements, there would be too many to fit into the Periodic Table. 

FIGURE 88 Dating methods. Shortly after the discovery of radioactivity, at the beginning of the twentieth century, it was found that the decay of radioactive elements could be used to keep track of time. Many of the dating techniques developed since then are, therefore, based on radioactive decay phenomena, but others, such as the hydration of obsidian, amino acid racemization, and dendrochronology, are based on other physical, chemical, or biological phenomena. 

This section started with the discovery of Soddy and Fajans on radioactive decay around 1910 and the relationship of radioactive decay to the periodic table. At this point in the history, we understand the periodic table and we understand the role of isotopes in the periodic table. We have not yet understood the structure of the modern Table, i.e. first row two elements, second row eight elements, etc. That understanding can be based on Bohr theory of the hydrogen atom originally developed in 1911 and is summarized in Bohr s famous article in Zeitschrift fur Physik (Bohr 1922). 

Marguerite Catherine Perey, an assistant to Marie Curie, is credited with the discovery of francium-223 in 1939. Perey discovered the sequence of radioactive decay of radium to actinium and then to several other unknown radioisotopes, one of which she identified as francium-223. Since half of her sample disappeared every 21 minutes, she did not have enough to continue her work, but a new element was discovered. 

Marie Curie discovered the element polonium, Po, in 1898. She named polonium after Poland, her homeland. Curie won two Nobel Prizes, one in Physics (1903) for sharing in the discovery of radioactivity, and one in Chemistry (1911) for the discovery of radium, which has been used to treat cancer. Radium-226 undergoes alpha decay to yield radon-222. 

Historically, the discovery of radioactivity dates back to 1896 when the French scientist Henri Becquerel believed that the afterglow observed in cathode ray tubes might be associated with phosphorescence, later realizing that this phenomenon was instead due to radiation. At first, this radiation was assumed to be similar to X-rays, but further research by Becquerel and a number of other notable scientists (including Marie Curie and Ernest Rutherford) revealed that the nature of this radiation was more complex. Subsequently, it emerged that there were three principal forms of radioactivity that result from different types of radioactive (nuclear) decay. 

The terms radioactive transmutation and radioactive decay arc synonymous-Gencrally, the term decay is preferred in the English literature. As already mentioned in section 2.1, many radionuclides were found after the discovery of radioactivity in 1896, These radionuclides were named UXi, UX2,.., or mesothorium 1, mesothorium 2,., or actinouranium,.., in order to indicate their genesis. Their atomic and mass numbers were determined later, after the concept of isotopes had been established. 

The fact that atoms were not eternally stable became evident in the discovery of radioactivity by Becquerel, followed by the extensive investigations of nuclear transformations by Marie Curie, Rutherford, Soddy, and others. Atoms thus are not indivisible eternal building blocks, but rather entities having a life history from their birth from simpler atomic nuclei by nuclear fusion to their decay or fusion to form other atoms. Radioactive decay is the conversion in time of some atoms into others and thus makes clear the inadequacy of using the atoms now known as starting points for the construction of chemistry. 

Ironically, nuclear transmutations were taking place virtually under the noses of the alchemists (or nnder their feet), bnt they had neither the methods to detect nor the knowledge to use these happenings. The discovery of the nuclear transmutation process was closely linked to the discovery of radioactivity by Henri Becqnerel in 1896. Nnclear transmutations occnr dnring the spontaneous radioactive decay of naturally occurring thorium and uranium (atomic numbers 90 and 92, respectively) and the radioactive... 

The idea of transmutation of elements in the natural decay chains did not accompany the discovery of radioactivity by Becquerel. However, Marie and Pierre Curie extended the investigations of Becquerel using a variety of... 

Finally, the last member of the noble gases, radon, was discovered by the German chemist Frederick Dom in 1900. A radioactive element and the heaviest elemental gas known, radon s discovery not only completed the Group 8A elements, but also advanced our understanding about the nature of radioactive decay and transmutation of elements. 

The discovery of radioactivity led to the eventual realization that the atom, which took its name from the idea that it was indivisible, could in fact be subdivided into more basic particles the proton, neutron, and electron. Rutherford was the first to try to split the atom, something he achieved by using one of the newly discovered products of radioactive decay, the alpha particle. 

The use of radioactive isotopes has had a profound effect on the practice of medicine. Radioisotopes were first used in medicine in the treatment of cancer. This treatment is based on the fact that rapidly dividing cells, such as those in cancer, are more adversely affected by radiation from radioactive substances than are cells that divide more slowly. Radium-226 and its decay product radon-222 were nsed for cancer therapy a few years after the discovery of radioactivity. Today gamma radiation from cobalt-60 is more commonly used. 

Mendeleev, like Dalton, regarded the atoms of a particular element as alike in respect of their weight, but that assumption was discarded when isotopy was discovered in the early twentieth century. That happened via the discovery of radioactivity by Henri Becquerel at the very end of the nineteenth century. It was not clear at first whether the underlying process was intrinsic to atoms as we have seen Mendeleev himself thought that it arose from a chemical interaction between heavy atoms and the ether. If it were a process of atomic disintegration, however, it made sense to analyse the decay products, new species whose place in the periodic table was unclear. Some of the decay products were found to be inseparable by chemical means from known elements, from which they had different atomic weights and radioactive properties (see [Soddy, 1966, 374-383]). In 1910 Soddy proposed that the new elements should occupy the same place... 

The same chemical separation research was done on thorium ores, leading to the discovery of a completely different set of radioactivities. Although the chemists made fundamental distinctions among the radioactivities based on chemical properties, it was often simpler to distinguish the radiation by the rate at which the radioactivity decayed. For uranium and thorium the level of radioactivity was independent of time. For most of the radioactivities separated from these elements, however, the activity showed an observable decrease with time and it was found that the rate of decrease was characteristic of each radioactive species. Each species had a unique half-life, ie, the time during which the activity was reduced to half of its initial value. 

The discoveries of Becquerel, Curie, and Rutherford and Rutherford s later development of the nuclear model of the atom (Section B) showed that radioactivity is produced by nuclear decay, the partial breakup of a nucleus. The change in the composition of a nucleus is called a nuclear reaction. Recall from Section B that nuclei are composed of protons and neutrons that are collectively called nucleons a specific nucleus with a given atomic number and mass number is called a nuclide. Thus, H, 2H, and lhO are three different nuclides the first two being isotopes of the same element. Nuclei that change their structure spontaneously and emit radiation are called radioactive. Often the result is a different nuclide. 

Shortly thereafter, Boltwood recognized that the Pb content of minerals increases with age and it became clear that Pb was the final product of radioactivity. Boltwood was also responsible for adding another substance to the decay series through his discovery of ionium ( °Th), and therefore for linking the U and Ra decay chains (Boltwood 1907). The discovery of initially known as UrII, followed in 1912. 

The fundamental work to establish the sequence of isotopes in the U and Th decay chains was therefore almost complete by 1913, only 17 years after the first discovery of radioactivity. It would be another 40 years before techniques for the routine measurement of some of these isotopes were developed (as detailed in Edwards et al. 2003) and the U-series isotopes started to see their widespread application to questions in the earth sciences. 

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