In 1896 Henri Bequerel, a French physicist, was studying the fluorescence of uranium compounds. He placed crystals of potassium uranyl sulfate on top of photographic film wrapped in dark paper and exposed the crystals to sunlight. Bequerel interpreted darkening of the film to be the result of penetration of the paper by the fluorescence of the uranium. A second experiment on this uranyl fluorescence was delayed for some days due to cloudy, wintry weather in Paris, leading Bequerel to decide to repeat the experiment, using new film. However, he developed the earlier film, expecting to see little, if any, darkening. To his surprise the film was as dark as if sunlight had been striking the uranium throughout the whole cloudy period. He concluded that uranium was spontaneously emitting high-energy rays that caused the observed darkening of the photographic plate and asked Madame Marie Curie, a research assistant in his laboratory, to join him in further studies of this new phenomenon. Curie named the spontaneous, high-energy radiation "radioactivity."
By 1898 Madame Curie and her husband Pierre, in collaboration with Bequerel, had isolated two new elements from the radioactive decay of uranium in pitchblende ore. Both were more radioactive than uranium itself. They named the first element polonium (Po) after Madame Curie's native land (Poland), and the second was named radium (Ra). Isolation of these two elements required chemical separation of very small amounts of Po and Ra from tons of pitchblende. Radium was found to be over 300,000 times more radioactive than uranium.
The French experiments attracted the attention of Ernest Rutherford, a physicist at the University of Manchester in England. Using an electrical field, Rutherford demonstrated that the radiation emitted from a radioactive sample could be separated into three types of rays, which he named alpha, beta, and gamma rays. The α rays (alpha rays) were positively charged, as they were deflected strongly to the negative side, whereas the negative β rays (beta rays) were deflected to the positive side. The γ rays (gamma rays) were not deflected and are uncharged high-energy electromagnetic radiation, similar to x rays and light rays. Gamma rays are the result of rearrangements of neutrons and protons in nuclei that yield lower-energy states and usually accompany other forms of radioactive decay.
Emission of an α -particle produces a new nucleus with a reduction in atomic number by two and in mass number by four. When a nucleus emits a β -particle, the atomic number of the new nucleus increases by one (over that of the decaying nucleus), but the mass numbers are unchanged. Some radioactive nuclei do not increase in atomic number in decay, but decrease by one unit of mass number due to the emission of a positron (a positively charged β ray). For example, in β decay with electron emission, is converted to , whereas in positron β decay, is converted to . An alternative process to β decay involves the absorption of an orbital electron by the nucleus in a process known as electron capture, which results in a decrease in the atomic number of the product nucleus, for example, 195 Au decaying to 195 Pt. Gamma-ray decay results in no change in either mass number or atomic number.
Another type of radioactive decay that is observed in the heaviest elements is spontaneous fission . In this process, a nucleus splits into two roughly equal parts, simultaneously releasing a large amount of energy. For example, for the nuclide , of every 100 nuclei that decay, approximately 97 do so by α decay and 3 undergo spontaneous fission.
The rate of radioactive decay is directly proportional to the amount of radioactive species present. A radioactive nucleus is characterized by its half-life, which is the amount of time it takes for 50 percent of the atoms present initially to decay. The half-life of 131 I is eight days: an original sample of 1 gram (0.035 ounces) of 131 I after eight days has only 0.5 grams (0.018 ounces) remaining; after sixteen days, only 0.25 grams (0.0088 ounces), and so on. The half-life is unaffected by differences in temperature, pressure, or chemical state. This constancy has made study of the half-lives of radioactive nuclei very useful to scientists engaged in dating archaeological and geological materials.
HANS GEIGER (1882–1945)
Hans Geiger worked in Ernest Rutherford's laboratory manually and meticulously counting α -particle scintillations for the famous experiments that led to the discovery of the nucleus. Because of this work, he developed an α -particle detector. After World War I, Geiger developed the modern Geiger–Mueller counter and worked until his death to increase its speed and sensitivity.
Since the discovery of radioactivity, radioactive nuclei serving as "tracers" have been of immense value to science, agriculture, medicine, and industry.
In the use of radioactive tracers it is assumed that the radioactive isotopes studied are identical in chemical behavior to the nonradioactive isotopes. The first experiments that used radioactive tracers were carried out in 1913 in Germany and were designed to measure the solubility of lead salts via the use of a radioactive isotope of lead. In industry, radionuclides have been used for analytical purposes, for measurements of flow in pipes, and as part of many other applications. Another example of an important tracer study has been the investigation of photosynthesis of carbohydrates from atmospheric CO 2 in the presence of light and chlorophyll . Scientists used , , and to identify the intermediate steps involved in the photo-synthesis of carbohydrates in plants that had been placed in an atmosphere composed of -labeled CO 2 and had been irradiated with light. The presence of the radioactive carbon in the synthesized carbohydrate was evidence that O 2 was involved in the synthesis .
The process of neutron activation analysis , in which radioactivity is induced in stable nuclei by their bombardment with neutrons, has allowed measurement of impurities on the level of less than one part per billion. Neutron activation analysis has been used in determining the authenticity of paintings, in criminology, in analyzing lunar soil, and in many other areas.
The largest single use of radionuclides has been in medical science. If a radioactive compound, such as a radioactively labeled amino acid, vitamin , or drug, is administered to a patient, the substance is incorporated in different organs to varying degrees. The substance undergoes chemical change within the body, and the movement of the radioactive atoms in the body can be followed with radiation detectors. Such information is of great diagnostic value toward identifying the presence of tumors and other diseases in different organs in the body. Radioactivity has also been used in medicine for therapeutic purposes (radiotherapy); for example, it attacks cancerous cells more efficiently than normal cells.
Choppin, Gregory R.; Liljenzin, Jan-Olov; and Rydberg, Jan (2001). Radiochemistry and Nuclear Chemistry, 3rd edition. Woburn, MA: Butterworth-Heinemann.
Ehmann, William D., and Vance, Diane E. (1991). Radiochemistry and Nuclear Methods of Analysis. New York: Wiley.
Friedlander, Gerhart, et al. (1981). Nuclear and Radiochemistry, 3rd edition. New York: Wiley.