The description of all currently-existing nuclei as ‘naturally-occurring’ is a fallacious one. In reality, all nuclei that existed, exist, or could exist can be said to be naturally-occurring. The erroneous distinction dates back to 1934, when Frédéric and Irène Joliot-Curie discovered a technique to recreate nuclei that were once naturally present, but had disappeared due to their radioactive decays. As a result, all these nuclei when recreated by man, have been referred to as “artificial”, though the term is somewhat misleading.
There are 287 different nuclei that can be found in nature. The total number of “artificial” nuclei, however, is currently over 2500; a number that keeps on growing as experiments continue. Some of these created nuclei are so unstable that they decay before having time to attract any electron, and thus cannot even be considered as real ‘atoms’.
Very few naturally-occurring nuclei and all lab-created nuclei are radioactive. Among them are a few celebrities: uranium used in nuclear reactors, Marie Curie’s radium, carbon-14 employed by archaeologists, radon the main source to exposition to radioactivity, potassium-40 present in the human body and tritium an heavy isotope of hydrogen. These are all radioactive nuclei that are found in nature. In contrast to these we have the famous artificial nuclei; the much-prized and much-feared plutonium, cobalt-60 with its many uses, technetium and thallium.
Nuclear fission is a process often used to to produce radioactive nuclei. A well-known fission product, Iodine-131, was also released by the Chernobyl disaster and believed to be responsible of thyroid cancers. Another product of nuclear fission, cesium-137, is today the main legacy of the radioactivity released into the atmosphere during the Tchernobyl and Fukushima accidents and by nuclear tests in the 1960s. It is used also as a radioactive tracer.
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