Radioisotopes
Atomic theory holds that the nucleus of an atom consists of protons and neutrons and describes the elements of the periodic table as atoms consisting of nuclei surrounded by electrons. Identical elements have the same atomic number, Z, which is equal to the number of protons in the nucleus, and the same mass number, A, which is equal to the number of protons plus the number of neutrons. Atoms with the same atomic number but a different number of neutrons, or a different mass number A, are called isotopes. For example, hydrogen has an atomic number of Z = 1, so it has one proton, but it occurs as three isotopes: 1-hydrogen (Z=1; A=1), deuterium or 2-hydrogen (Z=1, A=2), and tritium or 3-hydrogen (Z=1, A=3) in which the respective nuclei contain zero, one or two neutrons. The isotopes of an element are by convention prefixed by their mass number. The nuclei of elements can undergo radioactive decay, during which particles and radiation are emitted. The most common types of emitted radioactive particles are the following: alpha particles, which are 4-helium nuclei (two protons and two neutrons)]; beta particles, which are electrons traveling close to the speed of light (2.998 x 108 m/s) and which result from the breakdown of a neutron into a proton, an electron and a neutrino; and gamma particles, a form of electromagnetic radiation consisting of high-energy photons of very short wavelength. Isotopes are classified as to whether they decay spontaneously or not. The elements whose nuclei do not decay are termed stable isotopes and those isotopes whose nuclei are unstable and disintegrate spontaneously are called radioisotopes.
The first radioisotope experiments were performed in 1896 by the French physicist A.H. Becquerel who studied salts of uranium and observed they produced a latent image on a photographic plate that could be developed in the same way as a latent image produced by visible light.
The radiation emitted by the uranium salts could penetrate thin sheets of metal and discharge a charged oscilloscope.
Many radioisotopes occur naturally, such as the heavy atoms near the end of the periodic table. They belong to the uranium, thorium and actinium families. Each group is headed by a long-lived primary isotope: the thorium series starts with 232-thorium, the actinium series with 235-uranium, and the uranium-radium series with 238-uranium. The decay of the primary isotope leads to another radioisotope, which decays further. The exhaustion of the decay chain finally leads to formation of a stable isotope. The three families contain altogether some 44 radioisotopes and in each case, the final product is a stable lead isotope. Some lighter radioisotopes also occur. Natural potassium for example, besides stable 39-potassium and 41-potassium, also contains trace quantities (0.012%) of radioactive 40-potassium. Rubidium and samarium also contain radioisotopes. More than a thousand artificial radioisotopes are also known. They are produced by changing the original proton-neutron ratio of a stable nucleus by the initiation of nuclear disintegration processes, for instance by bombarding the nuclei with high-energy particles. Examples of artificially produced radioisotopes include over ten variants of the stable 127-iodine isotope and numerous variants of 22- and 24-sodium.
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