Radioactive isotopes
Radioactive isotopes, or radioisotopes, are unstable nuclides that decay over time, transitioning to stable forms by emitting radiation such as alpha, beta, or gamma rays. Found naturally in the Earth's crust, surface waters, and atmosphere, all known elements have at least one radioactive isotope, whether naturally occurring or artificially produced. These isotopes play a significant role in various fields, including medicine, where they are used for diagnostic imaging and treatment, as well as in nuclear power generation.
In scientific research and industry, radioisotopes serve as tracers to study metabolic pathways and transport processes, allowing for precise measurements of chemical interactions. The decay of certain isotopes, like carbon-14, is also utilized in dating ancient fossils, providing insights into historical timelines. The history of radioactivity includes notable figures like Marie Curie and Ernest Rutherford, who contributed to our understanding of different types of radiation. Overall, the applications of radioisotopes are diverse and impactful, ranging from healthcare to archaeology and industrial quality control.
Radioactive isotopes
Where Found
All the known elements have at least one radioactive isotope, either natural or artificially produced. Therefore, the radionuclides are found in the Earth’s crust, in its surface waters, and in the atmosphere.
Primary Uses
Radioisotopes are used in many areas of science and industry as tracers or as radiation sources. They provide fuel for the nuclear generation of electricity and have found both diagnostic and therapeutic uses in medicine.
Technical Definition
Radioactive isotopes are unstable nuclides that decay ultimately to stable nuclides by emission of alpha, beta, gamma, or proton radiation, by K capture, or by nuclear fission.
Description, Distribution, and Forms
Alpha, beta, and gamma radiation are the three types of naturally occurring radioactivity; they result in the transmutation of one chemical nucleus to another. Alpha decay is the ejection from the nucleus of a particle equivalent in size to a helium nucleus. The daughter nucleus has an atomic number (Z) two less than that of the parent and a mass number (A) four less than the parent. The equation below represents the emission of an alpha particle from a polonium nucleus to produce an of lead (a gamma ray is also emitted in rare cases).
Beta decay results from the change within the nucleus of a neutron into a proton. Z increases by one, while A is unchanged. The equation below illustrates beta emission by phosphorus to become sulfur.
In gamma decay, electromagnetic radiation is emitted as a nucleus drops to lower states from excited states. It is the nuclear equivalent of atomic line spectra that show wavelengths of visible light emitted by atoms when electrons drop from higher to lower energy levels. Nuclear fission is an extremely important process by which isotopes of the heavy elements such as uranium 235 capture a neutron and then split into fragments.
The neutrons produced are captured by other nuclei, which in turn fission, producing a chain reaction. This is the process that resulted in the first atomic bomb and is now used in nuclear plants to produce electric power.
History
The story of begins with Wilhelm Conrad Röntgen’s work with cathode-ray tubes. Roentgen allowed cathode rays to impinge on various metal surfaces and observed that highly penetrating radiations, which he called X-rays, were produced. He noted similarities between the X-rays and sunlight in that both could expose a photographic plate and could cause certain metals and salts to fluoresce.
This fluorescence was of interest to Antoine-Henri Becquerel, who discovered by accident that crystals of uranium salt left on a photographic plate in a drawer produced an intense silhouette of the crystals. Although his understanding of the phenomenon was limited at the time, what Becquerel had observed was the effect of uranium radioactivity.
Marie and Pierre Curie pursued the study of this phenomenon with other minerals. They worked to isolate and characterize the substances responsible and were able to isolate and purify samples of polonium and radium. Other scientists worked at the same time to characterize the radiations emitted. In 1903, Ernest Rutherford and Frederick Soddy proposed that the radiations were associated with the chemical changes that radiation produced, and they characterized three types of radiation: alpha (α), beta (β), and gamma (γ) rays.
Obtaining Radioisotopes
The use of nuclear fission to produce energy is based on a principle formulated by Albert Einstein, E = mc2. E is energy, m refers to mass, and c is a constant equal to 3.0 × 108m/c. The complete conversion of one gram of matter per second would produce energy at the rate of nine trillion watts.
The main particles contained in the nucleus of an atom are protons and neutrons. The mass of a given nucleus is less than the sum of the masses of the constituent protons and neutrons. This mass defect has been converted, according to the equation above, to energy (binding energy) in the process of forming the nucleus. The separation of the nucleus into its constituent particles would require replacement of this energy. The binding energy per nucleon is a measure of the stability of a particular nucleus. Those nuclei having mass numbers between 60 and 80 have the highest binding energy per nucleon and are therefore the most stable. A large nucleus such as uranium can split into fragments with sizes in the 60 to 80 mass range. When this happens, the excess binding energy is released.
Uses of Radioisotopes
Radioisotopes are used in a number of ways in the fields of chemistry and biology. Radioimmunoassay (RIA) is a type of isotopic dilution study in which labeled and unlabeled analytes compete for limited amounts of a molecule that binds the analyte very specifically. RIA is used worldwide in the determination of hormones, drugs, and viruses. The technique is so specific that concentrations in the picomolar region can be measured. Another major use of radioisotopes is as tracers that determine metabolic pathways, transport processes, and reaction mechanisms. A compound labeled with a radioactive isotope is introduced into the process, and the radioactivity allows the compound to be followed through the mechanism.
Pharmacokinetics is the study of the rates of movement and biotransformation of a drug and its metabolites in the body. Many kinetic parameters, such as a drug’s in the body, can be determined by using radiolabeled drugs and measuring radioactivity after some type of chromatographic separation of the parent drug from its metabolites.
Radiopharmaceuticals are substances labeled with radionuclides that are used in the visualization of organs, the location of tumors, and the imaging of biochemical processes. This usage is based on the fact that a substance that is found in a healthy cell at a certain concentration has a different concentration in damaged cells. The particular isotope used depends on the organ or biochemical process under study.
Radioisotopes are used in many ways in industry. Gamma rays from cobalt 60 are used to examine objects for cracks and other defects. Radioisotopes can be used to measure the thickness of all types of rolled materials and as tracers in locating leaks in pipes carrying liquids or gases. The fill level of closed containers is monitored by absorption or scattering of radiation.
In the chemical industry, radioisotopes are used to indicate the completeness of a precipitation reaction. A radioisotope of the to be precipitated is added to the solution to be precipitated. When the filtrate is free of radioactivity, precipitation is complete.
Radioisotopes are used in dating ancient rocks and fossils. Carbon is used in dating fossils. All living organisms are assumed to be in equilibrium with their environment, taking in carbon in food and expelling it through respiration and other processes. A living organism is assumed, when it dies, to have a certain percentage of carbon 14, radioactive carbon. As the fossil ages, the carbon 14 decays by beta emission, and its percentage is reduced. Since the decay rate is known, a reasonable age estimate can be obtained by measuring the rate of radioactive emission (proportional to percentage carbon 14) from the fossil. Uranium is used in a similar way to date samples that contain a mixture of uranium and lead, which is at the end of its decay chain.
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