Isotope Radioisotopes

Atoms that have the same atomic number, and hence are the same element, but have different masses are known as isotopes. Radioisotopes, or more correctly, radionuclides spontaneously and continuously emit characteristic types of radiation. They are particularly useful in analytical biochemistry, the unique nature of the radiation providing the basis for many specific and sensitive laboratory methods (Table 5.1). 

Although the multiple isotope method is most frequently used with stable isotopes (for example studies of oxygen KIE s in biophosphates used 1SN at a remote nitro group, or 13C on a remote carboxy group, as reporting isotopes), the technique is not restricted to stable isotopes radioisotopes have been used as reporting sites for stable isotopes. In a practical sense this is the only method that allows the measurement of isotope effects for elements that have only one stable isotope (e.g. fluorine and phosphorus). In these cases doubly radiolabeled material is used (see Section 7.4). 

A radioactive isotope (radioisotope) is an unstable isotope of an element that decays into a more stable isotope of the same element. They are of great use in medicine as tracers (to help monitor particular atoms in chemical and biological reactions) for the purpose of diagnosis (such as imaging) and treatment. Iodine (-131 and -123) and Technetium-99 are used for their short half-lives. 

The isotopes of a particular element have the same number of protons in the nucleus but a different number of neutrons, giving them the same proton number (atomic number) but a different nucleon number (mass number, i.e. number of protons + number of neutrons). Isotopes may be stable or radioactive. Radioactive isotopes (radioisotopes) disintegrate spontaneously at random to yield radiation and a decay product. 

The release of radiation by radioactive isotopes—radioisotopes, for short—is called decay. The nuclei of such radioisotopes are rmstable. However, not aU ruistable nuclei decay in the same way. Some give off more powerful radiation than others or different kinds of radiation. Between 1896 and 1903, scientists had discovered three types of nuclear radiation. Each type changes the nucleus in its own way. These three types were named after the first three letters of the Greek alphabet alpha (a), beta (/3), and gamma (y). 

Radioactive Isotopes (Radioisotopes) For more information about radioisotopes [Americium - 241 (Am-241) Celsium-137 (Cs-137) Cobalt-60 (CO-60) Iodine a-129 1-131) Plutonium-239 (Pu-239) Strontium-90 (Sr-90) Uranium-235 (U-235) and Uranium-238 (U-238)]see the Public Health Statement by the Agency for Toxic Substances and Disease Registry at http //w ww.atsdr.cdc.gov/toxprofiles, or visit the Environmental Protection Agency at littp //www.epa.gov/radiation/radionuclides/americiuni.htni. 

With this nomenclature, the notation 13-A1-27 represents stable aluminum as found in nature. If you look at a Chart of the Nuclides, you will find there are 11 different aluminum isotopes (all are aluminum because they have 13 protons, but they have masses from 23-33 They are all isotopes of aluminum, and all but 13-A1-27 are radioactive isotopes --radioisotopes). Since only aluminum has 13 protons in the nucleus, we often leave the Z value off the notation, such as Al-27. Some of our information sheets have the entries arranged in order of increasing Z, so it is helpful to remember about what the atomic numbers of different elements are to make it easier to find entries in tables. 

It was also discovered that radioactive isotopes (radioisotopes) frequently produce other elements. Rutherford found that radium, for... 

Isothiocyansaure isotope Isotop > unstable isotope/ radioisotope/ radioactive isotope instabiles Isotop, Radioisotop, Radionuclid, radioaktives Isotop isotope assay Isotopenversuch isotopic dilution Isotopenverdtmnimg isotropic isotrop,... 

Thirty isotopes are recognized. Only one stable isotope, 1271 is found in nature. The artificial radioisotope 1311, with a half-life of 8 days, has been used in treating the thyroid gland. The most common compounds are the iodides of sodium and potassium (KI) and the iodates (KIOs). Lack of iodine is the cause of goiter. 

Each element that has neither a stable isotope nor a characteristic natural isotopic composition is represented in this table by one of that element s commonly known radioisotopes identified by mass number and relative atomic mass. 

It is not necessary that there be two isotopes in both the sample and the spike. One isotope in the sample needs to be measured, but the spike can have one isotope of the same element that has been produced artificially. The latter is often a long-lived radioisotope. For example, and are radioactive and all occur naturally. The radioactive isotope does not occur naturally but is made artificially by irradiation of Th with neutrons. Since it is commercially available, this last isotope is often used as a spike for isotope-dilution analysis of natural uranium materials by comparison with the most abundant isotope ( U). 

Radiocarbon dating (43) has probably gained the widest general recognition (see Radioisotopes). Developed in the late 1940s, it depends on the formation of the radioactive isotope and its decay, with a half-life of 5730 yr. After forms in the upper stratosphere through nuclear reactions of... 

Unstable niobium isotopes that are produced in nuclear reactors or similar fission reactions have typical radiation hazards (see Radioisotopes). The metastable Nb, = 14 yr, decays by 0.03 MeV gamma emission to stable Nb Nb, = 35 d, a fission product of decays to stable Mo by... 

Many of the uranium fission fragments are radioactive. Of special interest are technetium-99 [14133-76-7] and iodine-129 [15046-84-1] having half-Hves of 2.13 X 10 yr and 1.7 x 10 yr, respectively. Data on all isotopes are found in Reference 6 (see also Radioisotopes). 

The isotope plutonium-238 [13981 -16-3] Pu, is of technical importance because of the high heat that accompanies its radioactive decay. This isotope has been and is being used as fuel in small terrestrial and space nuclear-powered sources (3,4). Tu-based radioisotope thermal generator systems dehvered 7 W/kg and cost 120,000/W in 1991 (3). For some time, %Pu was considered to be the most promising power source for the radioisotope-powered artificial heart and for cardiovascular pacemakers. Usage of plutonium was discontinued, however, after it was determined that adequate elimination of penetrating radiation was uncertain (5) (see PROSTHETIC AND BIOMEDICAL devices). 

AH of the 15 plutonium isotopes Hsted in Table 3 are synthetic and radioactive (see Radioisotopes). The lighter isotopes decay mainly by K-electron capture, thereby forming neptunium isotopes. With the exception of mass numbers 237 [15411-93-5] 241 [14119-32-5] and 243, the nine intermediate isotopes, ie, 236—244, are transformed into uranium isotopes by a-decay. The heaviest plutonium isotopes tend to undergo P-decay, thereby forming americium. Detailed reviews of the nuclear properties have been pubUshed (18). 

Uses of Plutonium. The fissile isotope Pu had its first use in fission weapons, beginning with the Trinity test at Alamogordo, New Mexico, on July 16, 1945, followed soon thereafter by the "Litde Boy" bomb dropped on Nagasaki on August 9, 1945. Its weapons use was extended as triggers for thermonuclear weapons. This isotope is produced in and consumed as fuel in breeder reactors. The short-Hved isotope Tu has been used in radioisotope electrical generators in unmanned space sateUites, lunar and interplanetary spaceships, heart pacemakers, and (as Tu—Be alloy) neutron sources (23). 

A large number of radiometric techniques have been developed for Pu analysis on tracer, biochemical, and environmental samples (119,120). In general the a-particles of most Pu isotopes are detected by gas-proportional, surface-barrier, or scintillation detectors. When the level of Pu is lower than 10 g/g sample, radiometric techniques must be enhanced by preliminary extraction of the Pu to concentrate the Pu and separate it from other radioisotopes (121,122). Alternatively, fission—fragment track detection can detect Pu at a level of 10 g/g sample or better (123). Chemical concentration of Pu from urine, neutron irradiation in a research reactor, followed by fission track detection, can achieve a sensitivity for Pu of better than 1 mBq/L (4 X 10 g/g sample) (124). 

Gas-flow counting is a method for detecting and quantitating radioisotopes on paper chromatography strips and thin-layer plates. Emissions are measured by interaction with an electrified wire in an inert gas atmosphere. AH isotopes are detectable however, tritium is detected at very low (- 1%) efficiency. 

A radioisotope is an atom the nucleus of which is not stable and which decays to a more stable state by the emission of various radiations. Radioactive isotopes, also called nucHdes or radionucHdes, are important to many areas of scientific research, as well as ia medical and iadustrial appHcations (see... 

Knowledge about the radiations from each isotope is important because as the uses of the radioisotopes have iacreased, it has become necessary to develop sensitive and accurate detection methods designed to determine both the presence of these materials and the amount present. These measurements determine the amount of radiation exposure of the human body or how much of the isotope is present ia various places ia the environment. For a discussion of detection methods used see References 1 and 2. 

Most areas of research and appHcations involving the use of radioisotopes require a knowledge of what radiations come from each isotope. The particular apphcation determines what type of information is needed. If the quantity of a radionuchde in a particular sample or at a particular location is to be deterrnined and this value is to be deterrnined from the y-ray spectmm, the half-life of the nucHde and the energies and intensities or emission probabiUties of the y-rays of interest must be known. Usually it is preferable to use the y-rays for an assay measurement because the d- and P-rays ate much more readily absorbed by the source material, and may not reach the sample surface having their original energies. Once these energies are altered they caimot be used to identify the parent radionuchde. 

Radioactivity is equal to the rate of decay of a given radioisotope. This quantity is proportional to the number of radioactive atoms present, so that for a single isotope,... 

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