Radioelements, Isotopes and Radionuclides

In laboratory experiments with radionuclides, knowledge of the mass of the radioactive substances is very important. For example, the mass of 1 MBq of ( i/2 = 14.3 d) is only about 10 ° g, and that of 1 MBq of Tc (ti/2 = 6.0 h) is only about 5 10 g. If there is no carrier present in the form of a large excess of inactive atoms of the same element in the same chemical state, these small amounts of radionuclides may easily be lost, for instance by adsorption on the walls. Whereas in the case of radioisotopes of stable elements the condition of the presence of carriers is often fulfilled due to the ubiquity of most stable elements, it is not fulfilled in case of short-lived isotopes of radioelements, and extraordinary behaviour may be observed (section 13.3). 

In the case of radioisotopes of stable elements, such as Co, and " Na, small amounts (traces) of these elements are always present, due to their ubiquity. The masses are, in general, higher than the masses of short-lived radioisotopes, and the omnipresent traces act as isotopic carriers of the short-lived radionuclides, provided that they are in the same chemical state. In the case of isotopes of radioelements, such as Ra, Ac, Po, Md or Lr, however, stable nuclides are absent and the masses of the radionuclides are identical with the masses of the elements, provided that the presence of longer-lived radioisotopes can be excluded. 

The term carrier-free is often used to indicate the absence of stable isotopes or longer-lived radioisotopes of the radionuclide considered. However, due to the omnipresence of most stable elements, carrier-free radioisotopes of stable elements are, in general, not available. The presence of stable isotopes or longer-lived radioisotopes has to be taken into account, and the specific activity is smaller than calculated by eq. (13.2). As long as the presence of such other isotopes cannot be excluded, it is more correct to distinguish no-carrier-added (n.c.a.) and carrier-added radionuclides. On the other hand, radioisotopes of radioelements are carrier-free if longer-lived radioisotopes are absent. 

Sources conventionally are prepared by precipitation to permit gravimetric measurement of stable isotopic carrier yield, as discussed in Section 6.3. The isotopic carrier must be in the same chemical state as the radionuclide of interest, or the sample must be processed to achieve this requirement when carrier is added. The precipitating agent is selected to obtain a pure precipitate of the radioelement with a large, but not necessarily quantitative, yield and reproducible weight. If this precipitate does not completely purify the sample, as is often the case, then previous separation steps should have done so. The various purification steps must eliminate extraneous solids that will add to the carrier yield and contaminant radionuclides that will add to the count rate. The purification steps must reduce such contaminants to a small fraction of the amount to be weighed and counted. Occurrence in nature of significant amounts of the isotopic carrier in the sample must be determined in control samples to correct the yield value. 

Each radioisotope formed in an element subject to a nuclear particle bombardment is uniquely characterized by its half-life (rate of decay) and the types and energies of the radiations it emits as it decays. Therefore, a positive identification of the radioelement is possible. The amount of element in the bombarded or activated sample can be determined directly from a measurement of the radionuclide s radioactivity because the induced radioactivity is directly proportional to the number of atoms of the stable isotope in the sample and to the intensity (flux) of the nuclear particles interacting with the stable nuclei. 

Man has artificially produced radioelements (technetium, promethium, transuranium elements) and also many radioactive isotopes of the naturally occurring elements. Thus, the natural radioactivity on the earth is to some extent increased by man-made radioactive materials. Many analytical research works focus on the determination of manmade radionuclides, their migration, pathways, and accumulation in the environment. The aim of this analytical research work is either to know about fate, pathway, and metabolism of materials or to obtain information about possible environmental protection problems. 

Mass number parity (A) Decay series name Header radionuclide (r, and ) Natural isotopic abundance Specific activity of parent radionuclide ( ) End stable nuclide ( ) Gaseous radioelement (emanation, old symbol)... 

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