Radioactive impurities

One of the first decisions to be made when designing an experiment is the method of detection to be used with a particular solute. If radiolabeled material is available, a simple method of analysis is to count the radiolabel appearing in the receiver compartment as a function of time. While convenient, this can be a dangerous practice. Depending upon the type of radioisotope, its position in the molecule, and its specific activity, radiolabeled compounds can be subject to a variety of chemical and solution-catalyzed degradation pathways. If the stock solution contains a significant amount of radioactive impurities or generates them as a result of solution instability, then the possibility for preferential transport of... 

There are a number of Physical Methods available but of all the Physical Methods, tracer analysis using C14, H3 or S35- labelled initiators or other substances has received considerable attention. However, this method is incapable of giving results specific to the actual functional group and is highly susceptible to interference by absorbed radioactive impurities. It is moreover hazardous, expensive and needs sophisticated apparatus. 

Carriers frequently are stable isotopes of the radionuclide of interest, but they need not be. Nonisotopic carriers are used in a variety of situations. Scavengers are nonisotopic carriers used in precipitations that carry/incorporate other radionuclides into their precipitates indiscriminately. For example, the precipitation of Fe (OH)3 frequently carries, quantitatively, many other cations that are absorbed on the surface of the gelatinous precipitate. Such scavengers are frequently used in chemical separations by precipitation in which a radionuclide is put in a soluble oxidation state, a scavenging precipitation is used to remove radioactive impurities, and then the nuclide is oxidized/reduced to an oxidation state where it can be precipitated. In such scavenging precipitations, holdback carriers are introduced to dilute the radionuclide atoms by inactive atoms and thus prevent them from being scavenged. 

I]GR 190028a was purified by HPLC from a non-radioactive impurity which was formed during the reaction, and isolated in a radiochemical purity of greater than 99% and a specific activity of 18 Ci/mmol in a yield of 72 % [21]. 

Finally, caution should be exerted when determining kinetic data with radiolabelled sugars. Trueb, et al. (32) found that labelled glucose from several manufacturers contained radioactive impurities of unknown structure which often reacted faster than glucose itself. Purification methods can be found in reference 31. 

An example of removing multiple interfering elements is strontium purification in the presence of fission products. Ferric ion is added as a holdback carrier for the rare earths (and other radionuclides) and then precipitated as Fe(OH)3, the scavenger that carries these radioactive impurities. This or any other step can be repeated for enhanced removal of impurities. 

For application as a tracer, a radioisotope is selected on the basis of availability, emitted radiation that can be distinguished from that of the analyte, an appropriately long half-life, and minimal radioactive impurities, including radioactive progeny. As discussed below, the tracer may either be purchased from a supplier as a standard or calibrated in the laboratory. An even safer procedure is to purchase a standard and check its reliability with a calibrated detector. 

In this experiment, tritiated water is purified by simple distillation, and the tritium beta particles in the condensate are measured with a liquid scintillation (LS) counter. Such distillation also can collect tritiated water samples from solids. Tritium in other forms must be processed before it can be counted like tritium in water for example, tritiated hydrogen gas and tritiated organic substances can be oxidized to form water. Additional separations may be needed if the liquid or solid sample contains radioactive gases or volatile substances other than tritium that may be collected with the distilled tritiated water. Such radioactive impurities can be identified in the data output from the LS counter of an energy spectrum that differs from that of pure tritium, or of counts in energy regions where tritium counts are not found. 

The sample is purified by distillation to separate the tritium-containing water from both non-radioactive and radioactive impurities. Various substances can cause scintillations by means other than radionuclide emission - by chemical fluorescence or luminescence - or interfere with ( quench ) detection of scintillations due to radionuclides. Even after purification, both processes are inevitable, but to a limited extent. Luminescence due to visible light will decay when the sample is stored in a darkened region of the LS system before the sample is counted. The degree of quenching, notably due to water in the sample, is determined instrumentally by reference to comparison sources and recorded, so that any deviation from the quenching observed for the tritium standard can be taken into account. 

This type of defect is very rare. Radioactive contamination is effective at destroying the transistors [11]. Some ceria particles may contain radioactive impurities depending on the source of raw materials. Therefore, it is important to screen for radioactive impurities for slurries that may contain radioactive raw materials. In addition, a radioactive contaminated packaging material may also cause irreversible damage if used to store CMP consumables. 

Optical microscope observations have shown that zircons often exhibit extremely complex microstructures (on the scale of 1-100 /tm), in which changes of birefringence correlate with the distribution of U and Th (Chakoumakos et al. 1987). However, no attempts appear to have been made to relate these microstructures to other impurities (such as water-related species) and crystal defects, both of which may significantly influence the processes of metamictization and recrystallization. Such a study involving TEM might also provide important information about the diffusion and leaching of radioactive impurities (and the products of their decay), processes that have important implications for ceramic nuclear-waste disposal and for techniques of age determination based on measurements of Pb/U isotopic ratios. 

The activity of naturally radioactive elements is a measure of their mass. Prerequisites of application of the correlation between mass and activity according to eq. (17.1) are that the isotopic composition of the element to be determined is constant and that interfering radioactive impurities are absent. If the daughter nuelides are also radioactive, radioactive equilibrium must be established or the daughter nuclides must be separated off quantitatively. Interference of radioactive impurities may be avoided by measuring the a or y spectrum of the radionuclide considered. 

Ra can be determined with high sensitivity by measuring Rn in radiochemical equilibrium with Ra. Ra is loaded into a closed receptacle, and after about 6 weeks Rn is transferred into an ionization chamber where its a activity is measured as a function of time. By separating the radon, the influence of radioactive impurities is excluded. In this way the very low Ra content in human bones, of the order of 10 g, can be determined. 

In the case of Th, the attainment of radioactive equilibrium with the daughter nuclides is very slow, because of the long half-life of Ra (q/2 = 5.75 y). Th can be determined directly by measuring its a radiation, but the measurement of Po is more sensitive (about 10 g Th can be determined in this way in 1 g of rock material). Other methods are based on the separation and measurement of Ra or Rn. In all determinations of Th, the possibility of the presence of radioactive impurities, mainly of members of the uranium and actinium families, has to be taken into account. 

Continual progress in rare-earth processing has reduced the radioactive impurities in rare-earth products substantially, so they are practically free of radioactivity today. Current processing technology for mineral recovery and for the subsequent lanthanide separation results in products that meet all regulatory requirements. 

Scavenging of Gaseous Chemically Active and Radioactive Impurities... 

The irradiation container is unloaded and the samples and standards prepared for radionuclide assay. After irradiation no care whatever need be taken to avoid contamination of the sample by non-radioactive impurities. Time performs the same function as a skilled chemist if the radionuclide to be measured has a longer half-life than the interferences. For example, trace Cr is easily measured in the presence of Ti even though the neutron-capture products of both elements emit gamma rays of exactly the same energy. Gram for gram, Tl emits 1000 times more photons per second than does Cr immediately after a 1-minute irradiation, but Cr is 2000 times more radioactive than Tl two hours later. 

Table 5.25 Radioactive impurities in yellow cake concentrate...

Radioactive impurity control. Prompt control of radioactive impurities in the primary sodium is made using three methods ... 

The high specificity of tetrodotoxin and saxitoxin for sodium channels in excitable ihembranes makes them suitable ligands with which to undertake binding studies on excitable membranes. From such studies, it is expected to demonstrate the presence of a saturable binding component with an affinity for the toxins comparable with the affinity determined indirectly from inhibition studies. This has proved to be so, although some early difficulties were experienced on two fronts. First, it is essential that an accurate specific activity is known for the radioactive toxin. If radioactive impurities (i.e. radioactivity un-... 

Behaviour of all other helium-components including heat exchangers and purification system (except radioactive impurities)... 

By means of an adequate regulation of the pressure levels within the chambers provided between the labyrinths seals it was ensured (for ftiture application to a nuclear heated system) that under no conditions of operation would primary helium cany radioactive impurities into the oil circuit. In order to seal the shaft penetration at the housing, a floating ring seal and a static seal were provided. This sealing principle was used for all 3 turbomachines. 

In order to gain experience, the helium purification-system of the fossil-fired EVO plant was designed in accordance with the requirements for a nuclear heated plant, except without provision for the removal of radioactive impurities. The design througlq>ut was 100 kg/h and the required cleanliness was < 1 ppm for any substance. The flow scheme of the system is given in Fig. 30. 

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