Radionuclides retention half-life

Radon-222, a decay product of the naturally occuring radioactive element uranium-238, emanates from soil and masonry materials and is released from coal-fired power plants. Even though Rn-222 is an inert gas, its decay products are chemically active. Rn-222 has a a half-life of 3.825 days and undergoes four succesive alpha and/or beta decays to Po-218 (RaA), Pb-214 (RaB), Bi-214 (RaC), and Po-214 (RaC ). These four decay products have short half-lifes and thus decay to 22.3 year Pb-210 (RaD). The radioactive decays products of Rn-222 have a tendency to attach to ambient aerosol particles. The size of the resulting radioactive particle depends on the available aerosol. The attachment of these radionuclides to small, respirable particles is an important mechanism for the retention of activity in air and the transport to people. 

The described techniques with blinking accelerator beam are quite cumbersome. Of course, mere observation that a short-lived radionuclide appears at the exit of the column indicates that the retention time is of similar order as the half-life. However, more precise measurements would require an independent determination of the initial activity released from the target. In the case of short-lived nuclides, the traditional simple catcher experiments can again be realized only by repeated bombardments and with the use of some technical tools. 

The manufacturing and handling of radiopharmaceuticals is potentially hazardous. The types of radiation emitted and the half-lives of the radioactive isotopes are parameters contributing to the level of risk. Particular attention must be paid to the prevention of cross-contamination, to the retention of radionuclide contaminants, and to waste disposal. Special consideration may be necessary with reference to the small batch sizes made frequently for many radiopharmaceuticals. Due to their short half-life some radiopharmaceuticals are released before completion of certain Quality Control tests. In this case, the continuous assessment of the effectiveness of the Quality Assurance system becomes very important. 

The second part of the quartz column serves as isothermal chromatography section. Volatile species pass this section whereby they undergo numerous sorption/desorption steps with retention times indicative of the volatility at the given temperature of the isothermal part of the column. The chemical yield of the volatile species is studied as a function of the temperature of the isothermal part of the column. The chemical yield rises steeply above a certain temperature and reaches a plateau at higher temperatures see O Fig. 20.11). The retention time of the volatile species is determined by using the nuclear half-life of the radionuclides as a clock at the temperature at which 50% of the plateau yield is observed, Tsoo/o, the retention time is equal to one half-life. 

The short lived wastes are essentially those which contain predominantly radionuclides having a half-life of not more than about 30 years (although limited quantities of longer-lived nuclides may be present ). The approach for the management of these wastes essentially involves retention of radionuclides for sufficient period to allow decay to itmocuous levels. Depending upon the volumes and the half-lives of the radionuclides, the retention may be in the as generated form or after volume reduction and conditioning. 

When the sampling period does not enable the biological half-life of the radionuclide to be estimated, assuming a long period of retention in the body for the purpose of dose assessment may result iu an underestimate of the intake, and hence of the committed effective dose. The degree of over- or under-estimation of the dose will depend upon the overall pattern of retention in the body. 

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