Retention radioactivity

The Natural Reactor. Some two biUion years ago, uranium had a much higher (ca 3%) fraction of U than that of modem times (0.7%). There is a difference in half-hves of the two principal uranium isotopes, U having a half-life of 7.08 x 10 yr and U 4.43 x 10 yr. A natural reactor existed, long before the dinosaurs were extinct and before humans appeared on the earth, in the African state of Gabon, near Oklo. Conditions were favorable for a neutron chain reaction involving only uranium and water. Evidence that this process continued intermittently over thousands of years is provided by concentration measurements of fission products and plutonium isotopes. Usehil information about retention or migration of radioactive wastes can be gleaned from studies of this natural reactor and its products (12). 

Further work on nickelocene and cobaltocene was done by Ross , who synthesized the respective compounds using Ni, Ni and " Co, which decay be E.C., jS and a fully converted isomeric transition, respectively, all producing radioactive cobalt isotopes. The results showed retentions, after sublimation, of 84%, 83% and 80%, respectively. The composition of the unsublimable residue was largely CoCp2, except for the highly converted "Co, where only 30% CoCpj could be detected. This was interpreted as showing that by internal conversion the molecules are totally destroyed, by the same sort of argument as was used by Riedel and Merz . 

It is important to note that since the amounts of radioactive material produced are so extremely small (some 10 % of the total is typical) it is usually necessary to add macro quantities—10-100 mg—of each compound expected to be present, in order to effect a good separation and to measure the chemical yield of the carrier. The yield measured is the radioactivity in each separated chemical species as a fraction of the total radioactivity in the sample, corrected to 100% chemical yield of each respective carrier. The term retention is commonly used to refer to the yield of the parent compound. This term has the disadvantage, however, of implying that the radioactive atom remained in the same molecule. Since it often appears that the molecule is only later reconstituted, the terms yield and parent yield are to be preferred. 

The ICRP (1994b, 1995) developed a Human Respiratory Tract Model for Radiological Protection, which contains respiratory tract deposition and clearance compartmental models for inhalation exposure that may be applied to particulate aerosols of americium compounds. The ICRP (1986, 1989) has a biokinetic model for human oral exposure that applies to americium. The National Council on Radiation Protection and Measurement (NCRP) has also developed a respiratory tract model for inhaled radionuclides (NCRP 1997). At this time, the NCRP recommends the use of the ICRP model for calculating exposures for radiation workers and the general public. Readers interested in this topic are referred to NCRP Report No. 125 Deposition, Retention and Dosimetry of Inhaled Radioactive Substances (NCRP 1997). In the appendix to the report, NCRP provides the animal testing clearance data and equations fitting the data that supported the development of the human mode for americium. 

Respiratory Tract Clearance. This portion of the model identifies the principal clearance pathways within the respiratory tract. The model was developed to predict the retention of various radioactive materials. Figure 3-4 presents the compartmental model and is linked to the deposition model (see Figure 3-2) and to reference values presented in Table 3-5. This table provides clearance rates, expressed as a fraction per day and also as half-time (Part A), and deposition fractions (Part B) for each compartment for insoluble... 

NCRP. 1997. Deposition, retention and dosimetry of inhaled radioactive substances. National Council on Radiation Protection and Measures. Bethesda, MD. Report No. 125. http //www.ncrp.com. 

The introduction of metabolizable ester functions may be a very useful means of obtaining retention of the radiotracer within the target, as known for 99mTc-ECD. It may also help to eliminate nontarget radioactivity more rapidly. Therefore, systematic studies have been performed, in order to understand the enzymatic hydrolysis of oxotechnetium(V) complexes with up to four ester groups in the complex [205]. Fig. 30 shows the partial hydrolysis by pig liver esterase as is was observed for 1 2 complexes of derivatized dimercaptosuccinic acid complexes. 

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. 

Radiocerium absorbed into the systemic circulation will be transported by blood proteins and be deposited predominantly in liver and bone. Deposition fractions will be about 0.45 for liver, 0.35 for bone, and 0.1 for other soft tissues with the remainder excreted in urine and feces. The retention times in liver and bone are long compared to the radioactive half-lives of the cerium isotopes. Therefore, their effective biological half-times in these organs will be approximately equal to their physical half-lives. Experimental data on internal organ distri-... 

Fig. 8. Summary of pulmonary retention of l44Ce in Beagle dogs after inhalation of labeled chloride, oxide, and aluminosilicate particles (heavy lines) as compared to Class W and Y compounds of l44Ce (light lines). These include radioactive decay.

Pulmonary retention functions are given as fractions of the initial pulmonary burden and are not corrected for radioactive decay of lMCe Ti/> = 285 d). 

Fig. 17. Biological model recommended for describing the uptake and retention of cerium by humans after inhalation or ingestion. Numbers in parentheses give the fractions of the material in the originating compartments which are cleared to the indicated sites of deposition. Clearance from the pulmonary region results from competition between mechanical clearances to the lymph nodes and gastrointestinal tract and absorption of soluble material into the systemic circulation. The fractions included in parentheses by the pulmonary compartment indicate the distribution of material subject to the two clearance rates however, these amounts will not be cleared in this manner if the material is previously absorbed into blood. Transfer rate constants or functions, S(t), are given in fractions per unit time. Dashed lines indicate clearance pathways which exist but occur at such slow rates as to be considered insignificant compared to radioactive decay of the cerium isotopes.

React for 10-15 minutes at room temperature. Optimization of the reaction time and the amount of 125I added to the reaction may have to be done to obtain the best radioactivity incorporation and retention of protein activity. 

The following procedure describes the iodination process for the Bolton-Hunter reagent and its subsequent use for the radiolabeling of protein molecules. Modification of other macromolecules can be done using the same general method. For particular labeling applications, optimization of the level of iodine incorporation may have to be done to obtain the best specific radioactivity with retention of biological activity. 

The second high-temperature study reviewed in Section 2 (Ichimiya and Furuichi, 1968) employed the radioactivity of tritium (3H) as a tool for measuring its uptake and retention in silicon wafers. This study yielded... 

The distribution of tritium in compounds 137-150 can be determined by tritium NMR spectroscopy without chemical manipulations115. The structure retention index relationship (SR IR)116 has been used for identification of unknown radioactive peaks and to differentiate by-products from radioimpurities from extraneous sources. 

Joyner, T. and R. Eisler. 1961. Retention and translocation of radioactive zinc by salmon fingerlings. Growth 25 151-156. 

The basic idea in radioactive age-dating of rocks (from the Earth, Moon and meteorites) is to find the ratio of daughter to parent in an isolated system. Thus the age inferred is usually the solidification age which is the time since the last occasion when chemical fractionation was halted by solidification. (K-Ar dating gives a gas-retention age which can be slightly shorter.)... 

Retention of Macromolecules and Exogenously Supplied Radioactivity Following Fixation and Washing Procedures"... 

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