Selenium nuclides

The abundances of krypton and xenon are determined exclusively from nucleosynthesis theory. They can be interpolated from the abundances of neighboring elements based on the observation that abundances of odd-mass-number nuclides vary smoothly with increasing mass numbers (Suess and Urey, 1956). The regular behavior of the s-process also provides a constraint (see Chapter 3). In a mature -process, the relative abundances of the stable nuclides are governed by the inverse of their neutron-capture cross-sections. Isotopes with large cross-sections have low abundance because they are easily destroyed, while the abundances of those with small cross-sections build up. Thus, one can estimate the abundances of krypton and xenon from the abundances of. v-only isotopes of neighboring elements (selenium, bromine, rubidium and strontium for krypton tellurium, iodine, cesium, and barium for xenon). 

Crocker sarcoma and Mecca lymphosarcoma tumors were found to accumulate the radioselenium slowly and continuously in contrast to rapid uptake and clearance of the label from most normal organs. Clinically, the affinity of tumors for selenium has been the basis for utilizing a radioactive nuclide of selenium as a tumor localizing agent (62, 64) The reason for the localization of selenium is not readily apparent but may reflect enhanced division rates, protein and chondroitin sulfate biosynthesis, or a decrease in the detoxification of selenium. An outgrowth of these observations has been to examine the in vitro effects of selenium supplementation on cellular propagation. 

Radioactive selenium-75, used to determine the shape of the pancreas, shifts to a more stable nuclide via electron capture. Write the nuclear equation for this change. 

In a full analysis i.e., the prompt as well as delayed y-emission analysis, the short-lived nuclides are determined first. Actually the most intense y-radiations are measured first and then observation about their decrease of intensity is made. (This will happen for y-radiations of those isotopes which have short half-lives). Using a fast rabbit system, each sample is irradiated separately (but together with a comparator in order to calculate the neutron flux) for 5-30 s. Sample and comparator are measured separately after a waiting time from seconds (the element selenium has a nuclide with a half-life time of 17.5 s and thus needs to be measured as quickly after irradiation as possible) to as much as 20 min. After the analysis has been completed for all samples, a waiting time of 5-7 days is required before irradiating them again in order to determine the other elements with longer half-life times. 

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