Isotope , background information

Understanding the behavior of radionuclides in estuaries, as the dynamic interface between the continental hydrochemical systems and the ocean basins, requires consideration of broader chemical cycling in the hydrosphere. In this volume, the behavior of U- and Th-series isotopes in rivers is discussed by Chabaux et al. (2003), that in groundwaters by Porcelli and Swarzenski (2003), and that in oceans by Cochran and Masque (2003). General background information is provided by Bourdon et al. (2003). 

The reader is referred to articles by Barrow,12 Garson,1314 and Moore15 for background information on the field of marine secondary metabolite biosynthesis. Early work in this field used radiochemical tracers such as 14C or 3H recently, studies with 13C have proved popular because of the ease of detection of this stable isotope by nuclear magnetic resonance (NMR). In particular, our understanding of marine microbial biosynthesis and of de novo biosynthesis in marine molluscs has progressed significantly as a consequence of the use of 13C-labeled precursors. 

A Mdssbauer resonance is known in at least one isotope of fourteen tran sition metals in addition to iron. However, none has been extensively used up to the present time. Several of them present extreme difficulties in measurement, but as this chapter will show, sufficient background information has been collected to assess the feasibility of chemical application. 

Exposure Levels in Humans. Although some data on the levels of americium in human tissues exposed to natural background levels (food, water, and air) are available, few measurements have been made on the americium content in human tissues. The principal source of information about occupationally exposed individuals is the U.S. Transuranium and Uranium Registries (USTUR) Tissue Program and database, established to document levels and distribution of uranium and transuranium isotopes in human tissues for occupationally exposed workers (USTUR 1999). Several major database files are available. 

The electronic transitions which give rise to X-ray emission spectra involve core electrons and are therefore relatively insensitive to the chemical and physical form of the determinant (Bertin, 1978). As a result, analyses can be performed with a minimum of sample preparation directly on materials in the condensed phase. This insensitivity of sample matrix applies to the wavelength of the emitted X-rays, not to their intensities and as quantitation is based on intensity measurement, closely matched standards are required. X-ray emission spectra can be excited by primary X-rays in a fluorescence experiment or by changed particles via collisional excitation. The cross sections for excitation of X-ray emission are rather low and this is combined with the low efficiency of collection, collimation, diffraction and detection of the emitted X-rays. This low overall efficiency leads to a relatively low sensitivity in some cases and is compounded by high backgrounds either from scattered primary radiation in a fluorescence experiment or due to bremsstrahlung in the charged-particle-excitation methods. Methods based on X-ray spectrometry do not provide isotopic information about the sample. Nonetheless, there are a number of radio analytical problems which can be solved by methods based on X-ray spectrometry. 

As you may know, some common everyday substances are radioactive. In this lab you will investigate the three naturally occurring potassium isotopes found in a common store-bought salt substitute. Two of potassium s isotopes, potassium-39 (93.1%) and potassium-41 (6.89%) are stable. However, potassium-40 (0.01%) decays by beta emission to form stable calcium-40. You will first measure the background radiation level, and then use that information to determine the radiation due to the beta decay of potassium-40. You will also measure radiation at various locations around your school. 

The assay for Sr and Sr together will require that standards be used to set up counting efficiencies in two regions of interest. Backgrounds for these regions of interest would also be required and the contribution of the Sr (1.49 MeV beta) to the Sr (0.546 MeV beta) count will be required. With this information, a single count of a mixture would yield information on the amount of both isotopes, the accuracy depending on the total count and the relative quantities of the two nuclides. 

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