Radioactivity, natural, rare

In most of diese applications, either thoriated tungsten is used (service temperature 1700-1800 °C), or impregnated cathodes, based on Ba0/Al203/Ca0 additions (so-called dispenser cathodes service temperature 900-1250 °C). However, because of its radioactive nature, thoriated tungsten is currently being more and more replaced by timgsten with rare earth (RE) oxide additions or W-Zr02 (see also Section 6.2.2). 

During the first decade of the 20th century the application of physics to chemistry once and for all established the reahty of atoms. Physicists even weighed the electrons that are now known to govern chemical properties. The discovery of radioactive particles alpha [a] and beta [P]) led to the earliest perceptions of isotopes. The remaining two natural rare earths were finally separated and identified, completing the lanthanide series of chemical elements. 

The silver gray metal can be cut with a knife, although it only melts at 1545 °C (for comparison, iron 1538 °C). It is the rarest of the "rare earths", but is nevertheless more abundant than iodine, mercury, and silver. Thulium has few applications, especially because it is relatively expensive. The element occurs naturally as a single isotope, namely 169Tm (compare bismuth). The artificial, radioactive 170Tm is a transportable source of X-rays for testing materials. Occasionally used in laser optics and microwave technology. 

It is good that we should be concerned about the environmental impact of what we, as chemists, do to our planet. But many environmental campaigners too easily confuse radioactive toxicity and chemical toxicity. For example, the radon gas emanating from naturally occurring granite rocks is chemically inert, because it is a rare gas, but it is toxic to humans because of its radioactivity. Conversely, sodium cyanide contains no radioactive constituents yet is chemically toxic. 

Thorium is a radioactive metal that occurs naturally in several minerals and rocks usually associated with uranium. However, it is approximately three times more abundant in nature than uranium. On average, soil contains 6 to 10 ppm of thorium. Thorium is most commonly found in the rare-earth thorium-phosphate mineral, monazite, which contains 8% 10% thorium. Current production of thorium is, therefore, linked to the production of monazite, which varies between 5500 and 6500 tonnes per year, with approximately 300 to 600 tonnes of thorium recovered (NEA/IAEA, 2006a). 

Francium (Fr, [Rn] . 1), name and symbol, after France, the country where it was prepared (Curie Institute in Paris). Discovered (1939) by Marguerite Perey. Very rare in nature, artificially prepared, highly radioactive. 

These two long rows of elements are traditionally moved to the base of the chart so the more important, lighter elements may be closer together for clarity. These two rows of metals each reflect the progressive addition of 14 electrons into an /-type subshell. The lanthanides occur in only trace amounts in nature and are often called rare earths. All of the actinides have large, unstable nuclei that undergo spontaneous radioactive decay. 

Linsalata P, Penna Franca E, Sachett I, et al. 1987. Radium, thorium, and the light rare earth elements in soils and vegetables grown in an area of high natural radioactivity. DOE Symp Ser 59 131-146. 

P xcept for tritium, carbon-14, and the long lived rare gases, the radio-active atoms produced by a nuclear detonation are accounted for completely within a population of radioactive particles. The nature of the particle population and the manner in which the individual radionuclides are distributed within it will vary with the conditions under which the detonation occurred. Characterization of the radioactive particle population requires ... 

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