Physical half-life

HALF-LIFE, EFFECTIVE Time required for a radioactive nuclide present in an organism to be diminished by 50% as a result of both radioactive decay and biological elimination. Symbol Teff Tcff = tb X L/(tb+L). (See also HALF-LIFE, BIOLOGICAL and HALF-LIFE, PHYSICAL)... 

HALF-LIFE, PHYSICAL (RADIOLOGICAL) Time required for a radioactive substance to decay to 50% of its initial activity. Each radionuclide has a unique half-life. Symbol L. 

The isotope produced was the 20-hour 255Fm. During 1953 and early 1954, while discovery of elements 99 and 100 was withheld from publication for security reasons, a group from the Nobel Institute of Physics in Stockholm bombarded 238U with 160 ions, and isolated a 30-min alpha-emitter, which they ascribed to 250-100, without claiming discovery of the element. This isotope has since been identified positively, and the 30-min half-life confirmed. 

Fourteen isotopes are now recognized. 258Md has a half-life of 2 months. This isotope has been produced by the bombardment of an isotope of einsteinium with ions of helium. Eventually enough 258Md should be made to determine its physical properties. 

Radon is the heaviest of the hehum-group elements and the heaviest of the normal gaseous elements. It is strongly radioactive. The most common isotope, Rn, has a half-life of 3.825 days (49). Radon s scarcity and radioactivity have severely limited the examination of its physical properties, and the values given ki Table 3 are much more uncertain than are the values Hsted for the other elements. 

Most uranium ore has a low, ca 1 part in 500, uranium content. Milling involves physical and chemical processing of the ore to extract the uranium. The mill tailings, which release gaseous radon-222 [13967-62-9] Ra, half-life 3.82 d, are placed in large piles and covered to prevent a local health problem. 

An extensive pesticide properties database was compiled, which includes six physical properties, ie, solubiUty, half-life, soil sorption, vapor pressure, acid pR and base pR for about 240 compounds (4). Because not all of the properties have been measured for all pesticides, some values had to be estimated. By early 1995, the Agricultural Research Service (ARS) had developed a computerized pesticide property database containing 17 physical properties for 330 pesticide compounds. The primary user of these data has been the USDA s Natural Resources Conservation Service (formerly the Soil Conservation Service) for leaching models to advise farmers on any combination of soil and pesticide properties that could potentially lead to substantial groundwater contamination. 

The alkali metals form a homogeneous group of extremely reactive elements which illustrate well the similarities and trends to be expected from the periodic classification, as discussed in Chapter 2. Their physical and chemical properties are readily interpreted in terms of their simple electronic configuration, ns, and for this reason they have been extensively studied by the full range of experimental and theoretical techniques. Compounds of sodium and potassium have been known from ancient times and both elements are essential for animal life. They are also major items of trade, commerce and chemical industry. Lithium was first recognized as a separate element at the beginning of the nineteenth eentury but did not assume major industrial importance until about 40 y ago. Rubidium and caesium are of considerable academic interest but so far have few industrial applications. Francium, the elusive element 87, has only fleeting existence in nature due to its very short radioactive half-life, and this delayed its discovery until 1939. 

Polonium, because of its very low abundance and very short half-life, is not obtained from natural sources. Virtually all our knowledge of the physical and chemical properties of the element come from studies on Po which is best made by neutron irradiation of in a nuclear reactor ... 

Half-life, Radioactive—Time required for a radioactive substance to lose 50% of its activity by decay. Each radio-nuclide has a unique physical half-life. Known also as physical half-time and symbolized as Tr or Trad. 

Tritium—The hydrogen isotope with one proton and two neutrons in the nucleus (Symbol 3H). It is radioactive and has a physical half-life of 12.3 years. 

Table 1 Physical constants Isotopic abundance Half-life...

FIGURE 10 The half-life. It is impossible to predict when a radioisotope or an unstable substance (molecule) will decay or be decomposed. On an average, however, only half of any type of radioisotope or unstable substance (molecule) remains after one half-life (A/2) one-quarter will remain after two half-lives (A/A), one-eighth after three half-lives (A/8), and so on. The half-life is characteristic of every radioisotope and unstable molecule that of radioisotopes is not affected in any way by the physical or chemical conditions to which the radioisotope may be subjected. Not so the half-life of chemically unstable molecules, which is altered by changes in temperature and by other physical and chemical conditions. 

Diamond, H., and R.F. Barnes Alpha half-life of Pu244 Physic. Rev. 101,... 

G. R., Biological half-life of tritium, Health Physics 1963, 9, 911-914. 

Retention of cerium deposited in these tissues is generally considered to be very long, with a biological half-time in excess of a thousand days. Therefore, the effective half-time of retention in these internal organs will be approximately equal to the physical half-life. 

Any alpha emitter with physical half-life 3.7 TBq (100 TpCi) 3... 

---