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                     Properties of Radiocesium and Potassium –40

Cesium is a soft gold coloured metal with a melting point of 28,5°C. It’s atomic number is 55 and has a relative atomic mass of 132.9. There are 35 known isotopes of the element cesium: Cs-114 to Cs-148 (SEELMANN EGGEBERT et al, 1981). The only naturally occurring isotope is the stable Cs-133. It is mainly found in the mineral pollucite at a concentration of up to 30% Cs20. All other Cs isotopes are artificial, the products of atomic bomb explosions or nuclear reactions.

Cesium as well as potassium belongs to the alkali metals and is the least inert and thus most reactive element in this group.  Its chemical and metabolic-physiological reactions are similar to those of potassium (DAVIS, 1963) which latter is essential for many organisms and is enriched intracellularly. However, Cs cannot replace K in its metabolic functions and is usually not taken up by organisms in the same proportion as potassium (KORNBERG, 1961):  The reason for this could be the differing ionic radii:  1.33
Å, for K+. 1.65 Å, for Cs+.  There is no proven biological significance of Cs for plants or animals.
 

 


In terms of radiation biology Cs-137 is the most significant isotope. It is formed with 6.2% splitting yield  in relatively large amounts in atomic bomb explosions   (KATCOFF, 1958) and with a physical halflife of 30.2 years remains in the environment for a long time. The decay scheme of the nucleid is presented in Figure 1.

Fig. 1 Decay scheme of Cesium-137 according to LEDERER et al (1967)


Cs-137 disentegrates with a probability of 6.5% directly and with a probability of 93.5% indirectly over the metastable Barium-137m into stable Barium –137. During indirect decay beta rays having an energy of 0.513 MeV are released.  The meta-stable Ba-137 disintegrates with a physical halflife of 2.55 minutes releasing gamma rays (0.662 MeV).  The determination of how active the Cs-137 is is deduced from the gamma rays. 

For radiation biological purposes Cs-134 is also, though of lesser importance than Cs-137.  Cs-134 decays with a physical halflife of 2.1 years emitting beta and gamma rays.  It originates mainly through neutron capture at the stable Cs-133 in nuclear reactors, and only occurs as a trace element in atomic bomb explosions.

In contrast to radiocesium Potassium-40 is a naturally occurring nucleid. It was formed together with the other elements during the creation of the earth. Due to its long halflife of 1.28 billion years it is still present on earth. K-40 is the only radioactive isotope of potassium and is present in an amount of 0.0119% in this natural element.  Further potassium include isotopes K-39 and K-41 with frequencies of respectively  93% and 6.9%. One gram of natural potassium contains 31.6 Bq K-40 (SEELMANN-EGGEBERT et al, 1981). Hence the activity of K-40 can be used to quantitatively determine total potassium. K-40 decays emitting ß-rays to stable Calcium-40 with a disintegration probability of 89%  and to stable Argon-40 emitting gamma rays at a rate of 11% (LEDERER et al, 1967). K-40 activity is determined by gamma ray decay at an energy of 1.461 keV. Potassium as well as K-40 are present in most terrestrial and biological substances, for ex. it is a macronutrient for plants. The body of a 70 kg person contains ca. 140 g of potassium and thus an activity of 4000 Bq K-40.  Due to its pesence in almost all foods this nucleid accounts for the greatest proportion of the naturally occurring radiation load through ingestion among people.


References:

Davis J. J., 1963: Cesium and ists relationships to potassium in ecology. in: Schultz V., Klement A. W. Jr. (eds.): Radioecology. ReinholdPpubl. Comp., New York: 539-556.
Lederer, M., Hollander, J.M., Perlmann, I., 1967: Table of isotopes. Wiley and Sons. New York.
Katcoff S., 1958: Fission product yields from U, Th, and Pu. Nucleonics 16: 78-85.
Kornberg H. A., 1961: The use of element-pairs in radiation hazard assessment. Health Phys. 6: 46-62.
Seelmann-Eggebert W., Pfennig G., Münzel H., 1981: Nuklidkarte. Gesellschaft für Kernforschung mbH. Karlsruhe, 5. Aufl..

 

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