Positron life-time measurement

The flow distribution in a model soil column (length 1 m, diameter 10 cm) with defined disturbing inserts (Fig. 2.1) was studied to demonstrate the feasibility of PET-studies. The measurements were carried out with a commercial PET-camera (Siemens ECAT EXACT HR (3D)). Kaliumfluoride, marked with the positron emitting isotope F-18 (half life time 110 min), was applied as hydrodynamic tracer. Fluorides of two-valent cations are only slightly soluble. The radio tracer would be absorbed in the col-... 

The life-times of positrons in a solid depend upon the particular mechanism of annihilation (e.g. through formation of positronium or positron-ion combinations). Consequently, measurements of positron annihilation rates give results which again may be interpreted in terms of electronic structure. 

A final, highly specialized procedure for monitoring carbon flow in plants involves the use of "CO . The use of this positron gamma-emitting isotope of C, with a half-life of 20.3 min, allowed several physiological parameters of my-corrhizal and non-mycorrhizal plants of Panicum coloratum to be measured simultaneously in real time (82). However, the technical problems associated with using "COi, particularly the ability to produce i.sotopes of carbon, and with handling this short half-life are likely to limit this approach to specialised facilities. 

In CF the superallowed transition of 1.6 sec half-life is the ground state decay. There is an isomeric state at 142 keV which was for some time thought to be the ground state, but which actually feeds it by an M3 transition whose internal conversion coefficient has been measured. This means that the ground state of Cl has spin 0 and isotopic spin 1 while the 142 keV level has spin y and isotopic spin zero. This level also decays by allowed positron emission to... 

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