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ADVENTURES IN EADIOISOTOPE EESEARCH 



Table 1 



atoms. The lead atoms on the contrary scarcely ever leave their places. 

 At room temperature, a lead atom in pure lead changes its place once 

 in 10 days while the silver atom in lead-silver alloy vacates its position 

 100 times per sec.^ As the temperature is raised the difference diminishes. 

 \Ve have already been able to demonstrate, in an earlier paper^, in the 

 case of silver ahoys that the speed of place exchange of the atomsde- 

 creases as the ideal metallic state is approached. The last step, however, 

 i.e. the measurement of self-diffusion in silver, was not practicable in 

 the systems mentioned. We have therefore applied our attention chiefly 

 to lead alloys because they presented an opportunity for determining 

 also the self-diffusion in lead. 



In determining the frequently very low velocity of diffusion, use was 

 made of quantitative optical-spectroscopic analysis which has been 

 found especially suitable for this purpose. Indeed this methods permits 

 both the determination of very small amounts of metal and the use of 

 very thin and strictly localized layers in the analysis. Samples of known 

 composition are first prepared, e.g., lead-thallium alloys with the 

 concentration varying between 3 and 0.01 per cent and the ratio of the 

 intensities of suitable lines of the two elements is determined. Layers, 

 each 0.05 mm thick, are cut after diffusion and these are analysed 

 spectroscopically by comparing with the test samples in accordance 

 with the method described by Gerlach^. The diffusion constant can be 

 calculated since the concentration, as a function of the distance from the 

 original boundary, is known. In the method described it is above all 

 necessary to ensure that the metal whose diffusion constant is to be 

 determined is distributed as atoms in the lead. If this does not appl} 

 1hen the analytically determined concentration is not identical Avith 



1 Concerning the calculation of the frequency of place exchange of an indi\i(lual 

 atom, refer to H. Braune, Z. phys. Chem. 110, 147 (1924) ; J. Frenkel, Z. Fhys. 

 35, 652 (1926) : J. A. M. van Liempt, Z. anorg. Chem. 195, 366 (1931). 



2G. Hevesv, Z. Elektrochem. 34. 463 (1928). 



3 W. Gerlach and E. Schweitzer, Chemical Emission-Spectrum Analysis, 

 Leipzig (1930). More details of the application of this method to measurements 

 of diffusion will shortly he described bv Guenther and Laird. 



