THE APPLICATION OF KADIOACTIVK KECOIL IN DIFFUSION MEASUREMENTS I'.i J 



The pellet can b(> hiought up to the experimental temperature for a 

 definite time by attaching at S a furnace containing a glass vessel with a 

 standard joint. In the glass vessel is a hollow iron block (1.5 kgm) into 

 which the pellet and the brass cylinder can be introduced such that the 

 direct contact with the metal facilitates rapid equalizationof the temper- 

 ature. The temperature is measured by Hoskins' method by means of 

 a thermoelement of high thermoelectric power fixed in a side hole in th(^ 

 iron block. This apparatus also is filled with nitrogen. 



The lead chloride was prepared from Kahlbaum purest lead chloride 

 by repeated recrystallization from hydrochloric acid solution. It was 

 dried by heating nearly to the melting point in a current of HCl. 



The lead iodide was obtained from an active acid solution of HI and 

 Pb(N03)2, purified by decantation and dried over PgOg. Sublimation 

 was avoided since Pbig prepared thus always contains traces of iodine. 



CALCULATION 



AVe are indebted to Professor R. Fiirth of Prague for the formulae 

 used below. 



The calculating procedure is as follows : If the recoil activity before 

 the experiment is equal to 1 and after the experiment to A, then 1—^ 

 atoms of lead have diffused so far in to the pellet that their recoil products 

 are no longer able to leave it. If all the recoil particles moved perpendi- 

 cular to the surface, then all those issuing from lead atoms which had not 

 diffused deeper than the range, a, of the recoil particles would be able 

 to escape from the surface. The relationship between the number A and 

 the diffusion constant D is therefore 







where Z is the time and x is the distance of the particle from the surface. 

 By using the Gaussian error function 



(2) 



the equation 



A — an _ 



2]/DZ 

 is obtained, whence D can be calculated 



A^yji-^] (3) 



