THE APPLICATION' OK RADIOACTIVE KKCOILI.N DirjrSlQN MEASUREMENTS 133 



and if also the retarding efl'ect due to the air column of the shutter is 

 equal 1o that of a Pbig layer witli a thickness b, then 



a—b 



J ]l{nDZ) ' 







In llie al)()\(' equal ion 



I = {a-h)!2]f{DZ) (11) 



and the expression is evaluated grapiiically. 



THE DIFFUSION OF LEAD IONS IN LEAD CHLORIDE 



If an at1(Mn])t is made to determine the diffusions of huid ions in lead 

 chloritle witli ihe ordinary apparatus of Stefan, by pressing together a 

 3 mm deep inactive lead chloride pellet and a 1 mm deep ThB-labelled 

 lead chloride layer, no diffusion of the radiactive lead ions can be detected 

 after 4 days at 480°C. It is not feasible to raise the experimental 

 temperature since the high vapour pressure of lead chloride at the 

 above-mentioned temperature (lO^i mm) already causes disturbance. 

 The interference can indeed be partly restrained by carrying out the 

 experiment in a pressure bomb under a nitrogen pressure cff 200 atm, 

 but cannot be wholly eliminated. For the reasons mentioned it also seemed 

 hopeless to prolong the duration of the experiment, possibly by replacing 

 ThB by the long-lived RaD. For the same reason, the determination of 

 the diffusion constant of lead chloride, in the vicinity of the melting point . 

 by means of the decrease in a-radiation after heating ThB chloride col- 

 lected on the surface of a PbCla pellet was also a failure. Even at 370( '° 

 the amount of ThB which evaporates can be detected by radioactive me- 

 thods. The diffusion constant of lead ions in lead chloride must therefore 

 be measured at lower temperatures at which only the very sensitive recoil 

 method can be considered. The results of the measurements obtained by 

 this method are shown in Fig. 2 and Table l.The circles relate to a series 

 of experiments in which the active deposit was treated with chlorine. 

 The circles combined with strokes relate to experiments where ThB 

 oxide or sulphide, instead of the chloride, was condensed on the PbClg 

 pellet. Another series of results not quoted here yield Ihe same graphical 

 pattern. 



The time of experiment was so chosen that the decrease in the recoil 

 yield after diffusion amounted to about 50 per cent. This circumstance 

 is most favourable both for carrjdng out the experiment and for calcula- 

 tion. It is necessary to know the range of the recoil rays in lead chloride 

 in order tobeable to ealeulate the diffusion constant from equation 8 on 



