138 ADVEXTURES IX RADIOISOTOPE RESEARCH 



Comment on papers 10 — 14 



When faced with tlie task of calculating the diffusion rate of gaseous oxygen 

 in gaseous nitrogen, to facilitate the calculation Maxw^ell made the assumption 

 that the molecules of oxygen and nitrogen have the same radius and same mass ; 

 he thus arrived at the notion of self -diffusion. The introduction of the labelling 

 principle made it possible to measure a diffusion rate close to self- diffusion. 



In connexion with the discussion of the interchange between lead atoms of a 

 lead foil and the surrounding lead ions the problem of the diffusion rate of solid 

 lead in lead was first raised in 1915 (paper 8). The first experiments in this field 

 described in paper 10 were, however, carried out a three years later only. Prior to 

 this investigation Groh and the wTiter measured the rate of diffusion of labelled 

 molten lead in non-labelled lead. Self-diffusion in liquids cannot be expected to 

 lead to results which cannot more or less be foreseen. The rate of diffusion of 

 molten lead in molten lead does not differ much from the diffusion rate of cadmiuiu 

 or thallium in molten lead. In contrast, the rate of seh-diffusion in solid metals 

 cannot be foreseen. The diffusion of a solid metal, even a closely related one, 

 in another metal produces changes in the crystalline state which may strongly 

 facilitate penetration. We found that the atoms of solid labelled lead diffused 

 into solid lead about 200 times slower than thallium atoms and about 10,000 

 times slower than gold atoms diffuse into solid lead. In the first investigation 

 on the diffusion in solids described in paper 9 labelled lead was soldered on a 

 non-radioactive lead rod. After keeping this system at 280°C for up to 400 days 

 shces from the rod close to the place of soldering were prepared and their radio- 

 activity compared. The figures obtained permitted the calculation of the upper 

 limit of the diffusion rate of lead in lead. In a later investigation (paper 11) 

 carried out with Mrs. Obrutsheva, wife of the weU-known Russian physical 

 chemist Frumkin, we increased the sensitivity of the method b;^- pressing in 

 vacuo a non-radioactive lead foil on one labelled with thorium B. The thickness 

 of the inactive foil was chosen slightly greater than the range of a-particles to 

 be measured ; therefore no scintillations originating from the radioactive lead 

 could be observed when investigating the inactive foil. But, on heating the aggre- 

 gate of the foils, a diffusion of the active lead into the inactive one took place 

 and the a-particles emitted by the succession products of ThB (ThB emits no 

 rx-rays, but comes rapidly into exchange equilibrium with a-particles emitting 

 desintegration products) produce scintillations on the observing screen. In further 

 investigations with Seith we replaced the counting of scintillations by ionization 

 measurements. The range of a-particles emitted by the disintegration product 

 of ThB in lead amounts to 3 x 10" ^ cm. A replacement within this thickness 

 of some of the ThB atoms by non-radioactive lead atoms due to an interchange 

 process leads to a decrease in the ionization measured. The range of the recoil 

 particles emitted by the ThC, the disintegration product of ThB, is stiU appreci- 

 ably (almost 1000 times) shorter than that of the a-particles. The measuring of the 

 decrease of the recoil yield with time of a with ThB covered surface perinits to 

 determine as low a diffusion rate as 10~i3 cm^day^ By making use of this me- 

 thod self-diffusion in solid lead taking place at 106°C or at a higher temperature 

 Avas measured (paper 12). From the change of the rate of self- diffusion with 

 temperature the value prevailing closely to the melting point was calculated 



