12(1 



ADVENTUJRES IN BADIOISOTOPE KESEARCH 



Table 3. — Diffusion Constant (D) of Thoriitm B in 

 Lkad Determined by the Reooil Method 



lallites demonstrated in those experiments could not be reproduced, 

 possibly because the single crystals in the preliminary experiments 

 were unavoidably exposed for a long time to contact with the air (during 

 counting of the scintillations). The present study shows much more 

 pointedly that neither the heat of relaxation of the lead lattice nor the 

 velocity of self-diffusion of lead atoms is structure-sensitive. This result 

 is possibly connected with the ready recrystallizability of lead since in 

 the molybdenum — tungsten system van LiemptI was unable to find a 

 structure dependence of Q, yet A and therefore the diffusion constant 

 showed such dependence. He found A to be about eight times as large 

 in polycrystalline material as in a single crystal, by measuring the veloc- 

 ity of diffusion of molybdenum in tungsten, and even earlier a structure 

 sensitivity of the electrolytic conductivity, which is closely related to 

 the self-diffusion, had been demonstrated in salts^. 



VAN LiEMPT calculates the constant A from the equation 



A = n .r2 r/6 



^\here x is the distance l)etween lattice planes and v the vibrational 

 frequency of the atom. In the case of molybdenum diffusing in tungsten 

 single crystals he finds remarkably good agreement between the observed 

 and calculated values of A. The value of A which we have measurerl 

 in lead is, on the contrary, about one thousand times the value calculated 



IS. A. M. VAN LiEMPT, Z. (tnorg. diem. 195, 3GG (1931): Rec. Trav. Chim. 

 51, 114 (1932). 



2G. Hevesy, Z. Phys. 10, 80 (1922) : G. Tammann and G. Veszi, Z. anorg. 

 Chcm. 150, 355 (1926) ; T. E. Phipps, W. D. Lansing and T. G. Cooke, J. Amcr. 

 (hem. Soc. 48, 112 (1926), eiv. 



