THE APPLICATION OF RADIOACTIVE RECOIL Ix\ DIFFUSION MEASIREMENJ .s 1 2i) 



collecting atoms of the lead istotope on a lead surface and counting the 

 number of scintillations shown by the infinitely thin radioactive coating 

 before and aft(>r diffusion in the heated metal. The decrease in the num- 

 ber of scintillations is a measure of the velocity with which the radioactive 

 lead atoms have passed, because of diffusion, outside the range of the 

 a-rays and into the interior of the metal. The layer thickness here required 

 toi- the diffusion calculation is the range of the a-rays in lead, which 

 amounts to about 1/30 mm. Diffusion constants down to lO-s cm'^ day-i 

 were measured with the help of this method. This sensitivity, however, 

 was inadequate in the present study and we therefore used the radioactive 

 recoil effect which can be expected to yield a considerable increase in 

 sensitivity for detection of the diffusion. The range of a-recoil in lead 

 amounts to only 3x10-^ mm and thus the application of the recoil 

 method permits measurement of the extraordinary small diffusion 

 constant of 10-^=^ cm^ day ^ The velocity of diffusion of the ions of the 

 lead isotope ThB (half-life 10.6 hr) was measured in different compounds 

 lead. The recoil yield thus the activity of ThC, was determined before 

 and after diffusion. The radioactivity measurements were made only 

 after the establishment of the equilibrium between ThB and ThC, since 

 the a-rays responsible for the recoil effect are derived not from thorium-B 

 but from its daughter product thorium-C (half-life 1 hr). 



EXPERIMENTAL METHOD 



The radioactive substance was condensed from the vapour phase on 

 the pellet to be used for measuring the velocity of diffusion. The pellets 

 were prepared by pressing very carefully purified lead chloride or iodide. 

 The pressure used was 1800 kgm/cm^ and was applied for 1 min. 



The pellet was pressed on to the front of a 14 mm diameter brass 

 cylinder and the two together were suspended in the apparatus (Fig. 1). 

 This apparatus consists of two chambers A and B connected by means of 

 a cock with a 2 cm bore. Each chamber can be separately evacuated and 

 filled with purified nitrogen. The brass cylinder with the pellet is fastened 

 l)y means of a silver wire to a screw, C, situated above the chamlx'r 

 .4. A phosphorus pentoxide tube is first attached to the standard joint. 

 8, and the whole apparatus is filled with nitrogen. 



The ThB chloride is condensed on to the lead chloride surface in t he 

 following way : the active deposit from thorium is collected on a plati- 

 num foil and the foil is then exposed to the action of chlorine gas. The 

 foil is transferred into the vaporization apparatus, which is attached at 

 S (see Fig. 1) while the pellet is situated in the chamber A. The vapor- 

 ization chamber consists of a brass tube K, which can be cooled, into 

 which the brass cylinder with the pellet just fits. The lower joint of 



9 Hevesy 



