CiiAP. 12] MISCELLANEOUS GEOPHYSICAL METHODS 877 



volts •sec~\ The equivalent Ra content is obtained by comparison with 

 a standard powder of identical thickness. If the standard weighs 3 g and 

 produces a decay of 10 scale divisions in 1 minute, 8 seconds, the reduced 

 decay is (750 — 3.5)/3 = 238.8 -10" volts -sec"^ per gram of substance. 

 Assuming the standard to contain 2.5-10" g Ra g"^, the equivalent radium 

 content of the specimen is (46.3/238.8) -2.5.10"' = 4.8G • 1 0"^ g Ra g~'. In 

 this method, therefore, the entire radiation (inclusive of that of thorimii) 

 is expressed by the equivalent Ra content. 



Measurement of the activity of the emanation imder (2) gives the Ra 

 content alone. ^° The specimen is ground up and about ^ g is fused with 



xx>z 



60Z 

 401 



Per Cent 

 t^adon 



Time In hours 



5 W ^0 40 60 too m 400 600 



Fig. 12-2. Percentage of radon (developed in confined space) as a function of time 



sodium carbonate. The alkaline and basic solutions are kept separately 

 in two sealed flasks for about twenty-five days. Within this time a maxi- 

 mum amount of radon has developed and is in radioactive "equilibrium" 

 with the Ra in the solution, since, from then on, as much radon is formed as 

 in turn decomposes. For strongly radioactive solutions it is not necessary 

 to wait twenty-five days, since the amount of radon present at any time 

 may be calculated in relation to the maximum amount developed (see 

 Fig. 12-2). One of the flasks is then placed on a shaking platform as 

 shown in Fig. 12-1, and air is forced through the solution by a pump or 

 aspirator whose intake connects to the ionization chamber outlet so that 



1" Within the time required for this experiment, the short-lived thoron (54 sec. 

 half-value time) has decayed completely. 



