307 



An Electroscope for Measuring the Radioactivity 



Soils. 



By R. R. Ramsey. 



In measuring the radioactivity of soils if extreme accuracy is desired it is 

 necessary to dissolve the sample and then determine the amount of radium 

 or thorium by means of the emanation method. The getting the sample in 

 solution is a long tedious process. For a description of this method I shall 

 refer to Joly's Radioactivity and Geology. 



For an approximate determination of the radioactivity one can use an a 

 ray electroscope provided that the sample is fairly active. The standard 

 being uranium oxide, U. 5 0$, a "thick" layer, one gram to 10 square centimeters 

 say. gives a current of o.SxlO- 13 amperes or 17.4xl0- 4 E.S.U. per square 

 centimeter surface if the plates of the electroscope are 4 cm. or more apart. 

 The amount of radium in the oxide may be determined by dissolving it and 

 then determining the amount of emanation in the solution after it has stood 

 30 days. The sample is placed in the a ray electroscope and compared 'with 

 the uranium oxide. It will be evident that an assumption is made here that 

 the absorption coefficient of all samples for a rays is the same as the absorp- 

 tion coefficient of uranium oxide for a rays. This assumption is only approx- 

 imately true. 



The radioactivity of soil is very slight and in order to get an appreciable 

 current a large area must be exposed. This necessitates large plates in the 

 ordinary form of a ray electroscope. The large plates increases the capacity 

 of the electroscope and thus diminishes the sensitiveness of the electroscope. 

 Instead of an ionization chamber with plates I have hit upon the plan of 

 using a cylindrical chamber with a central rod. The material to be tested is 

 packed between the wall of the cylinder and an inside cylinder made of wire 

 gauze. The space between the two walls is made as small as the ease of fill- 

 ing w-ill permit. One or two centimeters, say. 



In this form of electroscope the amount of surface exposed can be increased 

 at will by increasing the size of the cylinder, and as the diameter of the 

 cylinder is increased the capacity is decreased. Thus the sensitiveness of the 

 electroscope is increased in two ways as the ionization chamber is increased; 

 by increasing the surface exposed and by decreasing the capacity of the instru- 



