878 MISCELLANEOUS GEOPHYSICAL METHODS [Chap. 12 



air is continuously circulated through the apparatus. The air-radon mix- 

 ture then passes through a drying tube and ion trap, and then into the 

 ionization chamber. 



The calculation of the equivalent radium content is done as follows, 

 after a standard Ra solution has been treated in the same manner as the 

 acid and alkaline solutions of the sample. If | g of rock was treated and 

 the acid solution was left alone for 26 hours, 17.9 per cent of the maximum 

 radon was formed. When the air-gas mixture was forced through the 

 ionization chamber, the dispersion was 10 scale divisions in 2 minutes, 22 

 seconds. With a scale value of 51 volts and a normal dispersion of 3.5 

 millivolt • sec~\ this is 359 — 3.5 = 355.5 10"^ volts sec"^ and, reduced 

 to maximum emanation emission, 1985-10" volts -sec". The alkaline 

 solution was kept more than thirty days and produced, in 8 minutes, 10 

 seconds, a decay of 10 scale divisions. This yields, with the above scale 

 value and normal dispersion, 104 — 3.5 = 101.5-10"^ volts •sec~\ If this 

 is added to the decay of the acid solution and reduced to 1 g of substance, 

 the emanaxion yield of the specimen is proportional 4173-10" volts -sec" . 

 To obtain the equivalent Ra content, the specimen is now compared with 

 a standard solution of, say, 1.19-10"* g Ra, which, when treated in the 

 same manner as the two sample solutions, produces a dispersion of 200 • 10" 

 volts •sec"\ Hence, the specimen contained (1.19-10" •4173)/200 or 

 2.29 10"'gRag"\ 



In method (3) the radium content of a specimen is determined from its 

 penetrating radiation. The specimen is set up at a fixed distance from 

 the electroscope, which may be protected from the alpha and beta rays by 

 a lead screen about j inch thick. The equivalent Ra content may be 

 determined by comparison with a standard. The electroscope method is 

 suitable only for strongly radioactive minerals and rocks ; more universally 

 applicable are Geiger counters (see section c) . The specimen is placed at a 

 fixed distance from the counter and its radiation is compared with that 

 of a standard preparation. It is advantageous to surround the lower half 

 of the counter (axis horizontal) with a heavy lead shield and to expose 

 only the upper half of the counter tube" to the radiation so that the 

 effective counter area is the section of the tube. The ionization of the 

 gamma radiation from a milligram of Ra at a distance of 1 m amounts to 

 about 120 impulses^ per cm^ per minute of the effective counter area, and 

 it decreases with the square of the distance. 



C. Instruments and Procedure in Radioactivity Exploration 



Radioactivity exploration may be carried out in two ways: (1) by taking 

 soil samples and testing them in the laboratory for their radioactive prop- 



" A. K. Das and K. Wolcken, Phys. Zeit., 31, 136-139 (1930). 

 12 Ibid. 



