Chap. 12] 



MISCELLANEOUS GEOPHYSICAL METHODS 



881 



The type of emanometer described here may be applied also in meas- 

 uring the activity of spring and soil waters ("f on tactometer"). The water 

 is placed in a bottle similar in construction to that shown on the shaking 

 table of Fig. 12-1 and connected to a rubber bulb aspirator in such a 

 manner that the air circulates continuously through the water and the 

 ionization chamber. After the bottle has been shaken for about 1 or 2 

 minutes and the air been circulated for about the same time, the cocks of 

 the chamber are closed and the electroscope is read in the usual manner. 

 The dispersion is referred to a water volume of 1 

 liter, and the current is calculated from formula 

 (12-1) (in e.s.u.'s); multiplication by 1000 gives 

 the activity in Mache units. ^^ 



(c) The 'penetrating radiation is measured with 

 an ionization chamber which is shielded above 

 and on its sides against the "softer" radiations. ^^ 

 Theoretically, a minimum thickness of 2.93 inches 

 of (inactive) lead is required. Bogoiavlensky 

 used a chamber with brass walls 3 mm thick and 

 lead shields 10 mm thick. The bottom is closed 

 off by filters of varying thicknesses, which make 

 it possible to determine the absorption coefficient 

 and therefore the wave length of the radiation. 

 It is probable that the radiation increases in 

 hardness with the depth of the source. In Bogoiav- 

 lensky's apparatus the filters were each 2.5 mm 

 thick, the volume was 1650 cm , the capacity 

 0.725 cm, and the sensitivity 0.65 volts per divi- 

 sion. 



Another convenient method of measuring 

 gamma radiation is the use of a Geiger counter. 

 In its most widely used form it consists of a 

 wire surrounded by a cylindrical metal tube, 

 sealed in a glass tube in an atmosphere of 



argon under about 8 mm pressure. The cylinder is usually at a nega- 

 tive potential with respect to the wire. Alpha and beta radiation 

 is almost completely rejected by this type of counter. The gamma 

 radiation, however, passes readily through the glass and liberates electrons 



Fig. 12-4. Ambronn 

 emanometer. (1) Ioniza- 

 tion chamber, (2) elec- 

 trometer, (3) microscope, 

 (4) charging peg, (5) dry- 

 ing tube, (6) alcohol 

 burner, (7) tripod. 



1* Since one M.U. produces a saturation current of 1/1000 e.s.u.'s, see p. 873. 



19 L. N. Bogoiavlensky and A. Lomakin, Zeit. Geophys., 3, 87-92 (1927); U. S. 

 Bur. Mines, Circ. Inf. No. 6072, 1928; Inst. Pract. Geophys. Bull. 1, 57, 69, 184 (1925), 

 2, 184-195 (1926), 3, 87-112 (1927), 4, 165-178 (1928). 



