882 



MISCELLANEOUS GEOPHYSICAL METHODS 



[Chap. 12 



from the metal. These electrons, in rushing toward the wire, form positive 

 ions and new electrons, building up the current exponentially until the 

 potential difference drops to a point where ionization by collison can no 

 longer occur. Thence, the potential recovers at a rate depending on the 

 time constant of the circuit (current "pulse"). The current is practically 

 independent of the number of ions formed by the original electron f° the 

 same is true for open counters sensitive to alpha particles and beta rays. 

 The pulses may be counted by an arrangement shown in Fig. 12-5 in 





Counter 



V, 



Fig. 12-5. Geiger-Mueller tube, with amplifier and counter (after Neher). Ri 

 = 5 meg.; R2 = 1 meg.; R^ = Ri — Rs = h meg.; fls = i meg.; Rj = 10,000 ohms; 

 Rs = current-limiting resistor; Rg = 80,000 ohms; Ci = 50 mm /> d — 0.2 m/J C's = 

 0.1 nf; Vi = 100 volts; V2 = 250 volts. The tube following the counter is a 57 and 

 the output tube an 885. 



which the cathode, grid, and screen of the 57 tube and the wire of the 

 counter are all at a high positive potential. When a rush of electrons 

 passes the counter, the grid goes negative, blocking the current through 

 the tube. Cathode, grid, and so forth, drop rapidly to ground potential. 

 When the counter potential drops below the threshold value, the current 

 ceases. The negative charges leak off the grid; and the tube, counter, and 

 associated circuit are recovered for another count. The positive pulse thus 

 produced gives rise to a gas discharge in the 885 tube, actuating the re- 

 corder K and charging the condenser C2 which, by its effect on the bias, 

 stops the discharge in the tube. In a portable instrument, the high 

 voltage batteries required for a counter are annoying and may be elimi- 



2' H. V. Neher, in J. Strong, Procedures in Experimental Physics, p. 259, Pren- 

 tice-Hall, 1938). 



