1000 



EXPLORATION GEOPHYSICS 



number of pulses per unit of time. The "C" battery is omitted when subminiature tubes 



are used, but provision is made to vary the coupling constants Ri and Ci in two or three 



steps so that a single instrument may have several ranges of sensitivity (Figure 616). 



A great variety of small counting rate meters is available, but few of these instru- 



FiG. 613. — Portable Counter Circuit. Sym- 

 bols: G-M, Geiger-Miiller tube; V\, first stage 

 (normally on) ; F2, second stage (normally off) ; 

 R, recorder or rate meter connections; HV, high 

 voltage batteries; "B", "C", plate and bias bat- 

 teries. (Modified after O. H. Schmitt, Jnl. Sci. 

 Inst. 15. 24, 1938.) 



Tm 



Fig. 614. — Typical subminiature pentode 

 and socket, enlarged X 1.5. (Courtesy of 

 Raytheon Manufacturing Co.) 





ments are designed for mineral exploration. For accurate surveying, the instrument 

 should fulfill the following requirements : 



a. All scales should be approximately linear, i.e., the meter reading should be 

 directly proportional to the true counting rate. (The linearity may be checked by 

 tapping the pulses from the G-M tube directly into a scaler and comparing the scaler 

 count with the rate meter reading at various counting rates.) 



b. The factor between the various ranges 

 of sensitivity should be constant. A factor of 10 

 is generally used. (Its constancy is again best 

 checked with the aid of a scaler.) 



c. The reading of the instrument should not 

 vary with temperature and humidity. All high- 

 voltage connections should be well protected 

 against moisture, to prevent spurious "counting." 

 Although an external probe is frequently conve- 

 nient, a G-M tube built into the instrument case 

 (with short high voltage leads) is to be pre- 

 ferred for work in humid surroundings, especially underground. 



d. Provision should be made to check the B battery voltage easily. The reading 

 of all counting rate meters falls off with decreasing plate voltage, and a constant check 

 is essential if the measurement is to be reproducible. For the same reason, B batteries 

 should be large enough to maintain voltage for at least eight hours of continuous 

 operation. 



Mobile Field Equipment 



The measurement of radioactivity in drill holes and a iew other appli- 

 cations require integrating instruments of great sensitivity, good stability 



Fig. 615. — Capacity resistance type "stor- 

 age" circuit used in counting-rate meter. 



