RADIOACTIVITY METHODS 



999 



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reliable, but counting the Geiger impulses by ear is very tedious and limited 

 to low counting rates. The human ear cannot reliably follow more than 

 about 150 to 200 random counts per minute, and the human error is great 

 even at very low counting rates. Therefore it is desirable to eliminate the 

 necessity of counting by ear either by amplifying and recording the Geiger 

 pulses mechanically with an impulse register (relay-driven clock) or by 

 integrating the current flow resulting from the passage of amplified pulses. 

 Impulse recorders are most useful for low rates (up to about 300 counts per 

 minute) and rate meters are usually used for higher activities. 



The lowest potential conveniently used with conventional modern G-M tubes is 

 about 800 v. Various types of vibrators and oscillators have been used to supply this 

 voltage, but their output must necessarily be kept small to conserve battery power, and 

 proper voltage control therefore be- 

 comes difficult. Consequently oscil- 

 lator-powered high-voltage circuits 

 are not well suited to accurate quan- 

 titative work in the field, although 

 they are quite sufficient for rough 

 qualitative detection (Figure 612). 

 The pocket-type instrument shown 

 in this figure has a vibrator-powered 

 auditory counter. All power is sup- 

 plied by two flashlight cells. 



Dry cell batteries should supply 

 the high voltage in instruments to 

 be used in accurate surveys where 

 good reproducibility is all-impor- 

 tant. Small batteries are manufac- 

 tured commercially in 300-volt 

 packs. Three packs weigh about 3 

 lbs. 3 ozs., and occupy about 59 

 cubic inches. They are frequently 

 used as a 900-volt supply for stand- 

 ard G-M tubes in precision instru- 

 ments. 



A cathode-coupled multivibra- 

 tor circuit* (Figure 613) makes a 

 very good "amplifier" for Geiger 



pulses because a small trigger pulse fed into the circuit produces a much more powerful 

 "shaped" output pulse. The effective gain per tube is considerably higher than that 

 obtained from a conventional amplifier, resulting in economical power drain which in 

 turn permits the use of small batteries. 



The multivibrator may be adapted to drive a commercial impulse register by using 

 an R-F pentode (such as the 1N5) and a power amplifier tube (1T5, 1Q5, or similar) 

 for Vi and V2 respectively. The register is connected at R, and a 22J/2 v. "C" battery 

 provides bias to cut off the power tube. Most counting rate meters, on the other hand, 

 use subminiature tubes (Figure 614) for Vi and V2, since these tubes require as little 

 as 15 milliwatts of filament power per tube. A microammeter is then connected at R, 

 by-passed by a high-capacity electrolytic condenser to form a storage circuit (Figure 

 615) which smooths out the output pulses to give a steady current proportional to the 



Fig. 612. — Simple Geiger-Miiller counter with 

 vibrator high-voltage supply. (Courtesy of Nuclear 

 Instrument and Chemical Corporation.) 



* Also known as Schmitt's trigger circuit. For an extensive discussion of trigger circuits, see 

 O. S. Puckle, "Time Bases," Chapter 4 (John Wiley and Sons, New York, 1943). 



