1004 



EXPLORATION GEOPHYSICS 



This difficulty may be overcome with the vibrating-reed electrometer which 

 makes an excellent nonmicrophonic pulse amplifier for very low counting 

 rates (Figures 620 and 621), The collecting electrode is a circular grid, 

 parallel to the sample. Only the ions formed inside the cage are collected. 

 Most of the precise uranium and thorium measurements for the helium 

 method of geological age determination have been made by pulse chambers. 

 Proportional counters are instruments which utilize the principle of gas 

 amplification but operate in a voltage region too low for the Geiger discharge 

 (see Figure 611). They are usually filled with methane at atmospheric 

 pressure and require less powerful amplifiers than the pulse chambers. 



VIBRATING REED 



NITROGEN IN 



Fig. 620. — Pulse chamber for meas- 

 urement of very low a activities. 

 (Modified after Hurley.) 



Fig. 631. — ^(A) yibrating-reed (housed) and (B) asso- 

 ciated amplifying circuits. (Courtesy of Applied Physics 

 Corporation.) 



They will count )8's as well as a's. With the aid of a simple discriminator 

 it is possible to measure first the a activity of a sample and then the a and ji 

 activity by simply raising the voltage. A proportional counter with circular 

 center wire and the associated electronic equipment are shown in Figures 

 622 and 623. Although the proportional counter is finding its way into 

 geophysical research, it has not been used extensively in the past. 



The Geiger counter is the simplest radiation detector and therefore the 

 most commonly used in the laboratory. Depending on the construction, 

 it will count either y's alone of both /3's and y's, but the y counting efficiency 

 of mica-window y8-counters can be made very small, so that they count 

 essentially only yS's. The pulses from the counter are usually recorded with 

 a scaler that has a built-in regulated high-voltage supply for the G-M tube. 

 The counting equipment is identical for p and y counters. 



Laboratory counters are usually shielded with a few inches of lead or 

 iron to reduce the background radiation from softer components of cosmic 

 rays and radioactive contamination of the laboratory. 



