RADIOACTIVITY METHODS 1003 



fore the probe should be smaller than 1 inch overall outside diameter. Modern sub- 

 miniature tubes, small high-efficiency pulse transformers, and "hearing-aid" type con- 

 struction make it possible to make a probe about % inch in diameter, using a simple 

 G-M counter tube. The light casing (1 inch inside diameter) can be set and pulled by 

 hand down to about 200 feet. A pipe clamp is used to hold the string while sections 

 are added. 



The pulses from the G-M tube are integrated to give a continuous graph of the 

 pulse counting rate plotted against depth in the hole. The principle of a counting rate 

 meter is deceptively simple (Figure 615), but the detailed design is fairly difficult. The 

 pulses coming into the circuit pile up on the condenser, and slowly leak off through the 

 meter. If the meter reading is to have any significance, all the pulses must be equal in 

 the amount of energy that they transmit. Geiger pulses are not all strictly alike and 

 must be shaped before they can be metered. The shaping circuits are similar to that 

 shown in Figure 613. The shaper must have excellent stability and sufficient power to 

 put out a pulse large enough to deflect the meter and fast enough to trigger in time for 

 the next pulse to be received. The speed is governed by the product of RiCi, which is 

 the trigger coupling time constant (Figure 613). 



The rate meter resistance (R, Figure 615) must be so small that the voltage drop 

 across it at full meter deflection is still negligible compared to the plate voltage of the 

 feeding tube. The two principal causes of nonlinearity in counting rate meters are 

 insufficient speed of the shaping circuit and excessive meter resistance. 



The product of the meter resistance R and capacitance C determines the magnitude 

 of the time interval AT over which pulses are integrated. At a fixed counting rate, the 



statistical fluctuation of the needle will be proportional to y/AT. When a constant 

 source of radiation is placed near a counter feeding into a rate meter, the reading will 

 climb toward the equilibrium position at an exponentially decreasing rate. For most 

 practical purposes the needle reaches equilibrium in the approximate time 



T~3RC 



Where T is in seconds, R in ohms, and C in farads. The design of the logging instru- 

 ment must be such that the probe travels only a small distance, compared to the length 

 of the detector, in the time T. 



The plate voltage supply for a counting rate meter must be free from ripple and 

 well controlled to maintain constant amplification throughout the measurement. Fre- 

 quent calibration of rate meters is essential to guard against the error arising from 

 drift, usually caused by fluctuating plate voltage and the progressive decrease in cathode 

 emissivity of the rate-meter vacuum tube. 



Laboratory Instruments for Radioactivity Measurement 



A staggering variety of detectors and accessory equipment is available 

 to measure the radioactivity of samples in the laboratory. The principal 

 types of equipment are described, and the reader will find additional infor- 

 mation in the references. 



Detectors. — The pulse chamber is primarily an instrument for count- 

 ing a's. The usual shape of the chambers is broadly cylindrical with the 

 collecting electrode in the form of a grid or plate. It is usually filled with 

 argon, nitrogen, or air at atmospheric pressure. The chambers are so 

 designed that the a has sufficient room to expend most of its energy in the 

 gas. The resulting pulses are amplified electronically and counted or 

 recorded as "pips" on a strip chart. The first stages of the linear amplifiers 

 tend to be microphonic and must be guarded against noise and vibration. 



