284 INSTRUMENTATION IN SCIENTIFIC RESEARCH [Chap. 5 



Losses occur through recombinations. The rate of recombination 

 of positive and negative ions is by a factor of 10 4 larger than that of 

 positive ions and electrons. Negative ions are formed by electron 

 attachment, primarily in the presence of impurities. Gases of very 

 high purity (>99 per cent) should be used to reduce attachment. 



response to ionization varying with time. The current pass- 

 ing through an ionization chamber is frequently measured by the 

 voltage drop across a series resistor R, as shown in Fig. (5-2)3. The 

 time constant of the ionization-chamber circuit is then determined 

 by the product RC. If the resolving time of the subsequent detect- 

 ing equipment together with the RC network is t, the value of r 

 must be substituted for RC. 



The capacitor is charged to the voltage E. Any transition of 



charges dq within the chamber causes a discharge of the capacitor 



and a variation of the voltage across the chamber or across the 



resistance R of a value 



. ^ dq n 



AE =— = 1.6 x 10- 7 - 



e C 



where C is the capacitance of the chamber plus that of the amplifier 

 input, and n is the number of ion pairs formed. The capacitor is 

 recharged from the source E through the resistor R. Two cases 

 can be distinguished: 



1. Ion pulse chamber. The time constant RC is large compared 

 to the time required to collect the ions. The output pulse rises 

 rapidly at first until all electrons are collected (of the order of 1 

 ^sec) and then rises slowly until all ions are collected (about 1 msec). 

 The pulse then decays with the time constant RC. 



The maximum voltage of the pulse in this case is 



AE » = — c — = ~C 



where Qq and Qq are the positive and negative charges produced 

 by an ionizing particle, and C is the capacitance of the chamber 

 plus that of the input to the amplifier. N is the total number of 

 ions produced in the chamber. 



The ion pulse chamber thus furnishes information as to the total 

 number of ions produced in the chamber, but its response is slow, 

 so that the chamber is not suitable for fast counting. The response 

 characteristic of the subsequent amplifier must extend to low fre- 

 quencies, and the amplifier is, therefore, subject to microphonics of 

 the counter (see below) and a-c pickup. 



