356 ISOTOPIC TRACERS AND NUCLEAR RADIATIONS [Chap. 12 



b — average number of ion pairs produced in the chamber per cosmic 



ray 

 A = average number of alpha particles per unit time 

 B — average number of cosmic rays traversing chamber per unit time 

 In the presence of the radiation being measured, the standard deviation for a 

 single measurement of the radiation plus background over a time t T in terms of 

 charge units is 



<j r = — 7= (r 2 R + a 2 A + b 2 B)^ ion pairs/sec 



where r = average number of ion pairs produced in the chamber per incident 

 particle or ray 

 R = average number of events per unit time in chamber 

 The total standard deviation, therefore, is 



o" = (o"6 + °"?) ** ion pairs/sec 



Alternatively, a measurement can be expressed in millivolts per unit time 



by 



10 3 

 Vb = TT- (aA + bB) mv/sec 



Cq 



10 3 



v r = TT- {rR + aA + bB) mv/sec 



Cq 



where C = total capacity of collecting electrode plus its external connections; 



e.g., electrometer fiber and connections or vacuum tube grid and 



lead 

 q = number of ion pairs per coulomb 

 The standard deviations in terms of millivolts per second are then 



mv/sec 



mv/sec 



*t = [(°'r) 2 + (<r') 2 ] H mv/sec 



When a set of N measurements have been taken, the standard deviation from 

 their mean value v is given by 



mv/sec 



» = i 



where v* = values of individual determinations in units of mv per sec 

 This is calculated for N B determinations of the background and A 7 * deter- 



