354 



ISOTOPIC TRACERS AND NUCLEAR RADIATIONS [Chap. 12 



circuit and the total charge collected. It is important in this case to choose 

 electrometer tubes with extremely low grid currents and to use insulation of 

 the highest order. 



Direct-current amplifiers have had only limited use with ionization 

 chambers partly because of the technical difficulties in their construction 

 and operation and partly because of their susceptibility to microphonics, 

 long period drift and high noise level as compared with other detecting 

 devices. However, in recent years the use of 100 per cent feedback circuits 

 (current amplifiers) have eliminated many of their former shortcomings and 

 are finding more extensive applications to the detection of very small currents. 



d. Vibrating-reed Electrometer. The vibrating reed is a variable capacitor 

 for converting the d-c potential developed across a capacitor by the collected 

 charge from the ionization chamber into an oscillating voltage that can be 



WW- 



i — www- 



nnnn 



A.C. 



Ampi. 



M 



3 



B 



rmnn 



c~Ep 



A.C. Power 



Fig. 99. Schematic diagram of vibrating-reed electrometer. /, ionization chamber, 

 M, two-phase motor; P, slide-wire potentiometer operated by motor M ; R, chart paper 

 and recording pen; V, vibrating-reed capacitor and driving coil. 



amplified by conventional a-c amplifiers [6,12]. With a constant charge on 

 the collecting system, the voltage across the capacitor is 



V = 



Q _ Q(d -\- a -f- a sin ict) 

 C(t) kA + C {d + a + a sin cot) 



where Q — charge on electrode system 



C = capacitance of collecting electrodes 



k = dielectric constant 



A = area of reed 



d = minimum separation of vibrating reed electrodes 



a = amplitude of vibration of reed electrodes 



co = vibrational frequency of reed 



The oscillating component of the reed potential, which increases as charge 



accumulates, is applied to the input of a conventional linear a-c amplifier. 



After amplification it is rectified and the resulting d-c voltage measured by 



a recording potentiometer which automatically and continuously applies to 



