temperature transducer it may be a 'bimetallic 

 strip. In either case a displacement is gen- 

 erated by the mechanical sensor which controls 

 the shutter aperture, this displacement in turn 

 being determined by the measurand. 



The sensing head is a pulse rate transducer; 

 strictly speaking, the instrument is not a com- 

 plete digital transducer unless a counter and 

 time base generator are included. The sensing 

 head is also a displacement transducer in which 

 analog to pulse rate conversion is performed 

 essentially by mechanical means. The output sig- 

 nal is a representation of the physical variable 

 to be measured in which all the information is 

 contained in the pulse rate. 



NUCLEAR SOURCES 



The nuclear disintegration rate of a radio- 

 active source is absolutely independent of tem- 

 perature. Physico-mechanical effects inside the 

 source such as thermal expansion which might 

 alter the observed counting rate are negligible. 



Fig. 1. Block diagram of nuclear digital 

 transducer. 



Therefore, the data are effectively "frequentized" 

 at the source by non-electronic means and subse- 

 quent pulse recovery is almost completely insensi- 

 tive to electronic circuit stability. 



Alpha particles are preferred to betas for use 

 in this transducer. The reasons are apparent 

 from the source energy spectra shown in Fig. 2. 

 Alpha particles are essentially monoenergetic and 

 their differential spectrum shows a single peak. 

 The tailing off of this peak is due to slowing 

 down of a few alpha particles when they escape 

 from the source. The peak due to electrical noise 

 in the detection equipment is extremely well sep- 

 arated from the alpha peak and the discriminator 

 setting for elimination of the noise pulses is 

 not at all critical. This means that a simple 

 discriminator is sufficient to insure that each 

 alpha particle reaching the detector generates 

 one, and only one, pulse. 



In the case of the beta spectrum shown in 

 Fig. 2 the discriminator setting would be 

 extremely critical and also would prevent a 

 large fraction of the beta particles from being 

 counted. This arrangement does not allow very 

 good counting stability. Gamma rays also are not 

 preferred because, being very penetrating, their 

 use would require an inconveniently heavy shutter 

 and aperture. 



The variable aperture arrangement used in our 

 experimental transducers preserves the spectrum 

 of the source so that the alpha pulses are much 

 larger than the noise pulses and pulse discrim- 

 ination is very simple. This would not be true 



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PULSE HEIGHT 



PULSE HEIGHT 



ALPHA SPECTRUM 



BETA SPECTRUM 



Fig. 2. Source energy spectra. 



17l+ 



