if the alpha particles were to pass through some 

 absorbing material on their way to the detector, 

 thereby distorting the alpha spectrum. 



Nuclear decay is characterized by randomly 

 spaced pulses having approximately a Gaussian 

 frequency distribution. The statistics associ- 

 ated with nuclear counting are well known. A 

 statistical error in each accumulated count 

 exists such that the standard deviation of N 

 counts is equal to N?. This means that a trade- 

 off exists between accuracy of measurement and 

 sampling rate. For example, if the measurand is 

 to be sampled once per second and if the required 

 readout accuracy is 0.1$ of full scale, then at 

 least 10 basic counts must correspond to full 

 scale and the transducer must deliver at least 

 10 pulses per second to the counter or scaling 

 circuit. 



a corresponding dead time correction, n-r, of 

 approximately 20$*. At the present time a more 

 conservative estimate of the maximum pulse rate 

 would be 10° per second. The nonlinearity cor- 

 responding to the required dead time compensa- 

 tion 2 can be built either into the counter elec- 

 tronics or into the mechanical modulator as may 

 be appropriate. 



The transistorized preamplifier (Fig. 3) used 

 with the silicon junction detector is a modifica- 

 tion of a circuit described by Chase, Higinbotham 

 and Miller of Brookhaven National Laboratory, 7 

 to which we have added an extra stage of amplifi- 

 cation and a pulse inverter. The input circuit 

 contains a charge integrator so that the voltage 

 height of the output pulses is proportional to the 

 detector and therefore is proportional to the 

 incident particle energy. 



An interesting corollary of these nuclear 

 counting statistics is that for a fixed sampling 

 time the transducer readout error, expressed as 

 a percentage of full scale, decreases as the 

 scale reading decreases. In other words, more 

 accurate readings are obtained at the low end 

 of the scale. This contrasts with conventional 

 readout devices whose error is practically con- 

 stant at all points on the scale. 



OTHER DETECTORS 



Scintillation detectors or gas filled propor- 

 tional detectors theoretically could be used in 

 nuclear digital transducers. However, since 

 either of these types would be bulkier and its 

 resolving time longer than silicon junction 

 detectors, no further consideration will be given 

 to these types. 



There should be no hazard problem with respect 

 to nuclear sources for these transducers. Very 

 weak license-free sealed alpha sources have been 

 satisfactory for our experiments to date. If 

 much stronger activities should be needed, the 

 entire source chamber can be sealed. We have 

 used Radium D sources (the actual alpha emitter 

 is Po 210 ) which have a half -life of 20 years. 



In any final design a compromise between 

 source life and data rate, consistent with a 

 clean alpha spectrum, would be necessary. If 

 the source chosen for a particular application 

 should be relatively short-lived, the decay can 

 be compensated either by adjustment of the time 

 base or in the course of data processing. 



SILICON DETECTORS 



Silicon junction nuclear particle detectors^' 

 are particularly useful in nuclear digital trans- 

 ducers that employ alpha particles. They are 

 relatively insensitive to gammas. 



Geiger tubes are the simplest and cheapest 

 detectors available and may be interesting for 

 use when economy is a prime consideration. How- 

 ever, since Geiger tube pulses are exactly the 

 same for all types of radiation, any required 

 discrimination of radiation must be accomplished 

 by variations in shielding and in detector design. 

 Furthermore, the use of Geiger tubes, which 

 typically have resolving times of 20 to 1,000 

 microseconds, would limit the maximum pulse rate 

 to an undesirably low value for many transducer 

 applications . 



It is possible that silicon junction detectors 

 may be capable of operating in an avalanche mode 

 similar to a Geiger counter, thus resulting in 

 pulse amplification in the junction. This effect 

 should be most readily observable at cryogenic 

 temperatures. 3 If solid state avalanche detectors 

 should ever come into use, and if they should be 

 capable of room temperature operation with 

 reliable performance, they might be preferable to 

 Geiger tubes for use in low performance types of 

 transducers . 



The detector-limited pulse rise time, which 

 may range from 2 to 50 nanoseconds 5 should be 

 approachable by using very fast electronics." 

 At least one manufacturer markets detectors 

 having a 6 nanosecond rise time. If the 6 nano- 

 second figure is accepted as a reasonable limit 

 for the rise time, the minimum practical 

 resolving time of silicon junction detectors is 

 about 20 nanoseconds, leading to a maximum useful 

 random pulse rate of about 10? per second with 



TELEMETRY 



Counters with electronic scaling decades com- 

 monly provide a binary coded decimal output 

 suitable for telemetering. Therefore, if the 

 counter is located ahead of the telemetry link, 

 all the advantages of PCM telemetry are available. 

 PCM telemetry is most suitable when several 

 transducers are to be sampled serially and 



*As used in the formula n r 



n 

 1-nT 



of events recorded and t is counter recovery time 



where n is mean number of events occurring, n is mean number 



175 



