System was developed for use in astronomy and other research areas concerned with detection of faint-light images. Photometer system is comparable in performance to good photomultiplier tube array except that digital electronics are used instead of analog

Beaver, E. A.

Mcilwain, C. E.

English

NASA Technical Reports Server

NASA TECH BRIEF 
NASA Headquarters R .
November 1974	 B74-10200 
NASA Tech Briefs announce new technology derived from the U.S. space program. They are issued to encourage 
commercial application. Tech Briefs are available on a subscription basis from the National Technical Information 
Service, Springfield, Virginia 22151. Requests for individual copies or questions relating to the Tech Brief program may 
be directed to the Technology Utilization Office, NASA, Code KT, Washington, D.C. 20546. 
Digital Multichannel Photometer 
A new digital multichannel photometer system has 
been developed for use in astronomy and other research 
areas concerned with the detection of faint-light images. 
The photometer system is comparable in performance to 
a good photomultiplier tube array except that digital 
electronics are used instead of analog. The digital
circuits allow substantial reductions in cost, weight, and 
size of the system. 
The photometer system incorporates a conventional, 
single-stage, magnetically-focused diode tube as shown 
in Figure 1 Light coming from a faint image is 
converted into electron current at the cesium anti-
.
-20 kV	 15 cm	
•-I-J Solenoid 
Light
_	 I	 II	 I1t---.i) t 
Input
El_I- -
	 38 Channels 
Cesium	 Voltage Ring 
Antimonide 
Photocathode
I-
U, Ip• 
Silicon  17 	Element 
Substrate	 0.101-mm	 Face 
Detector 
Pitch	
i 
Figure 1. Diode Tube
(continued overleaf) 
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https://ntrs.nasa.gov/search.jsp?R=19740000200 2020-03-17T13:51:40+00:00Z
D Discriminator
16-Bit 
Accumulator Tape 
and Shift 
	
>	
Register
Recorder 
Charge-	 Voltage 
Sensitive Amplifier 
Preamplifier 
(38 Channels) 
rs.	 I 
38-Element Readout 
Diode Array  
0
CRT 
Display 
J 38 Shift Registers Commutated for 
Serial Readout 
Figure 2. Photometer System
. 
monide photocathode. These electrons are accelerated 
through a 20-kV potential, reaching energies of 20 keV. 
They are focused onto a linear diode array comprising 
38 diffused p-n diodes on a silicon wafer. 
Each diode is 0.089- by 0.089-mm square connected 
to a separate circuit channel (see Figure 2). Every 
channel includes a charge-sensitive preamplifier, an 
amplifier, a discriminator, and a digital accumulator 
for the detection of photoelectron arrivals. As each 
electron enters the diode depletion region, an electron-
hole pair is produced for every 3.5-eV energy loss. 
Thus, an electron losing 16 keV of energy in the 
depletion region produces a charge pulse of approxi-
mately 7.3x10 16 coulombs. This charge is easily de-
tected by the charge-sensitive preamplifier. 
The preamplifiers are constructed using low-noise 
field-effect transistors. However, there is a low-energy 
noise component in the preamplifier output which is 
eliminated in the discriminator. The output is fed to a 
16-bit accumulator and shift register which accumulates 
the image information and feeds it serially to a tape 
recorder or a cathode-ray tube (CRT) display, while the 
next accumulation is taking place. 
Results show that the tube dark current at 20 kV 
and a temperature of 35° C is approximately 200 
electrons/s/cm 2 , which suggests that most of the dark 
current is due to thermionic emission of the photo-
cathode. Since each detector is approximately 10 
cm  in area, the dark-current rate per diode is approxi-
mately 0.02 count/s. The present tube design can be 
expanded to include 1000 diode elements. In addition, 
different diode arrays can be generated for different 
applications. 
Note: 
Requests for further information may be directed to: 
Technology Utilization Officer 
NASA Headquarters 
Code KT 
Washington, D. C. 20546 
Reference: TSP74-10200 
Patent status: 
Title to this invention has been waived under the 
provisions of the National Aeronautics and Space Act 
[42 U.S.C. 2457(f)], to the University of California, 
Berkeley, California 94720. 
Source: Edward A. Beaver and 
Carl E. Mcllwain of
University of California
under contract to
NASA Headquarters
(HQN-1079 1)
. 
B74-10200	 Category 03, 02


Digital multichannel photometer

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