Data monitoring of gas leakage in satellites and spacecraft is facilitated by adding gaseous radioisotopes to cold compressed gas and mounting a detector in the thrust nozzle

Roselli-Lorenzini, F. G.

English

NASA Technical Reports Server

1972	 B72-10087 
NASA TECH BRIEF 
NASA Pasadena Office
	
A 
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Gas Leak-Detection System 
The problem: 
The attitude-control system for a satellite or a 
spacecraft uses compressed gas as a working fluid; 
solenoid-actuated valves allow the gas to flow through 
nozzles to provide the small thrust forces needed to 
change or maintain the position of the spacecraft. 
Since a perfectly-sealing jet valve is technically im-
probable, a certain amount of leakage is expected, 
and it would be desirable for the mission controller 
on earth to be provided with data on leakage rate. 
The solution: 
Add a gaseous radioisotope to the cold compressed 
gas and mount a detector in the thrust nozzle. 
How it's done: 
The radiation detector is located between the jet 
valve seat and the nozzle aperture, as indicated in 
the diagram. A radiotracer such as krypton (Kr-85) 
or carbon (C-14) is added in small quantity to the 
compressed gas. The sensitive element of the leakage 
detection system is an avalanche detector which con-
sists of an avalanche diode coupled with a tunnel 
diode amplifier. The avalanche diode is a silicon 
device with a gallium-diffused structure. When radia-
tion is absorbed in conventional silicon diodes, hole-
electron pairs are created which reach the PN 
junction and are swept across to provide a current. 
In contrast, the avalanche diode has a high internal 
field and when radiation produces a hole-electron 
pair, silicon atoms are ionized; if the internal field 
is high, a cascade of electrons is produced. Charge 
multiplications of the order of 100 to 1000 are realiz-
able when bias voltages are in the range of 1500 to 
1800 volts; if the signal is generated near the front
surface, it is multiplied to a greater extent than noise 
components resulting from the reverse-bias leakage 
current.
ster 
le 
It has been shown that a small avalanche detector 
can sense radioactive gas leaking through a gas jet 
nozzle, but only marginally. On the other hand, if 
the detector area can be increased, the lower limit 
for sensing leak rates is lowered proportionally and 
quantitative measurements of leak rate can be made. 
Reference: 
Locker, R. J.; and Huth, C. C.: A New Ionization 
Radiation Detection Concept Which Employs Semi-
conductor Avalanche Amplification and the Tunnel 
Diode Element. Applied Physics, Vol. 9, No. 6, 
page 227, 1966. 
DE
(continued overleaf) 
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Category 03
Patent status: 
No patent action is contemplated by NASA. 
Note: 
Requests for further information may be di-
rected to: S Source: Francesco G. Roselli-Lorenzini of 
Caltech/JPL
under contract to

NASA Pasadena Office 
(NPO-11405) 
Technology Utilization Officer 
NASA Pasadena Office 
4800 Oak Grove Drive 
Pasadena, California 91103 
Reference: TSP72-10087


Gas leak-detection system

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