Ionization gage in which internally generated X rays are minimized is described. This gage permits the measurement of gas pressures in ultrahigh systems of micro-pico torr /10-18/

Hudson, J. B.

Sears, G. W.

English

NASA Technical Reports Server

NASA TECH BRIEF 
n
Second P 
Ion Accel 
41
First Posh 
Ion Accell 
Tungsten Gage 
Electron Collector
Electron Multiplier 
Ird Photomultiplier Tube), 
age Body 
Tungsten Filament 
June 1964	 Brief 63-10597 
This NASA Tech Brief is issued by the Technology Utilization Division to acquaint industry 
with the technical content of an innovation derived from the NASA space program. 
Precision Gage Measures Ultrahigh Vacuum Levels 
The problem: Accurate measurement of pressures 
of less than 10-14 torr 
Ionization gages are the instruments most often 
used to measure gas pressures in ultrahigh systems. 
Such gages have a hot filament, usually tungsten, that 
maintains a constant electron current between it and a 
positively biased grid structure (the electron collector 
grid). Gas from the system under investigation is 
drawn through the grid structure where it is ionized 
by the electron current. The number of positive ions 
formed is directly proportional to the density of gas 
molecules and of the electron current. These positive 
ions are then collected on a negatively biased anode. 
The resulting current is directly proportional to the 
gas pressure.
The lowest pressure that the ionization gage can 
measure is determined by the quantity of soft X-rays 
formed during the process described above. When 
electrons from the filament strike the positively 
biased grid structure, soft X-rays are produced, the 
quantity being directly proportional to the strength 
of the electron current. Some of these X-rays strike 
the anode and cause secondary electron emission from 
the anode. Since the resulting secondary emission 
current cannot be distinguished from the positive ion 
current and is not pressure dependent, it sets a lower 
limit for the gage. 
The solution: An improved ionization gage in 
which internally generated X-rays are minimized, 
permitting gas pressures of 10 -18 torr to be measured. 
(continCied overleaf) 
This document was prepared under the sponsorship of the National 
Aeronautics and Space Administration. Neither the United States Govern. 
ment, nor NASA, nor any person acting on behalf of NASA: A. Makes 
any warranty or representation, express or implied, with respect to the 
accuracy, completeness, or usefulness of the information contained in
this document, or that the use of any information, apparatus, method, 
or process disclosed in this document may not infringe privately-owned 
rights; or B. Assumes any liabilities with respect to the use of, or for 
damages resulting from the use of, any information, apparatus, method, 
or process disclosed in this document.
https://ntrs.nasa.gov/search.jsp?R=19630000120 2020-03-11T16:48:07+00:00Z
How it's done: Two methods are combined to 
decrease the production of X-rays and, hence, the 
X-ray current. The first is to reduce the area of the 
anode and thus decrease its X-ray cross section. The 
second method is to reduce the electron current 
needed to give positive ion production by increasing 
the electron path between filament and grid, thus 
increasing the efficiency of a given electron as an ion 
former. To compensate for the lower positive ion 
current, an electron multiplier is used as the anode 
to increase the output current. 
Two accelerator grids are positioned between the 
collector grid and the dynode of the multiplier to 
facilitate the movement of positive ions from the 
collector grid to the dynode of the multiplier. With 
this design, the lower limit of gage is set by the 
"dark current" in the multiplier. Present multipliers 
have dark currents as low as 10-20 ampere, which is 
equivalent to a gas pressure of 10-18 torr in this gage
Notes: 
I. Where the range of gages currently used is ap-
proaching a lower limit, as in research laboratories, 
this gage would have definite utility. Industrial 
usefulness will require further development, since 
the life of the dynode coatings under vacuum bake-
out conditions is not well established. 
2. For further information about this innovation 
inquiries may be directed to: 
Technology Utilization Officer 
Goddard Space Flight Center 
Greenbelt, Maryland 20771 
Reference: B63-10597 
Patent status: NASA encourages the immediate 
commercial use of this invention. It is owned by 
NASA, and inquiries about obtaining royalty-free 
rights for its commercial use may be made to NASA 
Headquarters, Washington, D.C. 20546. 
Source: General Dynamics Corporation
under contract to

Goddard Space Flight Center (GSFC-1 14) 
Brief 63-10597


Precision gage measures ultrahigh vacuum levels

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