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Full text of "Detection techniques for tenuous planetary atmospheres first semiannual report, 1 jun. 1963-1 jan. 1964"

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First Semiannual Report 
NASA Contract NsG-458 
June 1, 1963 - January 1, 1964 



N64 



4 285 



DETECTION TECHNIQUES FOR TENUOUS PLANETARY ATMOSPHERES 



By 



Stuart A. Hcenig 
Melvin M. Eisenstadt 



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FIRST SEMIANNUAL REPORT ^ 
l(^Jun^ 1, 1963 -y^|iary 1, 1964J 



TO 



Dr. T. L. K. Smull 
Grants and Research Contracts Office Code SC 
National Aeronautics and Space Administration 
Washington, D. C. 20546 



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DETECTION TECHNIQUES FOR TENUOUS PLANETARY ATMOSPHERES 



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NsG-458 




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Principal Investigator 




llvin M. Eisenstadt 
Graduate Assistant 



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I . INTRODUCTION 

This program was set up to investigate chemisorption phenomena 
with the ultimate aim of developing devices for the investigation of ten- 
uous planetary atmospheres. Since the program, as originally proposed, 
involved several semi -independent studies, this report will be divided 
into several parts in order to discuss the work done in each of these 
areas . 

II. THE CHEMISORPTION DETECTOR FOR OXYGEN 

This device was first suggested about three years ago when 
Langmuir's work, on the effect of oxygen on tungsten, came to the. author's 
attention. Langmuir showed that chemisorption of oxygen raised the work 
function of tungsten and that this effect was highly specific for the gas 

and solid involved. This effect had been used for leak detection by Lang- 

(1 2) 
muir and others ' but no attempt was made to develop the device as a 

quantitative detector. 

With the kind assistance of Dr. G. P. Kuiper of the Lunar and 

Planetary Laboratory, a program was started to develop such a detector 

and investigate its behavior with mixtures of gases. The system finally 

settled upon was a tungsten filament cathode and a nickel anode, a drawing 

of the detector and a circuit schematic are shown in Fig. 1. For operation 

o 
the filament is flashed to 2300 K to remove adsorbed gases and then allowed 

to cool to 1500 K. The fall-off of the electron emission current is then 

a sensitive function of the ambient oxygen pressure since at 1500 K the 

oxygen can be chemisorbed by the tungsten and thereby raise the filament 

work function. Typical curves for a series of oxygen pressures are shown 



2 
in Fig. 2. Each curve is the mean of five runs and the spread in the data 
is shown by the horizontal bars. The ultimate pressure of operation is set 

by the time available for the experiment, but is expected in general, to be 

-11 -12 
about 10 - 10 torr. The device has proved to be quite reproducible 

and stable over long periods of time. 

Originally some question was raised about the behavior of the 
system in mixtures of gases. From the known data of chemisorption it was 
expected that the effects of other gases would be small, but as a check, 
experiments were run in oxygen/nitrogen and oxygen/carbon dioxide ambients. 
In each case the effects were as expected; the system was quite insensitive 
to nitrogen at nitrogen oxygen ratios as high as ten to one. This work was 
used for a M.S. thesis in Aerospace Engineering by Mr. Donald Collins and a 
paper discussing the work will be published in the Review of Scientific 
Instruments for January 1964. 

The work on COj is continuing but our first results indicate that 
there is no significant effect of CO- at CO2/O2 ratios as high as ten to one. 

Normally hot filament systems are monitored by a radiation pyro- 
meter which requires careful adjustment and an optical window in the vacuum 
system. By installation of a platinum-platinum rhodium thermocouple near 
the hot filament we have been able to obtain a calibration curve for fila- 
ment temperature vs. thermocouple output. Once this curve is available 
filament temperatures can be checked without using the optical pyrometer. 
This technique has proved to be quite sensitive and reproducible. The fila- 
ment to thermocouple calibration curve can be obtained in a simple vacuum 
system without opening the U-H-V system more often than necessary. This 
method of filament control should be quite valuable for operation of the 
detector in a field experiment where it may be very inconvenient to use a 



radiation pyrometer. 

III. THE THORIATED TUNGSTEN FILAMENT FOR OXYGEN DETECTION 

Langmuir's early work Indicated that a small amount of thorivim 
(1%) in tungsten substantially reduced the tungsten work function. He also 
noted that this thorium effect was easily destroyed by ambient oxygen. After 

discussions with Dr. Kuiper, we decided to investigate this effect for use 

-14 
as an ultra sensitive oxygen detector useful to pressures below 10 torr. 

The "first series of experiments have been quite favorable and indicate at 
least a three-order of magnitude gain in sensitivity over the bare tungsten 
detector. This sensitivity is gained at the expense of an increase in opera- 
tional complexity. For each run the thorium must be brought to the surface 
of the filament by careful heating. The filament is then flashed and allowed 
to cool to 1500 K, then the oxygen is admitted. The oxygen first combines 
with the thorium and thorium oxide is desorbed from the filament. The oxygen 
is then chemisorbed on the bare tungsten and the filament behaves as a 

standard chemisorption detector. The total change in emission current from 

3 
thoriated tungsten to tungsten with chemisorbed oxygen is about 10 times 

that for tungsten-oxygen alone. This leads to the increased sensitivity 
mentioned earlier. It must be emphasized that this type of device is a 
"one shot" affair. After a single high sensitivity exposure, the detector 
must be reactivated to bring new thorium to the surface. Further work is 
being done to determine the best operating conditions. The ultimate sensi- 
tivity should be well in the 10 - 10 torr region. 

IV. CHEMISORPTION DETECTOR FOR HYDROGEN 

The possibility exists of using a chemisorption type of detector 
for hydrogen. This would mean developing a new filament, probably palladium. 



4 
This work is just getting under way now because of the need for developing 
safety precautions when using hydrogen. It seems that palladium will work 
satisfactorily but its sensitivity to oxygen must be explored. Ultimately 
it might be possible to have a single anode with two filaments, one for 0^ 
and the other for H2; this would permit the experimenter to measure two of 
the most important components of a planetary atmosphere. 

V. THE MOLECULAR BEAM SYSTEM 

One of the most important ultimate uses of chemisorption detectors 
would be for measurement of slow protons and the neutral H /H ratio in space. 
This seems possible because of the expected difference in chemisorption be- 
tween H„ and H. Measurement of the H„/H ratio with mass-spectrometers is 
difficult because of the dissociation and excited states produced by the 
ionizing electron beam. The chemisorption detector would have the advantage 
of small size, simplicity and freedom from the disturbing effects of ioniz- 
ing electron beams. For investigation of this problem it was proposed that 
an ultra-high -vacuum molecular beam system be constructed with a furnace 
for production of H atoms. This work is now well under way and about one- 
half of the components are on hand at the present time. The system will 

be all stainless stell with copper seals, as shown in Fig. 3. The ultimate 

-9 
operational pressure is expected to be below 10 torr. 

To test some of the proposed U-H-V components and techniques a 

1/4 scale system was built during the past six months. This system is 

_9 
pumped by an 8 1/sec Vaclon pump and easily reaches the 10 torr range. 

This model system has proved quite valuable for component testing and for 

preliminary research programs. 

The large U-H-V system will be assembled in a new laboratory space 

generated by a NSF Facilities Grant. This space will be ready by June 1 and 



5 
assembly will start at that time. Components will be leak checked and 
cleaned for assembly as they are finished. 

VI. MOLECULAR BEAM DETECTORS 

As a preparation for our work with H- and H molecular beams we 
have been considering certain problems in beam detection and analysis. For 
beams of neutral species like H„ the only feasible detector at present is 
the system which first ionizes a small fraction of the beam by electron 
bombardment and then detects these ions by electrometer techniques . The 

problems of such systems are well-known and a review has been given recently 

(3) 
by Ramsey 

We have designed a new type of detector which can in principal 

detect a beam of any species and is not limited to sptecies of low ioniza- 

(4 5) 
tion potential. The device uses the technique first suggested by Muller ' 

for his field ion microscope. The system operates by means of a fine 

tungsten point or tip and an adjacent nickel ring. The point or tip is 

driven at about 25,000 volts DC positive with respect to the ring. Atoms 

or molecules passing near the tip have an induced dipole moment because 

of the strong field gradient, and are then drawn to the tip. At the tip 

the molecules yield an electron to the Fermi level of the tip and these 

positive ions are then drawn to the ring. The ion current is then detected 

via an electrometer system. 

Full details of the system will be reported when experiments have 

been performed to establish the best configuration. The effective cross 

sectional area for detection of an argon beam (a typical case) would be 

-4 2 2 

about 10 cm . If the beam intensity is (5) 10" particles/cm sec, which is 

a typical value for a molecular beam system, the detected current would be 



about (5) 10 amps. This is well within the limits of conventional elec- 
tronic techniques . 

Experiments are being planned to determine the actual values of 
the signal to noise ratio in a typical system. The importance of this 
detector lies in the fact that it will detect gases such as argon, helium, 
and hydrogen which are beyond the range of conventional surface ionization 
detectors. It can also be used for beams of metals such as copper. Copper 
is seldom used for molecular beam work because it cannot be detected by a 
standard surface ionization detector and the deposited copper short-circuits 
mass spectrometers. The field ion detector avoids both these problems quite 
easily. 

The parameters of the system are being discussed with the molecular 
beam research group in the Physics Department. A short note describing the 
device is being prepared for the Review of Scientific Instrimients. 

VII. FUTURE PLANS 

In the preceding sections we have indicated the direction of work 
for the second half of the contract year. First we wish to finish the runs 
on the oxygen detector with CO^/O, mixtures. The experimental procedure 
for testing the thoriated-tungsten oxygen detector is somewhat clumsy at 
present since the thorium must be "sweated" to the surface each time. It may 
actually be simpler to coat the thorium on the tungsten from an external 
source, but this will require further investigation which will start when 
the O2/CO2 work is done. 

The palladium detector for hydrogen will go into the vacuum system 
sometime in January and if the results are favorable the design for the 
atomic hydrogen oven will be put in the shop. 



7 
The large U-H-V system is in the fabrication stage and assembly 
will not begin until after June 1964 vhen space is available. Discussions 
h4ve been held with other on-campus groups regarding studies in the large 
U-H-V system. A study of simulated lunar soils will probably be done in 
cooperation with the Lunar and Planetary Laboratory but this will not be 
allowed to interfere with the hydrogen detector program. The development 
work on the molecular beam detector is just beginning and we are still in 
the process of refining the analysis. It does appear that the system will 
work as planned but more study will be needed to obtain the most efficient 
geometry. Experimental work will be done later in 1964 when a microammeter 
is available. 

It is expected that all the objectives of the program will be met 
within the available time and funds exttept for the assembly of the large 
U-H-V system. This U-H-V system will be delayed until the new laboratory 
space is available. 



REFERENCES 

1 . Jotmson, E. , Rev. Set. Inatr . , 21^ : IO84, : 1956 . 

2. Nelson, E., Rev. Sei. Instr .. 16_, 55, 1955. 

3. Ramsey, N,, Molecular Beams , Oxford University Press, 1956. 

4. Muller, E., "Field Ionization and Field Ion Microscopy," Advances in Elec- 
tronics and Electron Physics^ , Vol. XIII, pg. 83, Academic Press, N.Y., 1960. 

5. Good, R., Muller, E., Field Emission, Handbuch der Physik . 2nd ed. Vol. 21, 
pg. 185, Springer, Berlin, 1956. 



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