Simulated iron micrometeoroids were fired unto a capacitor-type micrometeoroid detector which responded to an impact with a spark. Large ion currents were extracted from the spark and analyzed in a crude ion time-of-flight mass spectrometer. The mass spectra show the elements of both detector and particle materials

Auer, S.

English

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https://ntrs.nasa.gov/search.jsp?R=19740023807 2020-03-23T04:07:14+00:00Z
X-672-74-240
A COSMIC DUST COMPOSITION ANALYZER
WITH A SPARK ION SOURCE
Siegfried Auer
August 1974
GODDARD SPACE FLIGHT CENTER
Greenbelt, Maryland
A COSMIC DUST COMPOSITION ANALYZER
WITH A SPARK ION SOURCE*
ABSTRACT
Simulated iron micrometeoroids were fired unto a capacitor-
type micrometeoroid detector which responded to an impact
with a spark. Large ion currents were extracted from the
spark and analyzed in a crude ion time-of-flight mass spec-
trometer. The mass spectra show the elements of both de-
tector and particle materials. They do not, however, show
the alkali metals which have been a problem with prior
composition analyzers.
*Patent Pending
iii
A COSMIC DUST COMPOSITION ANALYZER
WITH A SPARK ION SOURCE
Analyzing the chemical composition of cosmic dust particles is presently one of
the most attractive goals of the physics of interplanetary and interstellar matter,
particularly of cometary physics. A prior composition analyzer (Auer and Sitte,
1968, Friichtenicht et al., 1973, Dietzel et al., 1973) makes use of the plasma
that is emitted when a dust particle hits a solid surface. Ions can be extracted
from the plasma and analyzed in a time-of-flight mass spectrometer with re-
spect to their mass-to-charge ratios. That analyzer has one major disadvan-
tage: a mass spectrum taken of the ions represents the elemental species in
proportions different from those of the original dust particle; alkali metals,
particularly, are overrepresented whenever the impact velocities are below
about 20 km/s.
A new method is announced in this letter which offers the advantages of a spark
ion source combined with the sensitivity, reliability, and simplicity of a
capacitor-type meteoroid detector. The basic difference from the prior method
is in the fact that a large amount of external electric energy is added to the
kinetic energy of the dust particle in order to vaporize and ionize the atoms in
the particle with a high efficiency.
1
A spark ion source is known (see, e.g., Inghram, 1954) to have the following
major advantages:
1. It ionizes any element present in the sample, i.e., it handles both vola-
tile and involatile substances;
2. It has no blind spots, i.e., within one order of magnitude all elements
ionize with equal efficiency;
3. There is little fractionation of elements.
A major disadvantage of a spark ion source is that large spreads of energy are
always present and recourse to double focusing mass analyzers is necessary.
Another disadvantage is the flow of high currents with short rise times in the
spark which can result in conducted and radiated electromagnetic interference
with other equipment. Both disadvantages can be met with carefully designed
instrumentation.
Capacitor-type micrometeoroid detectors have been used on many spacecraft.
A very reliable metal-oxide-silicon (MOS) version is described by Kassel (1973).
Its construction is schematically shown in Figure 1. The detector is made from
a 51-mm-diameter, 0. 5 mm thick wafer of low resistivity, p-type (boron doped)
silicon which forms the inner electrode of the capacitor. A l m thick layer of
silicon dioxide is grown on the wafer by thermal oxidation to'form the dielectric
of the capacitor. A 0. lpm thick aluminum coating is then vapor deposited on
top of the SiO2 to form the outer electrode. An electric field with the strength
of the order of 106 v/cm is applied across the SiO2 dielectric. An impacting
2
dust particle can trigger a spark between the electrodes, giving rise to a volt-
age drop across the capacitor that can readily be detected and counted. Accord-
ing to Kassel (1973), an MOS detector with an SiO 2 thickness of 0.4pm is sen-
2.81
sitive to iron projectiles with diameters as small as d - -2 (m 3 s - 2 ), where
v is the particle velocity. For example, at v = 10km/s a particle with a di-
ameter of 0. 02 8pm or larger would normally trigger a spark. The detector is
sensitive to particles with low velocities, at least down to about 1km/s and
probably lower. Such low velocities have always been a problem with prior
plasma detectors. Capacitor-type detectors have until now only been used to
count dust particles, never to analyze them. The idea behind this work was to
make use of the inherent properties of the spark as a very efficient ion source
and to inject the ions into a mass spectrometer for analysis.
Experiments were carried out with iron microspheres from the hypervelocity
dust accelerator at the Goddard Space Flight Center. Most particles had sizes
of several m and velocities of several km/s. A MOS capacitor-type micro-
meteoroid detector, kindly provided by J. Alvarez and P. Kassel of the Langley
Research Center, was exposed to impacts of dust particles, one at a time. As
shown in Figure 1, the polarity of the bias voltage on the detector was such that
ions were accelerated from the back to the front electrode. An additional elec-
tric field was applied by means of a grid in front of the detector in order to ex-
tract ions from the spark and to accelerate them further (total acceleration volt-
age U = 265 volt). Some ions hit this grid, giving rise to a short and intense
3
current pulse (about 100lns wide, 1 Ampere peak current) which provided the
zero-time reference for a time-of-flight mass spectrum to be recorded on an
oscilloscope. Other ions passed through the grid, were deflected in a trans-
verse electric field and collected at a distance 1 = 47 cm from the detector.
The deflection was necessary in order to prevent neutral ejecta from hitting the
ion collector. Note that this very simple arrangement has all essential parts
of a time-of-flight mass spectrometer - ion source, accelerating field, drift
space, and ion collector. Correspondingly, the current flowing onto the ion
collector and being measured on an oscilloscope as a function of time represented
the time-of-flight mass spectrum of the ions that were extracted from the spark.
One example of about 40 essentially identical spectra is shown in Figure 2.
According to the equation of energy balance,
MA vA2 = eU, (1)
Figure 2 also shows the nominal arrival times tA = I/vA for mass numbers
A = 16 (0+), A = 27 (Al+), A = 28 (Si+), A = 56 (Fe+), and A = 60 (SiO+).
A large peak can be seen around mass numbers 27/28 which is probably aluminum
and silicon. Another large peak around mass number 56 is probably due to iron
and/or silicon dioxide. Oxygen is only weakly indicated if at all. Alkali metals
which have been a severe problem with the prior composition analyzer were not
detected at all. The first large signal should be disregarded; it was caused by
crosstalk between adjacent pins on the common vacuum feedthrough and essen-
tially disappeared when different pins were used. Clearly, the mass resolution
4
is poor. A better resolving mass spectrometer should be used for future experi-
ments in this area.
The total charge of the extracted ions can be estimated from the grid current
to be of the order of 10-6 As or 6 x 1012 elementary charges. If half of them
were ions from the projectile and half of them ions from the detector material,
the projectile must have had a radius of at least 2pm, which does not seem un-
reasonable. If half of the ions were from a dust particle, with a radius of approx-
imately 2 pm, it must have been vaporized and ionized by the spark with a very
high efficiency.
The method needs further development in three areas:
1. Experiments with different dust materials should be performed in order
to verify that all elements are ionized with essentially equal efficiency;
of particular interest are probable meteoritic materials such as graph-
ite or silicates.
2. Detectors should be made of materials which are normally not found in
meteoroids; particularly silicon and silicon dioxide should be avoided as
detector material in order to avoid an ambiguity with meteoritic silicon.
3. The mass spectroscopic resolution should be improved over that of the
very simple arrangement of Figure 1 such that at least major meteoritic-
elements can be identified with certainty and discriminated against ions
of the detector material.
5
In summary, it seems that a cosmic dust analyzer with a spark ion source will
be able to provide, for the first time, mass spectrograms which represent the
true proportions of elements and isotopes of which cosmic dust particles are
composed. It is expected that the method is applicable to dust particles with
high as well as low velocities (as low as 1 km/s) while the prior method was
severely limited at low impact velocities.
ACKNOWLEDGMENTS
It is a pleasure to thank O. E. Berg for continuous support and encouragement.
The National Academy of Sciences provided the author with a senior resident
research associateship.
REFERENCES
Auer, S., and K. Sitte, Detection Technique for Micrometeoroids Using Impact
Ionization, Earth and Planetary Science Letters, 4, 178-183, 1968
Dietzel, H., G. Eichhorn, H. Fechtig, E. Griin, H. J. Hoffmann, and J. Kissel,
The HEOS 2 and HELIOS Micrometeoroid Experiments, Journal of Physics E:
Scientific Instruments, 6, 209-217, 1973
Friichtenicht, J. F., N. L. Roy, and D. G. Becker, The Cosmic Dust Analyzer:
Experiment Evaluation of an Impact Ionization Model, in Evolutionary and Phy-
sical Properties of Meteroids, NASA SP-319, U.S. GovernmentPrinting Office,
Washington, D.C., pp.299-310, 1973
6
Inghram, M. G., and R. J. Hayden, in Mass Spectroscopy, Nuclear Science
Series Report Number 14, National Academy of Sciences - National Research
Council, Washington, D.C., p.3 2 , 1954
Kassel, P. C., Jr., Characteristics of Capacitor-Type Micrometeoroid Flux
Detectors when Impacted with Simulated Micrometeoroids, NASA TN D-7359,
National Technical Information Service, Springfield, Va., 1973
7
p-Si 0.5mm
SiO 2 1pm
Al 0.1pm
ION DEFLECTION PLATE
/0
CAPACITOR-TYPE o
MICROMETEOROID
DETECTOR
------ O IRON MICROSPHERE
0
-20V
1Mg
30V 235V
ION COLLECTOR
500 . TRIGGERPULSE
TIME-OF-FLIGHT ION MASS SPECTRUM
Figure 1. Schematic Diagram of the Experimental Arrangement
0+
Si+ SiO2
Figure 2. An Ion Time-of-Flight Mass Spectrum,
50mV, 5 )s/div
9


A cosmic dust composition analyzed with a spark ion source

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