The light scattering principle for particle detection is customary for the measurement of aerosols. Light scattering techniques can be applied to mixtures of particles (nephelometers) and to single particles as well. Measuring particle mixtures simplify detection because of the higher intensity level, however, information concerning the individual particle is lost. To provide well defined conditions over the whole rendezvous period, i.e., constant illumination beam and unchangeable scattering angle, the use of an artificial light source (instead of the sun) and a scattering volume located within the S/C is desirable. Considering this and the relatively low particle densities to be expected, the measurement of particle mixtures must be excluded

Giese, R. H.

Kneissel, B.

Zerull, R. H.

English

NASA Technical Reports Server

RESULTS TO BE EXPECTED FROM LIGHT SCATTERING
DUST ANALYZER DURING A RENDEZVOUS MISSION
R. H. Zerull, R. H. Giese and B. Kneissel
Ruhr Universitat
Bereich Extraterrestrische Physik
4630 Bochum
Federal Republic of Germany
1. General
The light scattering principle for particle detection is customary for the measurement of
aerosols (Hodkinson, 1966) and has also been used for space experiments ("Sysiphus" on board
Pioneer 10/11). Light scattering techniques can be applied to mixtures of particles
(nephelometers) and to single particles as well. Measuring particle mixtures, of course,
simplifies detection because of the higner intensity level, however, information concerning the
individual particle is lost. To provide well defined conditions over the whole rendezvous period,
i.e., constant illumination beam and unchangeable scattering angle, the use of an artificial light
source (instead of the sun) and a scattering volume located within the S/C is highly desirable.
Considering this and the relatively low particle densities to be expected, the measurement of
particle mixtures must be excluded.
2. Aspects of the Choice of Scattering Angle and Light Source
The scattering pattern not only indicates the evidence of a particle but also contains
information concerning its physical properties (size, refractive index, and structure), thus in
principle the measurement of the complete scattering diagram would be desirable. Weight and size
limits of space experiments, however, lead to the restrictions concerning the scattering angle
domain. For the selection of a most favorable scattering angle the following aspects must be
considered:
Near forward scattering provides maximum intensity, but contains only size information. Near
backward scattering produces less intensity and does not allow satisfactory size determination.
Additionally both forward and backward scattering measurements would require the highly parallel
beam of a laser. Detecting signals scattered by single particles always raises sensitivity
problems. These can be minimized by use of light sources with high UV-part, within the range of
the maximum quantum efficiency of photomultipliers. Shorter wavelengths are also favorable,
because the scattering efficiency of particles depends on the ratio of their size to the
illuminating wavelength (Kerker, 1969). Distinguishing features of different particle types also
depend favorably on this ratio. As we will see in the next section, for averaging reasons, too,
monochromatic light sources are not the optimum choice. All these considerations in connection
with weight and power aspects lead to the rejection of a laser source in favor of a customary
Hg-lamp. Consequently a scattering angle in the medium range must be chosen. For several reasons
the choice of 90° turns out to be optimum: In this range different types of particles have
different polarization properties. To take advantage of this effect, the scattered signal must be
split into two branches for separate measurement of the components parallel or perpendicular to
the scattering plane, respectively. The choice of 90°-scattenng angle furthermore simplifies
instrumentation for symmetry reasons.
\3. Averaging Concept
A scattering pattern of a single particle in one orientation illuminated by a monochromatic
light source contains many maxima and minima (especially in the case of dielectric particles, see
Kerker, 1969). Scattering analysis based on a distinct scattering angle would lead to unreliable
conclusions. Therefore the concept proposed provides three methods of averaging.
72
https://ntrs.nasa.gov/search.jsp?R=19820006126 2020-03-21T10:13:19+00:00Z
a) Average over Scattering Angles
scattering diagram is smoothed out by measuring an angular interval of scattering
angles around 90°. This step also increases the 6tilizable intensity, out misleading
conclusions can still not be excluded safely.
b) Average over Sizes
The scattering properties depend on the ratio of particle size and the wavelength of the
light source. Thus, one particle changes its scattering diagram if it is illuminated
with different wavelengths. The spectrum of the Hg-lamp selected contains many
utilizable lines from the UV to the red. This converts each particle into an artificial
polydisperse mixture.
4. Measurement Analysis
The quantities measured are the peak intensities I] and 13 registrated at the two sensors
responsible for the two directions of polarization, and the duration T of the scattered light
flash. Quantities derived from these data are the degree of linear polarization
the total intensity I = Ii + \2 (see Kerker, 1969) and the velocity of the particle v = a/T
(a is the dimension of the scattering volume in the direction of particle motion).
4.1 Criteria for Distinction Between Different Particle Types
The first step of the data evaluation are conclusions concerning the particle type. Such
conclusions are justified by measurements of the scattering properties of nonsphencal particle
(Zerull et al., 1979; Holland and Gagne, 1970; Pinmck et^ aj^ . , 1976; Perry et al., 1978; Giese
et aL,
The polarization measurements proposed allow distinctions concerning the refractive index of
the particle material (dielectric or absorbing) and the particle shape (spherical, irregular, or
"fluffy"). As a special type of particles, fluffy particles of dielectric and aosorbing
constituents (as collected by Brownlee, 1978), can also be identified.
4.2 Size Determination of Particles
The total intensity I = I] + 12 scattered at 90° is a measure for the size of the
particles. As the scattering efficiency at 90° depends on the type of particles, the
delimitation concerning particle type has to precede the size determination. Reliable size
information will be obtained using appropriate calibration curves for the particle type
registrated.
4.3 Velocity Determination
The duration of a light pulse registrated at the photomultipliers is inversely proportional to
the particle velocity.
5. CXjmpatiblity with Expected Flux Rates and Particle Velocities
The purpose of this section is to point out that the experiment proposed w i l l meet all
requirements due to extremely different flux rates and particle velocities to be expected during
the rendezvous period depending on the distance comet-sun and S/C-comet. To prove this, extensive
calculations have been carried out based on the conditions for the Tempel 2-mission using eitner
the nominal or extreme high model of Newburn (1979), considering the relevant area of the
scattering volume and the S/C trajectory proposed, following the procedures given oy Eddington
(1910), Wallace et aj_. (1958), and Mendis et^ aJL (1976). Typical excerpts are presented in
Table 1. The S/C coordinates x/y/x are centered at the nucleus of the comet with _x pointing to
the sun and x/y representing the orbital plane. N and E indicate the use of either Newburn' s
nominal or extreme high model.
73
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As Tempel 2 is no longer a serious rendezvous candidate, it should be pointed out that in case
of other candidates the S/C trajectory will probably be chosen appropriately to provide comparable
dust conditions.
The comparison of the performance limits of the instrument and the expected values for various
mission conditions suggests the following conclusions:
The allowable velocity range (v = < 500 ms-1, due to the limited sampling rate) is not even
exceeded in the extreme case of example No. 7. The minimum time requested between two events (for
data processing and to avoid overlapping of events, altogether about lOOpS) is also well
observed. On the other hand, the number of events to be expected during less active phases
(No. 1, 2, 3, 9, 10) turns out to be highly sufficient for reliable statistic conclusions.
Conditions for example No. 2 are chosen appropriately to demonstrate the capabilities of the
instrument near the apex distance of certain particles (in this case particles of 1.125y are
extremely dominant at velocities of only 15 ms~^). The especially high data rates expected in
the cases No. 5, 6, 7 can be mastered by appropriate choice of measuring intervals and use of
buffers for transitory data storage.
6. Summary of Problems of Cometary Physics Addressed by the Instrument
The Light Scattering Dust Analyzer will be able:
a) to determine the size distribution and number density of cometary dust as a function of
the position of the S/C within the coma and the comet's activity,
b) to determine the abundance of different bulk materials of the cometary dust,
c) to determine the bulk density of the cometary dust,
d) to measure the velocity of cometary dust particles,
e) to provide the necessary link to imaging experiments and remote measurements, to
investigations concerning the chemical structure (photometry of cometary emission, mass
spectrometer), and to dynamic studies in order to obtain consistent understanding of the
physical processes in comets and the interplay between cometary and interplanetary dust.
7. References
Brownlee, D. E. 1978, In Cosmic Dust (J. A. M. McDonnell, Ed.), Wiley, Chichester_New York-
Brisbane-Toronto, pp. 295-336.
Eddington, A. S. 1910, Monthly Notices of the Royal Astronomical Society 70, 442.
Giese, R. H., Weiss, K., Zerull, R. H. and Ono, T. 1978, Astronomy and Astrophysics 65, 265.
Hodkinson, 0. R. 1966, In Aerosol Science (C. N. Davies, Ed.), Academic Press, London/New York,
pp. 287.
Holland, A. C. and Gagne, G. 1970, Applied Optics 9, 1113.
Kerker, M. 1969, The Scattering of Light, Academic Press, New York.
Mendis, D. A. and Witt, G. 1976, Astrophysics & Space Science 39, 325.
Newburn, R. L. and Johnson, T. V. 1978, Icarus 35, 360-368.
Newburn, R. L., JPL Publication 79-60.
Perry, R. J., Hunt, A. J. and Huffman, D. R. 1978, Applied Optics 17, 2700.
75
Pinrnck, R. G., Carroll, D. E. and Hofman, D. H. 1976, Applied Optics 15, 384.
Wallace, L. V. and Miller, p. D. 1958, Astronomical Jour. j>3, 213.
Zerull, R. H., Giese, R. H., Schwill, S. and Weib, K. 1979, In Proc. of the Workshop on
Scattering by Nonspherical Particles, Albany, Plenum fubl. Corp., New York, 1980.
76


Results to be expected from light scattering dust analyzer during a rendezvous mission

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