CCSD3ZF0000100000001NJPL3IF0PDSX00000001 PDS_VERSION_ID = PDS3 RECORD_TYPE = STREAM OBJECT = INSTRUMENT INSTRUMENT_HOST_ID = ULY INSTRUMENT_ID = UDDS OBJECT = INSTRUMENT_INFORMATION INSTRUMENT_NAME = "ULYSSES DUST DETECTION SYSTEM" INSTRUMENT_TYPE = "DUST DETECTOR" INSTRUMENT_DESC = " Instrument Overview ------------------- The instrument consists of a 0.1 mm thick gold foil of hemispherical shape with three grids at the entrance (entrance grid, charge grid, and shield), as well as an ion collector and channeltron detector. The maximum sensitive area (for particles moving parallel to the sensor axis) is 0.1 m^2. Upon impact the particle produces a plasma, whose charge carriers are separated by an electric field between the target and the ion collector. Negative charges (mainly electrons) are collected at the target, the positive charges are collected partly by the ion collector and partly by a channeltron. The channeltron is used as it is insensitive to electric and vibrational noise. See [GRUENETAL1992B] for more information concerning the instrument. Science Objectives Summary -------------------------- The objective of the Ulysses dust experiment is to investigate the physical and dynamical properties of small dust particles (10^-16 - 10^-6g) as a function of ecliptic latitude and heliocentric distance, and the study of their interrelation with interplanetary/interstellar phenomena. The parameters to be determined include the mass, speed, flight direction and electric charge of individual particles. Specific objectives are: - To determine the impact rate, size frequency, and the distribution of flight directions and electric charges of interplanetary dust particles. - To classify particle orbits into bound orbits around the Sun or hyperbolic orbits leaving or entering the solar system. To study the distributions of orbital elements (semi-major axis, eccentricity, inclination ) of particles in bound orbits. - To determine as functions of heliocentric distance and ecliptic latitude the spatial density of the interplanetary large particle population which generally moves on bound orbits around the sun, and to determine the relative significance of comets and asteroids as sources for these zodiacal dust particles. - To measure the flux and velocity of particles coming on hyperbolic orbits from the general direction of the Sun. - To identify interstellar dust particles and perform direct measurements of the spatial density, heliocentric distribution, velocity and mass of interstellar grains transiting the solar system. - To observe enhancements of cometary dust particles during the transit of the spacecraft through the plane of a comet's orbit. - To investigate the spatial density of dust particles within the asteroid belt and determine the dust production by collisions in the asteroid belt. - To investigate the influence of the Jovian gravitational field on the interplanetary dust population. - To measure electric charges of dust particles and establish the relationship of these charges with properties of the ambient plasma (plasma density, energy spectrum), the solar radiation spectrum and magnetic fields. Instrument Measurements ----------------------- Positively or negatively charged particles entering the sensor are first detected via the charge which they induce to the charge grid while flying between the entrance and shield grids. The grids adjacent to the charge pick-up grid are kept at the same potential in order to minimize the susceptibility of the charge measurement to mechanical noise. All dust particles - charged or uncharged - are detected by the ionization they produce during the impact on the hemispherical impact sensor. After separation by an electric field, the ions and electrons of the plasma are accumulated by charge sensitive amplifiers (CSA), thus delivering two coincident pulses of opposite polarity. The rise times of the pulses, which are independent of the particle mass, decrease with increasing particle speed. From both the pulse heights and rise times, the mass and impact speed of the dust particles are derived by using empirical correlations between these four quantities. Detector Description -------------------- The sensor consists of a grid system for the measurement of the particle charge, an electrically grounded target ( hemisphere) and a negatively biased ion collector. A charged dust particle entering the sensor will induce a charge to the charge grid, which is connected to a charge sensitive amplifier. The output voltage of this amplifier rises until the particle passes this grid, and falls off to zero when it reaches the shield grid. The peak value (Q_p) is stored for a maximum of 600 microseconds and is only processed if an impact is detected by the impact ionization detector within this time. A dust particle hitting the hemispherical target produces electrons and ions, which are separated by the electric field between hemisphere and ion collector into negative charges (electrons and negative ions) and positive ions. The negative charges are collected at the hemisphere and measured by a charge sensitive amplifier (Q_e). Positive ions are collected and measured at the negatively biased ion collector with a charge sensitive amplifier (Q_i). Some of the ions penetrate the ion collector, which is partly transparent (total transmission approximately 40%), are further accelerated, and hit the entrance cone of an electron multiplier (channeltron). Secondary electrons are produced, amplified, and measured by a charge sensitive amplifier (Q_c). Other quantities measured are the rise times of both the positive and negative charge pulses. The measurement of the time delay between electron pulse and ion pulse serves as a means for distinuguishing impact events from noise. Impact events have time delays of 2-50 microseconds, while mechanical noise has a time delay of milliseconds. These signal amplitudes and times of a single recorded event are digitized and stored in an Experiment Data Frame (EDF). A measurement cycle is initiated if either the negative charge Q_e on the hemispherical target, or the positive charge on the ion-collector Q_i, or Q_c exceeds a threshold. Since the hemisphere has a large area which is directly exposed to interplanetary plasma and high-energy radiation, this may cause some interferences for the Q_e measurement. To avoid these interferences during high activity times, it is possible to switch by command to a mode in which a measurement cycle is initiated if only the charge on the ion collector Q_i (small area and not directly exposed) or channeltron signal Q_c exceeds the threshold. If more than one event occurs within the transmission time of one EDF, then these events are counted by several amplitude-dependent counters. The dead-time caused by the measurement cycles is 5 milliseconds. The signals from the sensor are conditioned and analysed. The microprocessor coordinates the experiment measurement cycle, collects the buffered measurement data and processes the data according to a program stored in the memory. Calibration Description ----------------------- Impact tests with iron, carbon, and silicate particles were performed at the Heidelberg dust accelerator facility. The particles were in the speed range from 1 to 70 km/s and in the mass range from 1.0E-15 to 1.0E-10 grams. In addition to the projectile material variation, calibrations for iron particles with varying impact angles were done. See [GOLLER&GRUEN1989] for more information. To obtain calibrations without information about the impact angle and the composition of an impacting micrometeoroid, a set of curves (one for each measurement channel) was calculated, which were averaged over three different materials (iron, carbon, and silicate) and over the range of relevant impact angles (20 to 53 degrees). The measurements were done at different angles with iron particles and at one fixed angle (20 degrees) with carbon and silicate projectiles. Difficulties in accelerating glass and carbon projectiles and the low acceleration rate made it impossible to do more than the tests at one angle. A computer simulation of the detector exposed to an isotropic particle flux leads to the result that 50% of the particles hit the detector under an angle of 32 degrees or lower, relative to the sensor axis. Its effective viewing cone covers a solid angle of 1.4 sr. As the target is curved (hemispherical) the impact angle, measured relative to the target normal at the point of impact, is generally different from the angle of incidence (relative to the sensor axis). The direction of travel of the impacting particle can not be determined. From the computer simulation the most probable impact angle is 28 degrees, the average angle is 36 degrees. This information, used with the pointing of the instrument, can be used to obtain a rough estimate of the particle trajectory. The particle's flight path inside the detector was obtained as 20 +/- 5 cm. There are three possibilities for the determination of a particle's speed (the risetimes and the ratio Q_c/Q_i). Using all three measurements and comparing them with the calibration curves, the speed can be determined with an accuracy of a factor of 1.6. Using only one the accuracy is given by a factor of 2. With a known particle speed the mass can be determined from the charge yields Q_i/m and Q_e/m. If the speed is known within a factor of 1.6 and both yields are used for mass measurements the value can be measured with an uncertainty of a factor of 6. The main part of this error is caused by the limited accuracy of the speed measurement. The smallest impact charge Q_i detectable is about 10^14 Coulomb which corresponds to a mass and speed dependent threshold that can be approximated by a power law (see [GRUENETAL1995C] equation (1)). Instrument Modes ---------------- Different instrument modes exist to alter the instrument's susceptibility to noise. These modes are changed by adjusting the thresholds of the detectors on-board the instrument. The thresholds are altered by telecommand from Earth. The threshold levels of the detectors are included within the dataset. Instrument Mounting ------------------- The instrument is located on the equipment platform of the spacecraft body and its axis is at an angle of 85 degrees with respect to the positive z axis, where the z axis is the rotation axis of the spacecraft and the positive direction is where the axis points roughly towards Earth. The Ulysses dust detector weighs 3.8 kg and consumes 2.2 W." 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