Instrument Information |
|
IDENTIFIER | urn:esa:psa:context:instrument:gio.pia::1.0 |
NAME |
PARTICULATE IMPACT ANALYZER |
TYPE |
DUST |
DESCRIPTION |
Instrument Overview =================== Dust is collected from the ram direction of the spacecraft (i.e., parallel to the spin axis) through an adjustable, elliptical aperture which has a collecting area variable between 5 cm^ and 1 mm^2 in order to limit the rate to < 500 per second, the maximum with which the instrument can cope. The particles, moving with roughly the encounter speed of 69 km/second, are then incident on an oblique target, the entrance aperture projecting to a circle adjustable up to 35 mm diameter. Particles moving this fast can cause: - a crater in the target - partial or complete destruction of the particle, depending on the speed - emission of secondary particles - a flash of light - release of neutrals, electrons, and positive and negative ions. The target is a 10-micron thick roll of platinum foil doped with 5% silver. The roll is 700mm by 55mm and is unrolled and transported across the entrance aperture at a rate of 0.6mm/sec whenever the count- rate of dust particles exceeds a certain level and can also be unrolled with commands in the telemetry from Earth. This transport ensures that the portion of the target in use is always clean. Four different detectors are used to sense the impact of a particle and initiate the collection of data for a mass spectrum. Two detectors, although shielded, are relatively exposed to the outside environment, while the other two are completely shielded from the outside environment. The impact of a particle onto the detector can cause measurable pulses on any of these four detectors but typically does not cause a detected pulse on all of them. 1. A photomultiplier (PMT) senses the light-pulse produced on the impact of a particle onto the target. The output of the PMT is fed through an rms noisemeter which is used to adjust the triggering threshold automatically. Although shielded, ambient light fluctuations can reach the PMT by scattering. (Signal PM) 2. Pulses of positive ions are sensed as a pulse at the target itself, which is held at +1kV. This sensor is also fed to an rms noise meter which adjusts the discriminator. Since the target is exposed to ambient plasma from the ram direction, it is sensitive to external fluctuations. (Signal TG) 3. Pulses of electrons are collected at the 'catcher channel', an anode to the side of the main path for ions. This detector is well shielded from the external environment so that pulses here should all correspond to real events. (Signal CA) 4. Pulses of positive ions are also sensed at the accelerating grid, held at -2kV, which accelerates the ions into the mass spectrometer. This sensor is also well isolated from the external environment. (Signal AC) Because a dynamic range of at least 4 decades was desired (to detect particles from 0.1 to 10 microns), the amplifiers on these front-end channels can be switched between two gain ranges, each three decades and separated by a factor 100 (i.e. gains 1-1000 and 100-100,000 in arbitrary units). The outputs of the amplifiers are each monitored by three single-channel discriminators set at relative levels of 1x, 10x, and 100x. The mass spectrometer operates on the time-of-flight principle, accelerating all ions to a constant energy, thereby sorting them to different velocities dependent on the mass. Since each particle produces a burst of ions in a very narrow interval of time, the arrival time of the ions at the detector is related directly to the mass of the ions. The positive ions, after passing the accelerator grid (-2 kV), are decelerated to an energy of 1 kV by a grid at the entrance to the first drift tube. While passing along this first drift tube, the ions are focussed by an ion lens to remove the transverse dispersion in the motions. The charge induced on the lens by the passage of a pulse of ions is recorded as an additional monitor of the existence of a real particle event. (Signal MO) The ions then enter a reflector which serves as a first order energy focussing device to remove the dispersion due to the initial energies which the ions have from the formation process (up to about 70 eV). They then pass along the second drift tube which contains a second ion lens. At the end of the second drift tube, the ions are incident on a multiplier with 20 dynodes. At this point, the ions from a single initial particle impact have been sorted in time by their masses with ions having masses from 1 to 110 amu arriving roughly from 4 to 42.5 microseconds, mass being proportional to the square of the arrival time. The output of the multiplier is recorded by converting the signals from dynodes 5, 8, 11, 14, 17, and 20 to a logarithmic scale and then summing these six signals. This yields an output signal with 5 decades of dynamic range. This signal is digitized and stored in a memory together with the digital output of a time^2 signal generator which provides an index of the mass of the ions. Instrument Manufacturer : Max-Planck-Institut-fuer-Kernphysik Platform or Mounting Name :Spacecraft body Science Objectives ================== The objective is the in-situ measurement of the chemical and physical properties of cometary dust particles. The chemical compositions and masses of individual particles will be measured. Their impact rate, determined as a function of position relative to the comet's nucleus, will be used to establish the mass distribution and production characteristics of cometary dust. In particular, the goals are to: - determine the elemental abundance of individual particles and to ascertain whether there are distinct particle classes that differ from each other chemically - investigate whether the elemental particle composition depends on the distance from the nucleus, and to look specifically for the effects of ice evaporation from particle surfaces - gain insight into the molecular composition of the impacting particles, with emphasis on possible evidence of organic matter - determine specific isotopic ratios, such as 6Li/7Li, 10B/11B, 12C/13C, and to try to establish the origin of the comet's particulate matter - study the mass distribution function of impacting particles, derive the total dust production rate in the measured mass range, and compare the latter with theoretical models - determine the extent of the dust envelope as a function of particle mass and analyze possible asymmetries in impact rate to model anisotropy effects in the dust's production. Calibration Description ======================= The multiplier is calibrated in flight by an EPID (Electronic Pulse Induced Desorption) ion generator. This provides ions of two masses with a known intensity ratio. The mass scale is calibrated against the value from the t^2 generator after receipt of the telemetry by identifying specific lines, typically the two lines due to silver produced from the target. Electronics =========== The experiment runs entirely autonomously and was designed to collect data for a period of 4 hours, with nearly all the events occurring during the 20 minutes centered on the time of closest approach. The control is implemented with an RCA 1802 microprocessor and several PROMs. A number of parameters about the previous measurements are maintained in order to adjust the operating modes of the system. The normal state of the system is one in which it is waiting for a particle impact to occur. If no impact occurs for a second, the internal parameters are updated. These parameters include: i. the impact rate for all events (rate IR, number of events per second) ii. the impact rate for large events, i.e. events in which the 100x threshold is exceeded in one of the front-end channels (rate IRL, number of events per second) iii. the impact rate for events with mass spectra, i.e. events in which a minimum number of ions are counted in the mass spectrum (various rates are stored, IR4 is the rate of events of classes 4 through 7 as defined below) iv. the onboard time counter v. the number of EDFs transmitted per second. After updating the parameters, whether after an event or after the 1-second interval without an event, the system processes any commands received, sets the spectrum mode (0 to 3), enables the trigger channels, sets the number of front-end coincidences required to trigger an event, adjusts the sensitivity settings, sets the high voltage on the multiplier, adjusts the shutter position, and moves the target if motion is required. All instrument parameters are preset when the instrument is turned on. Values stored in PROM are used when the instrument is turned on for the first time on spacecraft power. When the instrument is turned off but spacecraft power is still in place, all instrumental parameters are stored in RAM and these values are used when the instrument is reactivated. Since small particles may not trigger all of the front-end channels, at low count rates an event is triggered whenever any single front-end channel is triggered. At higher count rates additional front-end channels must be triggered. Specifically whenever IR4 > 10/second and IR > 2*IR4, a coincidence with an additional front-end channel is required. Whenever the triggering rate of a front-end channel exceeds the rate at which the instrument can process events, the noise-detection system on the front-end channel adjusts the triggering level upwards. If this level exceeds, 3.2 volts, that channel is disabled. Normally, the front-end channel amplifiers are set to the high sensitivity range. If all three front-end channels become disabled due to excessively high count rates, the amplifiers are automatically set to the low sensitivity range and reenabled. All front-end channels (AC, PM, TG, CA, and MO) are set to the low sensitivity mode if there are enough events to fill the telemetry capacity, specifically if IRL > TMR + 3. The mode in which spectra are recorded also depends on the counting rate. Every 26th event is always recorded in mode 0. The mode for the remaining spectra (1 to 3) is increased by 1 whenever IR4 >= 2*TMR or IR4 > 128. It is reduced by 1 whenever IR4 < 2*TMR. These mode changes are implemented at 4-second intervals, if required. The shutter aperture is also controlled by the microprocessor through a stepping motor which takes 71 steps to go from 500 mm^2 to 1 mm^2 with the default position ('zero' position) at 457 mm^2. The positions at 500 m^2 and 52.4 mm^2 are sensed by microswitches. The control algorithm is based on the value of IR4 (event rate for classes 4 to 7). IR4 < 15: open by 9 steps IR4 < 30: open by 6 steps IR4 < 60: open by 3 steps IR4 > 90: close by 3 steps IR4 > 180: close by 6 steps IR4 > 254: close by 9 steps Since only one motor is allowed to be moving at any given time, the shutter motor has priority over the target motor. The target motion is also controlled by the count rate, unless the shutter is also moving, starting when IR4 > 60 and stopping when IR4 < 30. The rate of motion is fixed at 0.6 mm/s. Events are classified by the data processing system as follows: 1. Events with only 1 front-end signal 2. Events with a coincidence between 2 front-end channels 3. Events that have a coincidence with all 3 front-end channels but still do not exhibit a mass spectrum 4. Events that have a minimum of 52, 46, or 10 entries in the spectrum for modes 1, 2, and 3, respectively 5. Events that have a minimum of 54/48/12 entries in the mass spectrum but not more than 228/148/140 entries and which also have a minimum of 26/21/6 entries in the mass range 6-58 amu. 6. Class 5 events that also have at least 2 entries of at least 0.2 full scale in the range 5-8 amu. 7. Class 5 events that have a minimum of 34/27/12 entries in the mass window but not more than 93/49/46 8. Events from the statistical sample 9. test pulses 10. events from the AHV routine (calibration) 0. Events that have already been transferred to the telemetry system. When a new event is classified, the controller searches first for an empty section of memory in which to store it. Failing that, it searches for an event of lower class to be overwritten. The high voltage applied to the multiplier controls the sensitivity and the dynamic range of the mass spectra. The value is set by telecommand to one of 128 discrete values between 0 and 4500 V. Instrument Operating Mode Descriptions ====================================== Mode 0 ------ 15 MHz Mode. In this mode, the output of the multiplier is digitized and recorded in the event memory every 66.66 nanoseconds. The output of the t^2 generator is not recorded in this mode. Mode 1 ------ Max/Min Mode. At each maximum of the signal appearing at the MP output, a sample is taken of the multiplier output and of the t^2 generator. An additional sample is taken 1/2 mass unit after each maximum in the multiplier output. This should correspond to the minimum between two adjacent masses. In addition, an sample is taken every 1.13 microseconds if no other maximum has occurred in that interval. Mode 2 ------ Max/Time Mode. For each maximum in the MP output, a sample is taken of the MP output and of the t^2 generator. In addition, a sample is taken every 1.13 microseconds if no other maximum has occurred in that interval. Mode 3 ------ Max Mode. This is the same as mode 2 except that samples are taken only after 8.6 microseconds. |
MODEL IDENTIFIER | |
NAIF INSTRUMENT IDENTIFIER |
not applicable |
SERIAL NUMBER |
not applicable |
REFERENCES |
Jessberger, E.K., and J. Kissel, Chemical Properties of Cometary Dust and a
Note on Carbon Isotopes, Comets in the Post-Halley Era (II), Newburn, R.,
M. Neugebauer, and J. Rahe (eds.), Kluwer Academic Publishers, Dordrecht,
1075-1092, 1991. Kissel, J., und F.R. Krueger, Die Chemische Zusammensetzung des Kometenstaubes bei P/Halley, Sterne und Weltraum 26, No. 4, 191-194, 1987. Kissel, J., und F.R. Krueger, Die Physik der Massenspektrometrischen Staubanalyse beim Kometen Halley, Phys. Bl. 43, 131, Nr. 5, 1987. Kissel, J., The Particulate Impact Analyzer, an Instrument to Analyze Small Particles Released by Halley's Comet, Proceedings of the International Meeting on the GIOTTO Mission, Noordwijkerhout, The Netherlands, ESA SP 169, 53-60, 1981. Kissel, J., Das Experiment PIA der ESA-Mission GIOTTO, Gerat zur Massenanalyse Kleinster Teilchen, die vom Halley'schen Kometen Freigesetzt Werden, Kleinheubacher Berichte Bd. 26, 83-86, 1983. Kissel, J., The Giotto Particulate Impact Analyser, ESA SP-1077, 67-83, 1986. Kissel, J., In Situ Investigation of Halley's Comet, Accademia Peloritana Dei Pericolanti Classe Di Science Fisiche Matematiche E Naturali, Supplemento N.1 64, 105-123, 1986. Kissel, J., Mass Spectrometric Studies of Halley's Comet, Advances in Mass Spectrometry 1985, 175-184, 1986. Kissel, J. (1986). 'The Giotto Particulate Impact Analyzer'. In 'The Giotto Mission - Its Scientific Investigations' (R. Reinhard and B. Battrick, Eds.) ESA SP-1077, pp. 67-83. ESA Publications Division, ESTEC, Noordwijk. Kissel, J., D.E. Brownlee, K. Buchler, B.C. Clark, H. Fechtig, E. Grun, K. Hornung, E.B. Igenbergs, E.K. Jessberger, F.R. Krueger, H. Kuczera, J.A.M. McDonnell, G.E. Morfill, J. Rahe, G.H. Schem, Z. Sekanina, N.G. Utterback, H.J. Volk, and H. Zook, Compostition of Comet Halley Dust Particles From GIOTTO Observations, Nature, Vol 321, No. 6067, 336-337, 1986. 6067, 280-282, 1986. Krueger, F.R., and J. Kissel, Experimental Investigations on Ion Emission with Dust Impact on Solid Surfaces, Proceedngs of the GIOTTO Plasma Environment Working Group Meeting, 10-12 April 1984, Bern, Switzerland, ESA SP-224, 43-48, 1984. Krueger, F.R., A. Korth, and J. Kissel, The Organic Matter of Comet Halley as Inferred by Joint Gas Phase and Solid Phase Analyses, Space Science Reviews, 56, 167-175, 1991. Langevin, Y., J. Kissel, J.L. Bertaux, and E. Chassefiere, First Statistical Analysis of 5000 Mass Spectra of Cometary Grains Obtained by PUMA 1 (VEGA 1) and PIA (GIOTTO) Impact Ionisation Mass Spectro Meters in the Compressed Modes, Astronomy and Astrophysics, 187, 761-766, 1987. Langevin, Y., J. Kissel, J.L. Bertaux, and E. Chassefiere, Impact Ionisation Mass Spectrometry of the Cometary Grains On-Board VEGA 1 and GIOTTO, VEGA 1, and VEGA 2 Spacecrafts, Lunar and Planetary Science XVIII, 533-534, 1987. 'The Giotto Dust Impact Detection System', J.A.M.McDonnell, W.M.Alexander, W.M.Burton, E. Bussoletti, D.H. Clark, G.C. EVANS, S.T. Evans, J.G.Firth, R.J.L.Grard, E. Grun, M.S. Hanner, D.W. Hughes, E. Igenbergs, H.Kuczera, B.A. Lindblad, J.-C. Mandeville, A. Minafra, D. Reading, A. Ridgeley, G.H. Schwehm, T.J. Stevenson, Z. Sekanina, R.F. Turner, M.K. Wallis, J.C. Zarnecki, in The Giotto Mission - It's Scientific Investigations, edited by R. Reinhard and B. Battrick, ESA SP1077, 1986. McDonnell, J.A.M., W.M. Alexander, W.M. Burton, E. Bussoletti, G.C. Evans, J.G. Firth, R.J.L. Grard, S.F. Green, E. Grun, M.S. Hanner, D.W. Hughes, E. Igenbergs, J. Kissel, H. Kuczera, B.A.Lindblad, Y. Langevin, J.C. Mandeville, S. Nappo, G.S.A. Pankiewicz, C.H. Perry, G.H. Schwehm, Z. Sekanina, T.J. Stevenson, R.F. Turner, U. Weishaupt, M.K. Wallis, and J.C. Zarnecki, The Dust Distribution Within the Inner Coma of Comet P/Halley 1982i: Encounter by GIOTTO's Impact Detectors, Astronomy and Astrophysics, 187, 719-741, 1987. McDonnell, J.A.M., S.F. Green, E. Grun, J. Kissel, S. Nappo, G.S. Pankiewicz, and C.H. Perry, In Situ Exploration of the Dusty Coma of Comet P/Halley at Giotto's Encounter: Flux Rates and Time Profiles from 10**-19 kg to 10**-5 kg, Advances in Space Research, Vol 9, No. 3, 277-280, 1989. 'Encounters with Comet Halley, The first results', Nature, Volume 321, No. 6067, 15 May 1986. |