PDS_VERSION_ID = PDS3 LABEL_REVISION_NOTE = " 2006-12-27 SOC:Carcich Initial version; 2016-10-31 SOC: All Updated text about PEPSSI values outside stated sensitivity limits, and about low-TOF difference between N2 and N3 PHA data; resolved liens from 2016-05 peer review. 2017-03-29 SOC: All Added details to the 'Data validity' section about what channels and mission phases should not be used for calibrated science. " RECORD_TYPE = STREAM OBJECT = INSTRUMENT INSTRUMENT_HOST_ID = "NH" INSTRUMENT_ID = "PEPSSI" OBJECT = INSTRUMENT_INFORMATION INSTRUMENT_NAME="PLUTO ENERGETIC PARTICLE SPECTROMETER SCIENCE INVESTIGATION" INSTRUMENT_TYPE = "CHARGED PARTICLE ANALYZER" INSTRUMENT_DESC = " ######################################################################## ######################################################################## REQUIRED READING: - McNutt et al. (2008) [MCNUTTETAL2008A] - SOC Instrument Interface Control Document ICD, originally delivered with this archive ######################################################################## ######################################################################## This PEPSSI description was adapted from the New Horizons website, the SOC Instrument ICD, and McNutt et al. (2008) [MCNUTTETAL2008A]. INSTRUMENT OVERVIEW =================== PEPSSI is a medium-energy particle spectrometer. SPECIFICATIONS -------------- NAME: PEPSSI DESCRIPTION: Medium energy particle spectrometer PRINCIPAL INVESTIGATOR: Ralph McNutt, APL ENERGY RANGE: 25-1000 keV (protons) 60-1000 keV (atomic ions) 100-1000 keV (molecular ions) 25-500 keV (electrons) FIELD OF VIEW: 160 x 12 deg ANGULAR RESOLUTION: 25 x 12 deg ENERGY/TOF RESOLUTION: 250 eV / 167 ps Granularity PEPSSI (Pluto Energetic Particles Spectrometer Science Investigation) is a hockey-puck-size (7.6cm diameter by 2.5cm thick), time-of-flight (TOF) spectrometer that measures ions and electrons over a broad range of energies and pitch angles. Particle composition and energy spectra are measured for H to Fe from ~25 keV/nucleon to 1 MeV/nucleon and for electrons from 25 keV to 500 keV. The PEPSSI instrument traces its heritage back to the MESSENGER Energetic Particle Sensor (EPS) instrument. EPS/PEPSSI was developed with the support of a NASA Planetary Instrument Definition and Development (PIDDP) grant aimed at designing a low-mass, low-power sensor that can measure energetic pickup ions produced near planets and comets (Andrews et al., 1998 [ANDREWSETAL1998]; McNutt et al., 1996 [MCNUTTETAL1996]). The overall PEPSSI instrument weighs 1.5 kg and uses a maximum of 1.4 W of power. Scientific Objectives ===================== Summary ------- 1. Determine the escape rate of Pluto's atmosphere. 2. Measure the interaction of the solar wind with Pluto's ionosphere. 3. Determine the source and nature of energetic particles found near Pluto. Details ------- PEPSSI's primary objective is to determine the mass, energy spectra, and directional distributions of energetic particles, with a resolution able to discriminate between the various types of species expected at Pluto. PEPSSI will also search for neutral atoms that escape Pluto's atmosphere and subsequently become charged by their interaction with the solar wind. Finally, PEPSSI will determine the composition and density of pick-up ions from Pluto, which indirectly addresses the atmospheric escape rate. Detector ======== Summary ------- A time-of-flight (TOF) section feeds a solid-state silicon detector (SSD; a.k.a. 'pixel') array. PEPSSI's field of view (FOV) is fan-like and measures 160 degrees x 12 degrees, divided into six angular sectors of 25 degrees x 12 degrees each. Each sector has two SSDs: 3 of the sectors have one electron SSD and one ion SSD each; three sectors have two ion SSDs each. Ions entering the PEPSSI FOV generate secondary electrons as they pass through entrance and exit foils in the TOF section, providing 'start' and 'stop' signals detected by a microchannel plate (MCP). Details ------- PEPSSI is a compact particle telescope with a time-of-flight (TOF) section and a solid-state detector (SSD; a.k.a. 'pixel') array. A mechanical collimator defines the acceptance angles for the incoming ions and electrons. The TOF section is axially symmetric; entrance and exit apertures are 6 mm wide with an azimuthal opening angle of 160 degrees. The entry and exit apertures are covered by thin (~7E-6 g/cm**2) polyimide/aluminum and (~10E-6 g/cm**2) palladium/carbon foil mounted on high-transmittance stainless-steel grids, respectively. The foil thickness and composition is a compromise to minimize the energy threshold, secondary electron production, and scattering of particles in the foil while blocking UV from the direct Sun and Lyman-alpha background. PEPSSI measures the ion TOF using secondary electrons generated as the ion passes through the entrance and exit foils in the spectrometer. Total energy is measured by the SSD array comprising six sectors. Each six sectors comprises two SSDs; sectors 2, 4, and 5 are dedicated for ion measurement with two ion SSDs each; sectors 1, 3, and 6 each have one ion SSD and one SSD covered with ~1E-6 m Al absorber, to block low energy ions and permit measurements of electrons. The fan-like collimator together with the internal geometry defines the acceptance angles. The FOV is 160 degrees by 12 degrees with six angular sectors of 25 degrees each; the total geometric factor is ~0.15 cm**2-sr. As an ion passes through the sensor, it is first accelerated by the potential of ~3 kV on the front foil prior to contact. The ion generates secondary electrons at the foils, which are then electrostatically steered to well-defined separate regions on a single micro channel plate (MCP), providing 'start' and 'stop' signals for the TOF measurements (from 1 ns to 320 ns). The segmented MCP anode, with one start segment for each of the six angular entrance segments, allows determination of the direction of travel even for lower-energy ions that do not give an SSD signal above threshold. The combination of measured energy and TOF provides unique particle identification by mass and particle energy in the range: for protons from 15 keV to 1 MeV; for heavy (CNO) ions from 80 keV to 1 MeV. Lower-energy (>3 keV) ion fluxes are measured by TOF and pulse-height analysis (PHA) of the signal they produce in the MCP, providing particle identification and velocity spectra at these energies as well. Molecular ions, expected from Pluto's atmosphere, will break up in the foil prior to their full detection, but will be detected as high-mass events. Internal event classification electronics determine the mass and produce an eight-point energy spectrum for each of four species for six arrival directions. Energetic electrons are measured simultaneously in the dedicated electron pixels in the range from 20 to 700 keV. Only protons with energies > 300 keV (expected to be very rare at Pluto) can penetrate the absorbers on these pixels, and even those would be eliminated by on-board MCP coincidence requirements and ground comparisons with the simultaneously measured ion flux. Electronics =========== Extensive uses of miniaturization and custom electronics in the design allows PEPSSI to weigh less than 1.5 kg and consume less than 1.4 W. PEPSSI is made up of six modular 10cm x 10cm slices. They consist of: 1) Energy board; 2) High Voltage Power Supply (HVPS); 3) TOF board; 4) Digital processing board; 5) Common event processor board; and 5) Low Voltage Power Supply (LVPS) board. See the SOC Instrument ICD, originally archived with this volume, and McNutt et al. (2008) [MCNUTTETAL2008A] for further details. Operational Modes ================= The PEPSSI instrument can operate in two modes: Normal and Diagnostic. On the spacecraft, each event generates a PHA record. This record is classified by event type: Electron, High-Energy Ion (or 'Hi-Ion' or 'Triple'), or low-energy ion (or 'Low-Ion,' 'Double,' or 'TOF-only'). In diagnostic mode, events are not classified; alternatively, all events are 'diagnostic events'. Events of a given type are further classified into 'Rate Boxes' by their energy and/or time of flight (TOF). Thus each event has a type, a rate box, and a detector in which it occurred. Data sampling and priority for TOF-only data -------------------------------------------- Data set users may notice a different TOF distribution for the 'high resolution' data (also called 'N2 data' or ApID 0x692 data) and 'low resolution' data (also called 'N3 data' or ApID 0x693, 0x694 data). For completeness there is also 'N1 data' (ApID 0x691), but it only produces 10 PHA events per hour during the Pluto encounter, it will be ignored here. Note that during the Pluto Encounter period the instrument recorded N1, N2, and N3 data at intervals of 1 hour, 1 second, and 2 hours, respectively. PEPSSI has two kinds of ion data, TOF vs. E (time-of-flight vs. energy) and TOF-Only (also known as 'high ion' and 'low ion' data, triple coincidence and double coincidence data, and are associated with B-rates and L-rates, respectively). The data are dominated by TOF-Only data because the triple coincidence TOF vs. E data requires a higher energy (thus a lower flux in nearly all situations) to trigger the measurement, has a lower background level (due to the additional coincidence logic), and a smaller geometric factor. For the TOF-Only data, the events with TOF < 20 ns are in a different priority group than the TOF > 20 ns. In the TOF-Only data the shorter TOFs are particles with enough energy to penetrate the SSDs, but they can also miss the SSDs, so some of them would be events that could be detected in the TOF vs. E mode and some not. Due to this ambiguity, and the fact that for higher energies there is the valuable addition of the solid state detector (SSD) measurement, the TOF < 20 ns data were judged not to be the main TOF-Only product. They were put in a different priority group so their relatively high count rate wouldn't suppress the more desirable TOF > 20 ns events. Additionally, the N3 PHA data uses a different priority scheme than the N2 PHA data (this may be changed in a future software update, but that hasn't happened as of June, 2016). In the N3 PHA the TOF < 20 ns priority group is only rarely (if ever) the top priority group, but there could be TOF < 20 ns from the rare triple coincidence data or in the TOF-Only data from the rare period when a very low probability event happens to be detected and not overwritten by the priority scheme. In the N2 PHA data the priority groups have rotating priority so that all data groups get representation; not so with the N3 data. Thus there are PHA events both above and below 20 ns in the N2 data, but almost no PHA events below 20 ns in the N3 data. Data validity ------------- Some subset of the PHA event data is noise or other instrumental artifacts. PHA events with parameters outside the stated instrument sensitivity limits (see the SPECIFICATIONS section above) should be ignored, or, at the very least, used with extreme caution. Only TOF vs. E event data following the TOF(E) curves/tracks (going from long TOF and small energy to short TOF and high energy) of the PEPSSI instrument are valid. Events with TOF < 5 ns and deposited energy > 1 MeV or < 40 keV are artifacts. Event data (both TOF vs. E and TOF-only) is not weighted by the instrumental efficiency and therefore cannot be used as-is to determine energy spectra in physical units. Most notably, the event-counts peak around 100ns in the TOF-only data is not an intensity peak. Some channels nominally respond to real particles, if they are present, but predominantly or completely respond to background during most or all of the flight. Therefore, they should not be used for scientific analysis: - After the Jupiter phase, the electron channels should not be used because penetrating radiation dominates the response. These are channels R00, R01, and R02, all sectors (S0-S5). - Background or electronic noise dominates the following triple coincidence channels: B00, B07, B08, B17, and B18. The B00 channel is a dump channel. B07 and B08 were designed to be He-3 channels, but there is no sign of this isotope so they contain only the wings of the He-4 distribution. The B17 and B18 channels are ultra heavies but have shown no evidence of responding to such such particles. - Other channels also have background (energetic particle measurements always have background). The ones listed above should be ignored because they show no signs of a foreground signal. PEPSSI has a degraded Am-241 alpha particle source on board for calibration, and these particles dominate specific channels and sectors. Therefore, the following channels should not be used for science: B07S04, B07S05, B08S04, B08S05 B09S04, B09S05, B10S04, and B10S05. These channels correspond to the ADU's as follows for dates after 2007-05-24 (see rateboxdefinitionplanes in calib for earlier dates): B07: Helium-3 (53,97) B08: Helium-3 (98,1023) B09: Helium (53,225) B10: Helium (226,1023) The PEPSSI measurements are very sensitive to spacecraft attitude. Particles, depending on energy and species, can be very anisotropic. The PEPSSI integration intervals change frequently, and multiple integration intervals are used simultaneously. The user must be aware of the current integration interval (the DT column in the FITS tables) when selecting an averaging interval to avoid effects such as aliasing. Also, be sure to select an averaging interval that results in sufficient statistics. Please refer to dataset.cat for additional details on the applicability and use of the Primary HDU and extensions of data products in this dataset. Bad Time Intervals (BTIs) ------------------------- Various instrument conditions can make the PEPSSI data difficult or impossible to use for scientific purposes. Powering down, ramping the high voltage power up or down, running in diagnostic mode, etc. will all make the PEPSSI data unusable for standard analysis. The PEPSSI_BTI.TAB file contains a table of 'Bad Time Intervals' which should not be used for science analysis. It should be noted that the entire 'Launch' phase of PEPSSI data is classified as a BTI. While not actually a BTI, the period between 2007 day 144 and day 178 should be treated with caution as well. The PEPSSI Rate Box tables were changed on day 144 and calibration and analysis of this period is still preliminary. See the SOC Instrument ICD, originally archived with this volume, and McNutt et al. (2008) [MCNUTTETAL2008A] for details. Measured Parameters =================== Particle energy information, measured by the SSD, is combined with TOF information to identify the particle's composition. Each particle's direction is determined by the particular 25 degrees sector in which it is detected. Event classification electronics determine incident mass and energy, with 12 channels of energy resolution. A typical measurement includes 8-point spectra for protons and electrons and reduced resolution energy spectra for heavier ions for all six look directions. In calibration, the rate, in counts/s, of each energy and/or TOF bin is converted to flux i.e. differential intensity (1/cm**2-sr-s-keV). Calibration =========== The calibration parameters would ideally be determined through a combination of all of the following: ground measurements; analysis of the in-flight calibration alpha-particle source; modeling; intercalibration with known measurements. Currently only the final method has been employed, which has the obvious drawback of not providing an independent determination of the absolute flux. Therefore the fluxes provided in CODMAC Level 3 data should not be used as is to conduct science that is relying on absolute fluxes for scientific interpretation unless the user determines the fluxes independently. Brief summaries of the flux and PHA calibrations are given here. See McNutt et al. (2008) [MCNUTTETAL2008A] and the SOC Instrument Interface Control Document (distributed with this archive) for details. Flux Calibration ---------------- The calibration quantities are energy pass-band (dE = Ehi - Elo, lower and upper limit of the energies of the particles measured), measurement efficiency (N, the fraction of valid incident particles that are actually measured), the geometry factor (G, the measurement of the physical detector size and solid angle subtended by the field of view). These values are all given and applied with uncertainties in the CODMAC Level 3 files. The differential intensity, j (1/cm**2-sr-s-keV), is calculated in terms of the counts C, time coverage T (s), geometric factor G (cm**2-sr), upper and lower energy bounds Ehi and Elo (keV), and detection efficiency N: j = (C/T)/(G * dE * N), where dE = Ehi - Elo. The uncertainty values assume Poisson statistics for C, no error in T, absolute errors in G, Ehi, Elo and relative error in N. I.e., formally the counts are C = C +/- deltaC, the energies are E = E +/- deltaE, the geometry factor is G = G +/- deltaG. The efficiency is N = [N * epsilon or N / epsilon], where epsilon = deltaN/N, to one sigma confidence. In the initial delivery of the PEPSSI data from the Launch and Jupiter phases of the New Horizons mission these values are supplied to convert the instrument specific data (e.g., count rates) into physical instrument-independent units (e.g., differential intensity), as well as computing the physical quantities themselves. It must be stressed that these are preliminary values that should not be used without significant effort from the user to understand their limitations (see the SOC Instrument ICD, provided with this archive, and McNutt et al. (2008) [MCNUTTETAL2008A] for a description of the method used to calculate differential intensity, also called flux). PHA Event Calibration --------------- The following quantities are provided in the calibrated data products. The linear calibration constants are in the data labels; see the SOC Instrument ICD and McNutt et al. (2008) [MCNUTTETAL2008A] for details. * Calibrated Deposited Energy and/or TOF values * Speed column from the TOF assuming a 6.0cm flight path. * The PHA_HIGH_ION calibrated data contain additional quantities, where each value indicates the Incident energy assuming that the event is of that (H, He, O, or S) species: - H_Incident_Energy - He_Incident_Energy - O_Incident_Energy - S_Incident_Energy Events with the multi-hit (cross talk) flag set have been excluded. Quantities of limited usefulness (such as Heavy Ion Discriminator triggers) have been excluded. Because of the difficulty of removing priority scheme biases from non-N2 PHA data, only N2 (APID == 0x692) PHA data is present in the calibrated PHA data. Priority group artifacts have been removed from quantities in the Rate_Normalized_Weight column of the PHA_HIGH_ION extension using the procedure described in the SOC Instrument ICD. This column is usually used in making histograms of the High Energy Ion PHA data. Filters, Optics, Locations, Subsystems ====================================== N/A " END_OBJECT = INSTRUMENT_INFORMATION OBJECT = INSTRUMENT_REFERENCE_INFO REFERENCE_KEY_ID = "MCNUTTETAL2008A" END_OBJECT = INSTRUMENT_REFERENCE_INFO OBJECT = INSTRUMENT_REFERENCE_INFO REFERENCE_KEY_ID = "MCNUTTETAL1996" END_OBJECT = INSTRUMENT_REFERENCE_INFO OBJECT = INSTRUMENT_REFERENCE_INFO REFERENCE_KEY_ID = "ANDREWSETAL1998" END_OBJECT = INSTRUMENT_REFERENCE_INFO END_OBJECT = INSTRUMENT END