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.
2017-07-26 PEPSSI: PK: Updated data validity, calibration, and instrument
description.
2018-06-22 SOC: TF Added to data validity section regarding dataset
applicability.
"
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 = "
########################################################################
Recommended Reading - these documents describe how PEPSSI operates, needed
for analyzing the data properly:
- 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 to 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 apertures are covered by a thin
polyimide/aluminum/polyimide foil. The stop foil is a
polyimide/palladium/ polyimide foil. The foils are mounted on
high-transmittance stainless-steel grids. 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 to 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.
Also, 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. These tracks roughly follow the
channel boxes shown in Fig. 11-6 in the SOC to Instrument Pipeline ICD.
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 after 2007 DOY 144: 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 particles. Also B06, nominally measuring
high-energy protons, is unreliable. Beyond these channels, the
channels B14, B15, B16 are nominally measuring sulfur but are
dominated by noise in the interplanetary medium far from Jupiter.
Before 2007 DOY 144, the similar issues with background and noise
as described above exist but the channel definitions are different.
Background or electronic noise dominates the dump channels B00 and
B18. The B05 channel measuring high energy protons is at
background, except in near-Jupiter environment.
- 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.
B rates in sectors 4 and 5 (B*S04, B*S05) are contaminated or
dominated by counts from the internal alpha source of the instrument
and are therefore only useful for diagnostic purposes of the
instrument. B rates in the affected sectors (like B01S04, B01S05,
etc) should therefore not be used for science. Make sure to not
average over all sectors. Also any L*S04 and L*S05 channels should
be ignored due to contamination by the internal alpha source.
Background or electronic noise dominates the following B channels
after 2007 DOY 144:
B00, B06, B07, B08, B17, and B18.
B06, B14, B15, and B16 should only be used close to Jupiter with
significant foreground of energetic protons and sulfur. Before
2007 DOY 144, similar issues exist but the channel definitions are
different: B00 and B18 show noise. B05 measures energetic protons
only during significant foreground. The channels measuring sulfur
respond well during this period.
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 the raw
and calibrated PEPSSI datasets. Upper level derived dataset products
(Level 4) do not use these extensions, but it is helpful to read the
SOC to Instrument ICD in addition to understand the operation of PEPSSI
and the different types of channels.
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 to 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 quantities in the current version are now physically
meaningful for the B channels. They might still change in the course of
calibration efforts of the team.
The R channels nominally measure electrons and the calibration quantities
assume this. This is applicable in the Jupiter environment. At all other
times, the R channel count rates were found to be dominated by
penetrating particles consistent with cosmic rays. Therefore electron
fluxes cannot be provided for this period.
The L channels do not distinguish ion species. Attempts to deconvolve
their response are ongoing. We currently do not provide detection
efficiencies for the L channels and only correct for their time dependent
efficiency. The PEPSSI sectors of the L channels that nominally
distinguish different look directions show electronic cross talk that
makes it difficult to determine the true intensity distribution as a
function of direction. Comparisons different sectors are therefore not
meaningful, independent if count rates or the provided formal fluxes are
used.
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.
The PEPSSI measurements throughout the mission are supplied both as
instrument specific data (e.g., count rates) as well as physical
instrument-independent units (e.g., differential intensity). It must
be stressed that these are preliminary values that should not be used
without effort from the user to understand their limitations (see the
SOC to Instrument ICD, provided with this archive).
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 to 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
|