The Galileo Energetic Particles Detector

From Space Science Reviews

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INSTRUMENT_HOST_ID             = GO
INSTRUMENT_ID                  = EPD

OBJECT     = INSTRUMENT_INFORMATION
INSTRUMENT_NAME                = "ENERGETIC PARTICLES DETECTOR"
INSTRUMENT_TYPE                = "ENERGETIC PARTICLES DETECTOR"
INSTRUMENT_DESC                = "

	The Galileo Energetic Particles Detector is fully described by
Williams et al [WILLIAMSETAL1992].

INTRODUCTION

	Jupiter possesses the largest planetary magnetosphere in the
solar system.  It is the largest in spatial dimension, has the highest
trapped particle energies and intensities, has the greatest
compositional variety in its major particle populations, displays the
largest co-rotational effects and has the largest number of moons
within the magnetosphere that provide both strong sources for and
losses of the observed particle populations.  These characteristics,
uncovered by the Pioneer and Voyager flybys demand an instrument
design capable of accommodating the great range in parametric values
established by these extremes.
	Within the Jovian magnetosphere, the energetic (>=20 keV)
particle populations play an important dual role.  First, they
represent a major factor in determining the size, shape, and dynamics
of the system.  For example, observations of energetic particle
intensities and corresponding energy densities show that these
populations are important in (1) standing off the solar wind and
thereby determining magnetopause position; (2) determining the general
magnetic field configuration in the evening magnetosphere and (3)
establishing the bulk of the ring current responsible for the
magnetodisk configuration of the middle-Jovian magnetosphere.
	Secondly the energetic particles play an important diagnostic
role in the determination of energization, transport, and loss
processes active in the Jovian magnetosphere.  In this role they also
provide a remote sensing capability for identifying magnetospheric
structures through finite gyroradius effects and for diagnosing remote
processes through field-aligned flow, E x B drift, and magnetic drift
effects.
	The Galileo EPD will provide major extensions to the Jovian
energetic particle data base obtained from the Pioneer and Voyager
flybys.  For example:
(1)  Galileo will be placed into a highly elliptical orbit around
Jupiter.  The nominal two-year mission lifetime will allow both a
direct measure of time variations in the Jovian magnetosphere and a
significantly larger spatial sample of the system than has been
possible with the previous flybys.
(2) The nominal mission includes several close ( < 1000km) flybys of
the Galilean satellites thereby providing the best opportunity to date
to observe details of the satellite/magnetospheric interactions.
(3)  The EPD provides the first 4-pi steradian angular coverage for
Jovian energetic particles, thereby assuring that the necessary
energetic particle measurements will be obtained independent of
satellite orientation and magnetic field direction.
(4)  The low-energy thresholds of the EPD effectively close the energy
gap between plasma and energetic particle measurements that has
existed in previous observations and assures that processes thought to
operate in that gap will be tested by direct observation.  For
example, it has been suggested that the particles powering Jovian
aurora are ions of energies <=100 keV/nucl, a composition energy range
to be measured by Galileo instrumentation at Jupiter.

EPD OVERVIEW

	The EPD instrument is the result of a joint effort between The
Johns Hopkins University Applied Physics Laboratory (JHU/APL), The
Max-Planck-Institute fur Aeronomie (MPAe) and The National Oceanic and
Atmospheric Administration Space Environment Laboratory (NOAA/SEL).
Proposed in 1976 with initial funds received in late 1977, the EPD was
launched onboard the Galileo spacecraft on October 12, 1989.  The MPAe
was responsible for the detector heads and three analog circuit boards
associated with those heads.  The NOAA/SEL was responsible for the
original time-of-flight (TOF) circuitry.  The TOF circuitry employed
in the upgraded TOF detector actually flown (and described in the
composition measurement system, CMS, section) was the joint
responsibility of MPAe and JHU/APL.  The JHU/APL was responsible for
all remaining electronics, the scanning motor, the data system,
instrument power, structure test, instrument integration, and
spacecraft integration.  Calibrations were performed by JHU/APL and
MPAe.
	The general characteristics of the EPD are listed in the
following table:


-----------------------------------------------------------------------
      Galileo Energetic Particle Detector (EPD) characteristics
-----------------------------------------------------------------------
Mass: 10.5kg	Power: 6W electronics; 4W heaters    Bit rate: 912bps
Size: 19.5cm x 27cm x 36.1cm

Two bi-directional telescopes mounted on stepper platform

4pi steradian coverage with 52 to 420 samples every 7 S/C spins (~140s)

Geometric factors: 6E-03 - 5E-01 cm^2/ster, dependent on detector head

Time resolution: 0.33-2.67 s dependant on rate channel

Magnetic deflection, deltaE x E, and time-of-flight systems

Energy coverage: (Mev/nucl)
0.02-55		Z>=1
0.025-15.5	Helium
0.012-10.7	Oxygen
0.01-13		Sulfur
0.01-15		Iron
0.015-11	Electrons

64 rate channels plus pulse height analysis
------------------------------------------------------------------------


	The two bi-directional solid-state detector telescopes are the
Low Energy Magnetospheric Measurement System (LEMMS) and the
Composition Measurement System (CMS).  These detector heads are
mounted on a platform and rotated by a stepper motor contained in the
main electronics box.  The combination of the satellite spin and the
stepper motor rotation (nominally stepping to the next position after
each spacecraft spin) provides 4 pi steradian coverage of the unit
sphere.  The 0 degree ends of the two telescopes have a clear field of
view over the unit sphere and also can be positioned behind a
foreground shield/source holder for background measurements and
in-flight calibrations.  The 180 degree ends experience obscuration
effects in motor positions 4, 5, and 6 caused by the magnetometer boom
and foreground shield.
	The zero degree end of the LEMMS unit uses magnetic deflection
to separate electrons from ions and provides, from detectors A and B,
total-ion energy above ~20keV and from detectors E1, E2 and F1, F2
electron spectra above ~15keV.  The 180 degree end of LEMMS uses
absorbers in combination with detectors C and D to provide
measurements of ions >~16Mev and electrons >~2Mev.
	The zero degree end of the CMS telescope employs a
time-of-flight (TOF) versus total energy technique to measure
elemental energy spectra above ~10keV/nucl for helium through iron.  A
sweeping magnet in the entrance collimator prevents electrons with
energies <~256keV from entering the system.  TOF start and stop pulses
are generated as the incoming ions pass, respectively, through a thin
entrance foil and impinge on the detector KT.  Electrons released form
the foil and the detector are accelerated and deflected through a
series of grids and are detected by the microchannel places, MCP1 and
MCP2.  The time difference between the start pulse, MCP1, and the stop
pulse, MCP2, is then obtained, along with the ion total energy from
KT.  Knowing the ion total energy and its travel time through the
system (which gives its velocity), the ion mass is determined.
	The 180 degree end of the CMS telescope measures the ion
energy loss, deltaE, as the ions pass through detectors Ja and Jb and
the ion residual energy E=E(total) - deltaE, as they impact detectors
Ka and Kb.  The resulting deltaE and E measurement provides a measure
of ion composition for energies >~200keV/nucl.
	The planned norman mode of EPD operation is to have both the
telescopes powered and to step the stepper platform once each
satellite spin.  This will yield a 4-pi scan of the unit sphere
approximately every 140s.  Many other scanning modes are available and
will be used for special circumstances.  For example, during satellite
encounters, the EPD will be configured to scan particular directions
such as the expected direction of the magnetic flux tube, the
direction of the Galilean satellite wakes as they travel through the
Jovian magnetosphere, and the direction of the E x B drift paths.


	The following table contains the channel energy ranges and
geometric factors for the detectors on the LEMMS telescope.

Channel         Species         Energy Range    Geometric Factor
Name                            (MeV)           (cm**2 sr)
--------------------------------------------------------------------------------
A0              Z >= 1          0.022-  0.042   0.006
A1              Z >= 1          0.042-  0.065   0.006
A2              Z >= 1          0.065-  0.120   0.006
A3              Z >= 1          0.120-  0.280   0.006
A4              Z >= 1          0.280-  0.515   0.006
A5              Z >= 1          0.515-  0.825   0.006
A6              Z >= 1          0.825-  1.68    0.006
A7              Z >= 1          1.68 -  3.20    0.006
A8              Z >= 2          3.50 - 12.4     0.006
B0		Z  = 1		3.20 - 10.1     0.006
B1		electrons      ~1.5  - 10.5	0.006
B2		Z  = 2	       16.0  -100.	0.006
DC0		Z >= 1	       14.5  - 33.5     0.5
DC1		Z >= 1	       51.   - 59. 	0.5
DC2		electrons	     >~ 2.	0.5
DC3		electrons	     >~11.	0.5
E0              electrons       0.015-  0.029   0.006*
E1              electrons       0.029-  0.042   0.020*
E2              electrons       0.042-  0.055   0.030*
E3              electrons       0.055-  0.093   0.033*
F0              electrons       0.093-  0.188   0.028*
F1              electrons       0.174-  0.304   0.007*
F2              electrons       0.304-  0.527   0.016*
F3              electrons       0.527-  0.884   0.018*
AS		singles		all counts	0.006
				in detector
BS		singles		all counts	0.006
				in detector
CS		singles		all counts	0.5
				in detector
DS		singles		all counts	0.5
				in detector
EB1		background	sidewise penetrators
EB2		background	E1E2 coincidences
FB1		background	Sidewise penetrators
FB2		background	F1F2 coincidences

*  Geometric factor determined from table in paper by Y. Wu, T.P.
   Armstrong [WUARMSTRONG87].


	The following table contains the channel energy ranges and
geometric factors for the detectors on the CMS telescope.

Channel         Species         Energy Range    Geometric Factor
Name                            (MeV nucl^-1)     (cm**2 sr)
------------------------------------------------------------------------
TOF x E
------------------------------------------------------------------------
TP1		protons		0.08-0.22	0.007
TP2		protons		0.22-0.54	0.007
TP3		protons		0.54-1.25	0.007
TA1		alphas		0.027-0.155	0.007
TA2		alphas		0.155-1.00	0.007
TO1		medium nuclei	0.012-0.026	0.007
TO2		medium nuclei	0.026-0.051	0.007
TO3		medium nuclei	0.051-0.112	0.007
TO4		medium nuclei	0.112-0.562	0.007
TS1		intermediate	0.016-0.030	0.007
TS2		intermediate	0.030-0.062	0.007
TS3		intermediate	0.062-0.31	0.007
TH1		heavy nuclei	0.02 -0.20	0.007
TACS		singles
STARTS		rates
KtS		

------------------------------------------------------------------------
Delta E x E
------------------------------------------------------------------------
CA1		alphas		0.17- 0.49
CA3		alphas		0.49- 0.68
CA4		alphas		0.68- 1.4
CM1		medium nuclei	0.16- 0.55
CM3		medium nuclei	0.55- 1.1
CM4		medium nuclei	1.1 - 2.9
CM5		medium nuclei	2.9 -10.7
CN0		intermediate	1.0 - 2.2
CN1		intermediate	2.2 -11.7
CH1		heavy nuclei	0.22- 0.33
CH3		heavy nuclei	0.33- 0.67
CH4		heavy nuclei	0.67- 1.3
CH5		heavy nuclei	1.3 -15.0
JaS		singles rates
JbS		singles rates
KS		singles rates

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