The Galileo Energetic Particles Detector

From Space Science Reviews

LEMMS

 The Low Energy Magnetospheric Measurements System

The LEMMS telescopes are designed to measure low to medium energy ion and electron fluxes with wide dynamic range and high angular (<20 deg) and temporal (1/3 to 4/3 s) resolution. As stated previously, a full 4pi coverage of the unit sphere is obtained by the use of a stepping platform in conjunction with satellite spin.

The LEMMS detector head is a double-ended telescope containing eight heavily shielded silicon solid state surface barrier totally depleted detectors providing measurements of electrons from 15 keV to > 11 MeV, and ios from 22 keV to eV to ~55 MeV, in 32 rate channels.

Detail of the EPD LEMMS detector head.

Graphical summary of EPD energy and species rate channel coverage. Additional high energy resolution and detailed species identification are obtained from pulse height analysis data gathered simultaneously with the rate channel data show.

LEMMS Channel Descriptions

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.

The geometric factor of the collimator and magnetic optics system has been analyzed by extensive computer modeling (Wu and Armstrong, 1988). The energy dependent aperature-integrated geometry factors for detectors E1 and F1 are shown here.

Response of the EPD LEMMS low energy electron detectors. The geometric factor, as calculated from a detailed simulation shown as a function of energy.

A diagram of the LEMMS analog electronics is included next. The unbuffered preamp tail-pulse output signals are sent down the polytwist wiring harness to three LEMMS analog boards, where the signals are amplified and shaped. All signal processing in the LEMMS channels is done in the linear domain. Comparator-like discriminator circuits assigned to each of the eight channels fire whe pre-defined threshold levels are exceeded.

The resulting discriminator outputs are sent to the rate logic electronics, where combinatorial logic is used to define 32 rate channels (event bins).

LEMMS data also are processed through a pulse height analyzer (PHA) that produces 13 46-channel energy spectra per major frame.

Block diagram of LEMMS analog electronics.

The detector heads are mounted on a platform and rotated by a stepper motor contained in the main electronics box. The stepper motor positions as seen looking down onto the top of the EPD along the motor rotation axis can be seen in the figure below

View looking down EPD along stepper motor rotation axis showing EPD viewing positions. Galileo spin axis direction is parallel to plane of figure and from right to left. Up to 60 samples per spin for the nominal spin period of 20s are obtained, giving good angular resolution over the full 4pi steradian of the unit sphere.

The combination of the satellite spin and the stepper motor rotation (nominally stepping to the next position after each spacecraft spin) provides 4pi steradian coverage of the unit sphere. The zero 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 primary elements of the system are the LEMMS/CMS detector heads with their analog electronics, the motorized scanning system, the digital support electronics, and the data system. The following figure shows a functional block diagram of the EPD.

EPD functional block diagram.

Return to main Galileo Table of Contents Page.
Return to Fundamental Technologies Home Page.

QUICK FACTS

RELATED LINKS