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Instrument: EPAC : Energetic Particles Composition Instrument (Ulysses)
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Associated Platforms

Spectral/Frequency Information
Wavelength Keyword: Gamma Ray
Number Channels: 4
Spectral/Frequency Coverage/Range: 0.1 MeV - 1.5 MeV

Related Data Sets
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The Energetic PArticles Composition instrument EPAC pn Ulysses was designed to
provide information on the flux, anisotropy and chemical composition of
energetic particles in interplanetary space. EPAC consists of four identical
telescopes inclined under angles of 22.5 deg, 67.5 deg, 112.5 deg and 157.5 deg
with respect to the spacecraft spin axis. This design, together with spin
sectorisation, allows us to sample 80% of the sphere in 32 bins and therefore
get a fully three-dimensional resolution of angular distributions. In each of
the telescopes we used the so-called "E-dE/dx" technique, which requires a
particle to traverse a very thin detector and then stop it in a second, much
thicker detector. Particles of higher energies can traverse the two detector
stack, but are eliminated by a third "veto" detector. Each telescope has a
geometric factor of about 0.08 cm"sr and has a field-of-view with a full angle
of 35 deg.

As a front detector we used an very thin semiconductor detector with a
thickness of 5 um and 25 mm" sensitive area (detector A). Such detectors became
available in reliable technique by the time when we started to build the
instrument. The energy detector B was 100 ým thick. A third detector (C) of
much larger area provided veto signals from penetrating particles. The detector
stack is surrounded by a massive platinum shield. Further background rejection
is realised by using multiparameter analysis. The front detectors are protected
against sunlight by 80 ug/cm" Al-layers. By using hybrid electronic technology
we were able to make this fairly complicated instrument with very low weight
(2685 g) and power demand (3.43 W).

The telescopes based on this design allowed clear separation of Hydrogen,
Helium and the heavier nuclei up to iron. 14 different categories of data are
transmitted to the ground, using different time and angular resolutions for the
various categories. In flight a functional performance test using a pulse
generator, which on command produced sequences of coincident and non-coincident
pulses is initiated from time to time. The sensor was designed to operate at
temperatures between +10C near Earth and -35C at Jupiter.
A full description can be found in: E. Keppler et al., Astron. Astrophys.
Suppl. Ser. 92, 317-331, 1992

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Online Resources

Instrument Logistics
Data Rate: 0.012 kbps
Instrument Start Date: 1990-10-06
Instrument Owner: Max Planck Institute