Energetic Particle Composition Experiment (EPAC)

The Energetic PArticles Composition instrument EPAC 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°, 67.5°, 112.5° and 157.5° 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°.

As a front detector we used an very thin semiconductor detector with a thickness of 5 µm 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 µm 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 µg/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 +10°C near Earth and -35°C at Jupiter.

A full description can be found in: E. Keppler et al., Astron. Astrophys. Suppl. Ser. 92, 317-331, 1992

EPAC (Energetic PArticles Composition) Team Home Page

Back to Description of Instruments