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Energetic PArticles Composition / Interstellar Neutral Gas

The two sensor systems EPAC and GAS are dedicated to the interplanetary ions and neutral gas respectively. The GAS detector was originally to fly on the NASA satellite of the dual-spacecraft ISPM (International Solar-Polar Mission), but after cancellation of the US satellite it was incorporated in the Ulysses payload, closely interfaced with the EPAC instrument.

EPAC - Energetic PArticles Composition

The EPAC sensor is designed to measure the fluxes, angular distributions, energy spectra, and composition of ions in the energy range from 300 keV/nucleon to 25 MeV/nucleon. It comprises four telescopes that each use the dE/dx - E technique, where particles traverse a thin detector and then stop it in a second, much thicker detector. Particles that traverse the two detector stack, are eliminated by a third veto-detector.

The four telescopes (T1-T4) are mounted such that their central axes include angles of 22.5°, 67.5°, 112.5°, and 157.5°, with respect to the spacecraft spin axis. Each telescope has a field-of-view with a full angle of 35°. Thus the instrument covers 80% of the full sphere during one spacecraft rotation. Each telescope has a geometric factor of about 0.08 cm² sr.

Each telescope consists of three Si-surface barrier detectors, A, B, and C surrounded by a massive platinum shield. Background rejection is realised by using multiparameter analysis. All four telescopes operate in a self-calibrating dE/dx (detector A) versus E (detector B) mode, wherein particle tracks can be used to obtain conclusive absolute calibrations. Each telescope and associated electronics is able to measure the elemental composition of low-energy nuclei from hydrogen to iron.

Detector Area
Voltage (V)
A (dE/dx) 25 5 10
B (E) 80 100 30
C (veto) 300 300 70
Detector dimensions for all four telescopes

The front detectors (A) are protected against sunlight by Al-layers, 80 µg cm-2 thick, and are used with the Al-side facing the incoming particles. The B-detectors are installed with their Au-side facing the front detector to minimize radiation damage.

GAS - Interstellar Neutral Gas

The Solar System moves through the local interstellar medium (IM) with a relative velocity of 20 kms-1. The Sun's magnetic field prevents the charged component of the IM from entering the Solar System, but not the neutral component. The properties of the local interstellar gas, represented by neutral helium penetrating the heliosphere, can therefore be measured in-situ by the Ulysses GAS instrument.

The neutral helium particles are detected via the secondary electrons or ions which are emitted upon particle impact from a freshly deposited lithium-fluoride (LiF) layer in the GAS detector.

Two nearly identical detector channels are housed in a vacuum-tight box of 9.1 × 5.6 × 3.4 cm. The fields of view are limited by two circular apertures each to full opening angles of 4.9° in channel (I) and 7.4° for channel (II). The outer apertures are protected by a simple, asymmetric baffle against direct sunlight.

Incoming particles first pass electrostatic deflection systems, which serve as filters against charged particles up to energies per charge of ~80 kV in channel (I) and ~50 kV in channel (II) due to a DC-voltage between the plates. The conversion plates consisting of reduced (black and conductive) lead glass are mounted on ceramic thick- film resistors, used as heaters to bake out the conversion plates at temperatures up to 200 °C. The plates, with an effective area of about 8 × 10 mm, are inclined by 45° to the optical axis towards the channeltrons and by 28° towards the furnace, which is mounted in the middle between the two channels.

The tiny furnace is filled with about 2 mm³ of LiF. On telecommand a helix (1 mm diameter, 3 mm length) of platinum wire heats the LiF efficiently up to about 600 °C, where a mild evaporation of the LiF starts. The evaporated LiF is deposited simultaneously on both the conversion plates and a small quartz crystal used to monitor the thickness of the deposited layers. The supply is sufficient for a total thickness of 150 nm of the deposited layers, or about 20 evaporation processes.

Secondary particles released from the conversion plates after particle impact are accelerated towards the funnel of the two (one for each channel) Channel Electron Multipliers (CEMs) by an electric field between the conversion plate and the plane grid in front of the CEM. Here the number of secondary particles is measured. The acceleration potential can be selected by telecommands to select the detection mode. It is about -2700 V for detection of the positive secondary ions or about +420 V for detection of secondary electrons. The two channels (I) and (II) are always operated simultaneously in the same mode (electron or ion detection).

The sensor head is mounted on a turntable, with the rotation axis oriented perpendicular to the spin axis of the spacecraft. This way the elevation angle between the spin axis and the optical axis of the sensor can be varied between 0° and 180° with a minimum step width of 1°. Then, together with the rotation of the spacecraft, the whole celestial sphere can be scanned.

Summary of Objectives

Among the range of investigations the EPAC measurements contribute to, are the studies of:

  • Variations in the elemental abundances of low-energy charged particle populations close to the Sun:
    - The influence of various coronal structures upon coronal transport
    - The energy dependence of coronal storage
    - The overall characteristics of interplanetary propagation of these particles at different heliographic latitudes
    - The influence of the interplanetary magnetic field and its fluctuations on particle propagation (diffusion)
    - The effects of solar-wind streams (convection and adiabatic deceleration)
    - The importance of gradient and curvature drifts
    - The influence of the induced electric-field drift on low- energy particles
    - The dependence on heliographic latitude of all of the above components
  • The acceleration mechanism of interplanetary energetic particles that are accelerated in interplanetary space out of the thermal and suprathermal tails of the ion energy distributions by for example interplanetary-propagating shock waves or corotating (with the Sun) interaction regions
  • The occurence and latitudinal dependence of the anomalous cosmic ray (ACR) component (ACR particles originate at the edge of the heliopause, as opposed to from within our galaxy like most of the galactic cosmic rays). ACR are predominantly singly charged, moderate-velocity, high-rigidity ions that have their origin as interstellar neutrals which have been ionized in interplanetary space, convected outward by the solar wind and accelerated by the termination shock
  • The penetration and streaming of galactic cosmic rays into the heliosphere

The main goal of the GAS instrument is to determine the density, bulk velocity relative to the Solar System, and temperature of the interstellar particles penetrating the Solar System.

Last Update: 08 December 2006

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