Galileo Heavy Ion Counter

Science Requirements Document
Section 1 of 4
Next Section

Thomas L. Garrard

Objectives and Overview

The Galileo Heavy Ion Counter (HIC) was added onto the Galileo mission for the purpose of monitoring highly ionizing energetic particles capable of causing "single event upsets" in the memory chips, etc. of the spacecraft in the Jovian magnetosphere. The instrument is optimized to measure the energy spectra and charge composition of oxygen, sulfur, and sodium in the Jovian magnetosphere from ~ 5 MeV/nucleon to ~ 200 MeV/nucleon. We planned to monitor penetrating galactic carbon, nitrogen, and oxygen during the cruise phase and in the outer magnetosphere because these nuclei can also cause "upsets". The capability to cover a charge range up to Fe at low energies is desirable for the cruise phase and for the outer magnetosphere for scientific reasons.

The radial distance range covered by Galileo extends from ~ 4 RJ to more than 100 RJ. The telescope is constrained to function in the flux maxima around 4 RJ and around 7-8 RJ. It should survive radiation damage from fluences calculated for mission lifetimes of more than a year. The instrument also had to be be built within cost, weight, power, volume, time, et cetera limitation imposed by its nature as an add-on to the existing mission.

These constraints were met by using the existing Voyager CRS PTM (Proof-Test Model) instrument. The LET telescopes have demonstrated the ability to resolve O, Na, and S in the Jovian magnetosphere. Eight of these telescopes were exposed to the radiation environment with shielding of only 3 microns Aluminum and only one detector was lost. We adapted the instrument to extend its energy range and to improve its resolution by rerouting LET detector signals into the HET electronics and providing better collimation and a thicker window. To conserve telemetry and emphasize the heavies we raised all thresholds above proton and alpha signals. One of the parallel HET/LET "blocks" of electronics was extracted from the PTM and repackaged to meet volume and shielding constraints.

Relevant flux levels are quite small. The sensitive area of one of the IC's used on Galileo is ~ 3.4 x 10^^-3 cm^^2. AW is ~ 0.01 cm^^2 sr. For a circuit to have a 2% chance of seeing an oxygen nucleus, the fluence is 2 Oxygen/cm^^2 sr. To observe 10 oxygens we must have a geometry factor of 5 cm^^2 sr. Thus one telescope, LET E, has a widened acceptance geometry.

In order to interface signals to the Galileo spacecraft an adapter was added which receives the Voyager data and outputs Galileo data. This adapter also translates Galileo commands for the CRS Voyager interface.

HIC is mounted on top of Bay 2 in the spinning portion of the spacecraft. This location is shown in the JPL Galileo Spacecraft Mechanical Configuration, drawing number 10084461. (See also JPL ICD 10086786.) The telescopes are oriented about 10° "below" the normal to the spin axis, in order to keep the sunshade out of the field of view. In spacecraft coordinates, our boresight is TBD.

Analysis Conditions

Events are generated when particles generate signals satisfying the requirements shown below. Table 1 shows the "physically relevant" conditions for analysis of events of various types.

Table 1 -- Events
Event condition geometry
energy range
LETB: LB1.LB2.LB3.LB4* 0.4293 ~ 4.8 to 17.5 for oxygen including L1.L2
DUBL: LE1.LE2.LE3* 0.435 ~ 17 to 18 for oxygen
TRPL: LE1.LE2.LE3.LE4* 0.435 ~18 to 24 nuc for oxygen.
WDSTP: LE2.LE3.LE4.LE5* ~4.006 ~30** to 48 for oxygen.
WDPEN: LE2.LE3.LE4.LE5 ~4.006 ~48 to 185 for oxygen. Cutoff at 185 due to LE1 threshold for oxygen but not sulfur.
HGPEN: LE2.LE3.LE4.LE5.HG ~4.006 Cosmic ray carbon and heavier. Carbon from ~41 MeV/nuc up.
** Energy assignment depends on whether L1 fired.

Next Section