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Heavy Ion Counter (HIC)

NSSDC ID: 1989-084B-29
Mission Name: Galileo Orbiter
Principal Investigator: Prof. Edward C. Stone, Jr.


The Heavy Ion Counter (HIC) experiment on Galileo was included on the payload originally as an engineering experiment. Its purpose was to monitor the flux of energetic heavy ions in order to: (1) provide basic information on a form of radiation to which electronic circuitry is highly susceptible; and, (2) provide the basis for the design of better radiation-hardened electronics for future missions and applications. It was quickly realized, however, that the scientific utility of such data would complement not only the observations made by other Galileo instruments, but would improve on and extend observations made during earlier missions (i.e., Voyager 1/2, Pioneer 10/11).

The HIC was designed to detect ions from 6C to 28Ni with energies from about 6 MeV/nucleon to more than 200 MeV/nucleon. The experiment consisted of two solid-state detector telescopes called Low-Energy Telescopes (LETs). The two LETs, located on the spun section of the spacecraft, were used so that a wide-range of energies could be sampled while minimizing the possibility of "pulse pile-up", the process by which the flux of lower-energy particles overwhelm the detector. One LET (LET E) was used for the detection of ions with energies up to 200 MeV/nucleon, but its thicker aperture barriers prevented not only the entrance of low-energy protons, but also lower energy oxygen and sulfur ions.This effectively prevented the detection of heavy ions below an energy of 15 MeV/nucleon. The other particle telescope (LET B) was therefore used to detect ions down to 6 MeV/nucleon. In order to accomplish this, LET B had a more narrowly collimated aperture, to prevent the entrance of low-energy protons and helium, but thinner window barriers, to permit the detection lower-energy heavy ions. This particular telescope (LET B) was an improved version of the Voyager Cosmic Ray System (CRS) LETs, with additional aperture collimation and thicker windows to prevent contamination of the signal by background protons. Indeed, the electronics for the HIC, except for the interface to the spacecraft, were originally part of the test model for the Voyager CRS as well.

Both LETs were oriented so that their aperture axes were near the spin plane of the spacecraft to take advantage of the nominal 3 rpm rotation and permit pitch-angle sampling of the incoming ions. The time-resolution of the HIC detectors was between 2/3 s and 2 s. This translated into an angular resolution of the pitch-angle distribution of between 12 and 36 degrees, compared to the aperture opening angles of 25 degrees (narrow geometry mode) and 46 degrees (wide geometry mode).

Alternate Names

  • HIC

Facts in Brief

Mass: 8.33 kg
Power (avg): 2.8 W

Funding Agency

  • NASA-Office of Space Science (United States)


  • Engineering: None assigned
  • Planetary Science: Fields and Particles

Additional Information

Questions or comments about this experiment can be directed to: Dr. Edwin V. Bell, II.



NameRoleOriginal AffiliationE-mail
Prof. Edward C. Stone, Jr.General ContactCalifornia Institute of

Selected References

Garrard, T. L., et al., The Galileo heavy element monitor, Space Sci. Rev., 60, No. 1-4, 305-315, May 1992.

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