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NRL began studies of cosmic rays in 1949. For the next ten years NRL physicists developed detectors for identifying relativistic particles in cosmic rays. The detectors consisted of stacks of sensitive emulsions, and after a particle passed through, the path could be traced and analyzed. In the 1960s and 1970s, the NRL cosmic-ray-physics program shifted from particle studies to the stars, the origin of these cosmic rays. During the Gemini XI mission, NRL had several detector trays on the spacecraft that yielded important information on the origin and history of cosmic rays. NRL's principal vehicle for studying cosmic rays, however, was the Skyhook balloon, a 10-million-cubic-foot plastic balloon that carried detectors to an altitude of 140,000 feet.
From April 7, 1984, to January 12, 1990, NRL's Heavy Ions in Space (HIIS) experiment flew aboard NASA's Long Duration Exposure Facility (LDEF). The HIIS detectors were mounted on the space-facing end of the LDEF and comprised large stacks of plastic track detectors with a total geometry factor of 2.0 m2-sr. HIIS is one of the largest cosmic ray experiments ever flown.
The following are some of the scientific results from HIIS:
- The HIIS experiment provided the first measurements of the ionic charge state of solar energetic Fe ions at very high energies.
- HIIS, in conjunction with other cosmic ray detectors aboard LDEF, provided new observations of trapped anomalous cosmic rays. In addition, the LDEF experiments have provided evidence of additional species (Mg, Si, and Fe) whose origin is not yet understood.
- HIIS collected a sample of "ultraheavy" galactic cosmic rays (UHGCRs) roughly three times larger than accumulated in previous experiments. Analysis of the HIIS UHGCRs is still in progress.