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Heavy Ions in Space

HIIS and the Long Duration Exposure Facility HIIS compaired to other experiments Detector Design Detector Calibration Scientific Results from HIIS

HIIS and the Long Duration Exposure Facility (LDEF)

The Heavy Ions in Space (HIIS) experiment flew aboard NASA's Long Duration Exposure Facility (LDEF) for 5.8 years, from 7 April 1984 to 12 January 1990. LDEF was a 3-axis stabilized spacecraft deployed in a nearly-circular 28.4 degree orbit at a mean altitude of 476 km for most of its mission. The orbit decayed rapidly in the final year and the satellite was retrieved at ~330 km. The HIIS detectors (circled in the above photograph) were mounted on the space-facing end of LDEF and comprised large stacks of plastic track detectors with a total geometry factor of 2.0 m^2-sr.

HIIS Compared to Other Cosmic Ray Experiments

HIIS is one of the largest cosmic ray experiments ever flown. In this figure, the collecting powers of ultraheavy Galactic cosmic ray experiments, as measured by the total number of collected Galactic cosmic ray iron nuclei vs. minimum kinetic energy, are compared. Skylab, HEAO, ARIEL VI, TREK, HIIS, and UHCRE (solid curves) have been flown. HNC is envisioned for the International Space Station but will not return data in this century. The only flown experiment larger than HIIS is another LDEF experiment, the UltraHeavy Cosmic Ray Experiment (UHCRE) from the Dublin Institute of Advanced Studies. HIIS and UHCRE are complementary experiments, with HIIS ultraheavy measurements beginning at Z=45 and the larger collecting power of UHCRE's primary detectors sensitive only to the rarest cosmic rays at Z>65.

The figure compares the collecting power of instruments which detect cosmic rays by bringing them to rest. The ISEE-3, Ulysses, CRRES, TRIS, Spacelab I, Spacelab III, and HIIS instruments have been flown. SAMPEX, Wind, and Geotail are currently in orbit. ACE (dashed lines) is planned for flight. The two spacelab missions and TRIS were flown less than a year, so for these missions the total number of Si events is plotted. A second flight of TRIS is planned for March 1996.

Detector Design

The HIIS experiment was located in two trays on LDEF's space-facing end. Each tray contained four detector modules. Each module comprised two separate stacks of plastic track detectors, a main stack which was sealed in one atmosphere of dry air and a top stack which was in vacuum. Most of the sheets in the main stacks were CR39 (with DOP), which was manufactured according to a special process designed for producing highly-uniform, detector-quality material. The top stacks and some sheets in the main stacks were Lexan. The total vertical thickness of the detector module was 12 g/cm^2. The total number of detector sheets is 2782, each with an area of 1064 cm^2.

To extend the detector's range to lower energies, one of the eight modules had a special design in which both stacks were sealed in one atmosphere of dry air and the honeycomb lid in the figure was replaced with a thin Kapton window. This module lost its air when the Kapton window was punctured by a micrometeoroid.

For a detailed description of the HIIS detectors, see: James H. Adams, Jr. et al., Proceedings of the First LDEF Post-Retrieval Symposium (NASA CP-3134), 1, 377-391 (1991).

Detector Calibration

Because of the extended duration we have used an internal calibration, derived from the cosmic ray tracks themselves. The figure shows the measurements from 97 stopping heavy-ion tracks located in CR39 sheets near the top of one module. The data organize themselves into densely populated bands, with few tracks above the topmost band. Since nuclei heavier than Fe are rare, the strong accumulation of tracks near the top of the plot was assumed to be Fe. To derive an internal calibration, we used a subset of these Fe tracks as well as a few lighter tracks, which extended the calibration to more lightly-ionizing species. The curves in the figure, which were derived from this calibration, show the detector response for elements with atomic numbers Z=14-28, with solid and dashed curves for even and odd elements, respectively.

This figure shows the charge resolution we obtained for the HIIS stopping ions. It is a histogram of the fitted atomic numbers of ions found throughout the stack (not just the top section, from which the calibration was derived.) The clear Fe peak is fit to a Gaussian with standard deviation of 0.43 +/- 0.04 charge units.

For more information on the HIIS calibration, see: James H. Adams, Jr. et al. Proceedings of the Second LDEF Post-Retrival Symposium (NASA CP-3194), 1, 247-259 (1993). Allan J. Tylka, et al., Proceedings of the Third LDEF Post-Retrieval Symposim (NASA CP-3275), 1, 113-127 (1993).

Scientific Results from HIIS

Ionic Charge States of Solar Energetic Heavy Ions:

Ionic charge state measurements are one of the most powerful probes of the source plasma and acceleration mechanisms which give rise to solar energetic particles (SEPs). Prior to the flight of HIIS, our only knowledge of SEP charge states consisted of direct measurements at ~1 MeV/nuc and indirect suggestions that these charge states continued to ~10 MeV/nuc. The HIIS experiment has provided the first measurements of the ionic charge state of solar energetic Fe ions at very high energies, ~200 - 600 MeV/nuc. More information on HIIS studies of SEP charge states can be found in the following papers:

1. Tylka, A.J., et al. 1995, ApJ Letters, 444, L109-L113, 'The Mean Ionic Charge State of Solar Energetic Fe Ions Above 200 MeV per Nucleon'

        Abstract   Complete Paper
   

2. Tylka, A.J., et al. Proceedings of the High Energy Solar Phenomena (HESP) Workshop, NASA/Goddard Space Flight Center, Greenbelt, MD, 16-18 August 1995, 'HIIS Results on the Mean Ionic Charge State of SEP Fe above 200 MeV per nucleon' (AIP, in press)

        Abstract   Complete Paper
   

3. Boberg, P.R., et al. 1995, Proc. 24th Int. Cosmic Ray Conf. (Rome), 4, pages 466-469, 'The Source Plasma of Solar Energetic Particles in Gradual Events'

        Abstract   Complete Paper
   

4. Boberg, P.R., et al. 1995, Geophys. Res. Lett., 22, 1133-1136, 'Geomagnetic Transmission of Solar Energetic Protons During the Geomagnetic Disturbances of October 1989'

        Abstract   Complete Paper
   

Partially-ionized Galactic Cosmic Rays:

Galactic cosmic ray nuclei are generally believed to be completely stripped of electrons because of their extended journey through the interstellar medium. Cosmic ray nuclei with some electrons still attached would indicate a nearby source of Galactic cosmic rays. In recent years, several experiments have reported observations which have been interpreted as evidence of partially-ionized Galactic cosmic rays. The HIIS experiment does not confirm these earlier reports and places stringent upper limits on the flux of partially ionized Galactic cosmic rays. Details about this result can be found in:

1. Tylka, A.J., et al. 1995, ApJ Letters, 438, L83-L86, 'On Purported Observations of Partially Ionized Galactic Cosmic Rays'

        Abstract   Complete Paper
   

Geomagnetically-Trapped Energetic Heavy Ions:

One of the most important results in low-energy cosmic ray physics in recent years has been the discovery of geomagnetically trapped anomalous cosmic rays in Earth's inner magnetosphere. This distinct trapped population, which some have referred to as a "third radiation belt", consists primarily of N, O, and Ne nuclei with energies of ~5-30 MeV per nucleon. HIIS, in conjunction with other cosmic ray detectors aboard LDEF, has provided new observations of these 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. More information about LDEF observations of trapped heavy ions can be found in:

1. Kleis, T., et al. 1996, Adv. Space Res., 17, (2)163-(2)166, 'Low Energy Ions in the Heavy Ions In Space (HIIS) Experiment on LDEF'

        Abstract   Complete Paper
   

2. Kleis, T., et al. 1995, Proc. 24th Int. Cosmic Ray Conf. (Rome), 4, pages 481-484, 'Trapped Low-Energy Heavy Ion Results from LDEF'

        Abstract   Complete Paper
   

Ultraheavy Galactic Cosmic Rays:

Measurements of the relative abundances of cosmic ray nuclei heavier than Fe provide valuable clues on the origin and propagation history of Galactic cosmic rays. Because of its large geometry factor and extended exposure, HIIS collected a sample of these "ultraheavy" Galactic cosmic rays (UHGCRs) roughly three times larger than accumulated in previous experiments. Analysis of the HIIS UHGCRs is still in progress. Click HERE for more information on the HIIS UHGCR measurements.