Gamma-Ray Imaging Spectrometer

History

GRIS followed earlier gamma ray spectroscopy work by Bell Labs/Sandia National Laboratories and co-investigators Marvin Leventhal and Bonnard Teegarden, including the LEGS spectrometer. GRIS was selected for a balloon program after the removal of a high-resolution gamma-ray spectrometer from the payload of what would become the Compton Gamma Ray Observatory.

GRIS first flew in May 1988 from Alice Springs, Australia. During its first several flights, the instrument definitively measured the gamma ray lines from Supernova 1987A, including that of ⁵⁶Co, and the positron annihilation line from the Galactic Center at 511 keV, elucidating the nature of these emissions. These measurements resulted in two letters in Nature and three in The Astrophysical Journal, and earned the John Lindsay Memorial Award for Science from Goddard Space Flight Center.

GRIS was flown a total of nine times between 1988 and 1995, with a total flight time of 223 hours. In a configuration that included a wide-field collimator and blocking crystal mechanism, GRIS measured the diffuse galactic and cosmic gamma-ray spectra, yielding insight into the production of ²⁶Al in the galaxy. During its final two flights from Alice Springs, GRIS carried the PoRTIA instrument, which yielded measurements of the CdZnTe detector background for use in future instrument design.

Following the program, the Goddard team proposed transferring the instrument to the University of Maryland to be refurbished for the Long Duration Balloon program, which would entail reconfiguring the instrument for wide field-of-view studies of diffuse emissions. The program would involve graduate and undergraduate student researchers, and would address observation regimes inaccessible to the INTEGRAL mission.