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The 12-Detector Germanium Spectrometer and its balloon gondola, together known as HEXAGONE, was developed for observations of cosmic gamma ray sources at the highest feasible energy resolution and sensitivity using innovative Ge detector systems over the 20 keV to 10 MeV range, and for the development of techniques and technologies required to implement similar instruments on space missions. The program was led by UCSD in collaboration with UC Berkeley, and CESR and CEA in France. Its first flight was on May 22, 1989 as part of the SN1987A campaign.

HEXAGONE’s 5.6 cm diameter x 5.6 cm long detectors had closed-end coaxial geometry and a reversed electrode configuration where positive bias was supplied to an outer, very thin electrode. This produced a negligible dead layer and allowed for good efficiency down to nearly 10 keV. The energy resolution was typically 2.3 keV at 1.3 MeV and 1.2 keV at 122 keV. It achieved low background and high sensitivity by combining 1) Ge detector segmentation and pulse shape discrimination to discriminate against beta decay radioactivity while accepting gamma rays, 2) an array of 12 detectors based upon a modular concept to obtain a large detector volume, and 3) a 5 cm thick BGO anti-coincidence shield to reduce the measured flux of ambient gamma rays.

The Ge detectors were contained in three cryostats, each of which contained 4 detectors. Each cryostat had its own 10 liter liquid nitrogen Dewar with 3 to 4 day holding time. Surrounding the detectors was an anti-coincidence shield made of 240 kg of BGO and 60 kg of CsI, viewed by 61 photomultipliers. The 5 cm BGO thickness gave 1% transmission of 511 keV gamma rays. This reduced the atmospheric 511 keV line flux transmitted to the detectors to the same level as the flux from the Galactic Center.

The balloon gondola carried the instrument in an elevation gimbal. For azimuth pointing, the entire gondola was servoed by a rotor located 3.3 meters above the instrument. The rotor used a simple worm gear drive, which torqued against the balloon through the parachute/load line. The problem of instabilities introduced by the torsionally soft coupling of parachute/load line to the massive inertia of the balloon was overcome by the introduction of a torque bridge. The bridge measured the torque input to the balloon gondola by the parachute/load line, and its signal along with the gondola azimuth offset and angular velocity provided the information necessary for a stable servo. The system achieved 0.1 degree stability.

The May, 1989 flight was a scientific and technical success providing upper limits on cobalt radioactive lines in 10 hours of observations of SN1987A, a measurement of the 511 keV line from the Galactic Center in 6.3 hours of observing, and 1.3 hours on the Crab Nebula and the X-ray transient A0535+26. A subsequent flight from Australia resulted in a mechanical failure associated with parachute shock at the end of the flight and the gondola and instrument free fell from 90,000 feet. The system was completely destroyed upon impact.

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