HEXAGONE (High Energy X-ray And Gamma ray Observatory for Nuclear Emissions)

HEXAGONE was a high-resolution balloon-borne gamma-ray spectrometer developed to study astrophysical sources of gamma-ray line and continuum emission with exceptional spectral resolution and improved sensitivity. It was designed and built by a collaboration between American and French institutions, including the Center for Astrophysics and Space Sciences at the University of California, San Diego; the Space Sciences Laboratory and Lawrence Berkeley Laboratory at the University of California, Berkeley; the Centre d'Étude Spatiale des Rayonnements in Toulouse, France; and the Service d'Astrophysique at the Centre d'Études Nucléaires de Saclay, France. The project's primary goal was to observe phenomena such as electron-positron annihilation and radioactive decay lines in regions like the Galactic Center (GC) and to test novel instrumental techniques that could later be applied to future satellite missions.

In the image at left we can see an scheme of the instrument (click for more details). HEXAGONE detected gamma rays using an array of twelve high-purity germanium detectors, each about 55 mm in length and diameter. These detectors were cooled to 85 K using liquid nitrogen, allowing for energy resolutions of 1 keV below 100 keV and 2 keV at 1 MeV in laboratory conditions. During the balloon flight, the resolution degraded slightly to 2.2 keV at 511 keV. The total detector volume was 1568 cm�, and the active area was 285 cm². Five of the detectors were equipped with segmentation and pulse shape discrimination electronics to suppress background events caused by β-decay, particularly from induced radioactivity in the detectors themselves. Though these advanced capabilities were installed, they were not utilized for the data analysis presented from the flight. The detected energy losses were processed using dual-range 4096-channel pulse height analyzers, providing resolutions of 0.67 keV/channel up to 2.74 MeV and 3.35 keV/channel up to 13.7 MeV. Accepted events were telemetered in an event-by-event format at 40 kbps.

Surrounding the detectors was an active anticoincidence shield system designed to reject charged particles and secondary photons. The shield was composed of 240 kg of 5 cm thick bismuth germanate (BGO) scintillators at the sides and rear, and 55 kg of 10 cm thick CsI(Na) scintillators at the front. The CsI front shield had apertures to define the instrument's field of view (FWHM) to 18 degrees. Sixty-one photomultiplier tubes viewed the scintillators and allowed an anticoincidence threshold of 30 keV.

The total mass of the instrument was 540 kg, and it was mounted on a balloon gondola capable of alt-azimuth pointing control, with the complete gondola system total weight was 1134 kg. The gondola was designed to provide stable and accurate pointing for the spectrometer and included a star camera for aspect verification during observations.

Balloon launched on: 6/1/1992 at 7:47 utc
Launch site: Australian Balloon Launching Station, Alice Springs, Australia  
Balloon launched by: National Scientific Balloon Facility (NSBF)
Balloon manufacturer/size/composition: Zero Pressure Balloon SF3-480.10-080-NSCH-04-ST
Balloon serial number: W39.96-3-10
Flight identification number: 335N
End of flight (L for landing time, W for last contact, otherwise termination time): 6/1/1992 at 19:34 utc
Balloon flight duration (F: time at float only, otherwise total flight time in d:days / h:hours or m:minutes - ): 10 h
Landing site: In Mantuan Downs, Queensland, Australia

The experiment experienced a free-fall and resulted destroyed.

• High energy astronomy program at UCSD Advances in Space Research Volume 14, Issue 2, Pag. 151
• Information about HEXAGONE Gamma-Ray Astronomy group of the University of California website
• NASA Balloon Flights (1989-1998) in NASA Historical Data Book, Vol. VII: NASA Launch Systems, Space Transportation, Human Spaceflight, and Space Science, 1989-1998