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: 5/22/1989 at 20:45 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 N29-8/8/8T.28.40
Balloon serial number: R28.40-2-118
Flight identification number: 270N
End of flight (L for landing time, W for last contact, otherwise termination time): 5/23/1989 at 16:56 utc
Balloon flight duration (F: time at float only, otherwise total flight time in d:days / h:hours or m:minutes - ): 5 h
Landing site: 130 miles WSW of Mackay, Queensland, Australia

This was the first flight of HEXAGONE. The 28 million cubic feet balloon was launched on 22 May 1989 from Alice Springs, Australia, and lasted for 17.4 hours at altitudes corresponding to 3.5-4.7 g/cm² of atmospheric depth. During the flight, the instrument observed SN1987A for 9.9 hours, the Galactic Center for 6.3 hours, and the Crab Nebula along with the transient x-ray source 0535+26 for 1.3 hours. Observations alternated between target and background positions, each lasting 20 minutes, and the pointing was verified with a star camera. Of the twelve detectors, three showed abnormal behavior and did not produce usable data, resulting in nine detectors contributing to the final analysis.

• Annihilation Radiation and Associated Continuum from the Galactic Center Region 23rd International Cosmic Ray Conference, Vol. 1, p.148
• Compton-backscattered annihilation radiation from the Galactic Center region Astrophysical Journal, Part 1, vol. 414, no. 1, p. 165-177
• HEXAGONE observation of the Galactic center gamma-ray continuum Astronomy and Astrophysics Supplement Series, vol. 97, no. 1, p. 199-203
• High energy astronomy program at UCSD Advances in Space Research Volume 14, Issue 2, Pag. 151
• High energy observation of the Galactic center region 511 keV and Al-26 lines with HEXAGONE Astronomy and Astrophysics Supplement Series, vol. 97, no. 1, p. 185-187
• 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
• Observation of SN 1987A with the gamma-ray spectrometer HEXAGONE Astrophysical Journal, Part 1, vol. 403, no. 1, p. 332-335.
• Observation of the 511 keV annihilation line in the direction of the galactic center with HEXAGONE AIP Conf. Proc. 232, 52 (1991)
• Observations of SN1987A and the Galactic Center with a New High Resolution Gamma-Ray Spectrometer 21st International Cosmic Ray Conference. Volume 2 (OG Sessions), p.174
• Positrons annihilation and the Galactic center Thesis by Wallyn, Pierre - Universite de Paris
• Traitement des donnees de l'experience HEXAGONE: Regions centrales de la Galaxie et SN1987 A. Interpretation Thesis by Chapuis, Claude - Universite de Paris