LOW ENERGY GAMMA RAY TELESCOPE

The objective of the flight was to perform gamma-ray observations of the galactic center region using a GAMMA-RAY TELESCOPE, with a specific focus on detecting and verifying the presence of a spectral line in the low-energy gamma-ray spectrum, particularly around 476 keV. The experiment aimed to measure both the continuum gamma-ray flux and any superimposed spectral features, especially during a lunar occultation of the X-ray source GX 3+1, to clarify the celestial origin of previously detected anomalies and to study the characteristics of gamma-ray emission from the galactic center in the energy range from 30 keV to approximately 2.4 MeV. The experiment was developed by the Department of Space Science at Rice University, Houston, Texas.

In the image at left we can see the instrument mounted on its gondola (click to enlarge). The primary detection elements were two identical directional gamma-ray spectrometers, each centered around a sodium iodide (NaI(Tl)) scintillation crystal measuring two inches in thickness and four inches in diameter. Each central detector was surrounded by a well-type NaI(Tl) guard crystal viewed by six photomultiplier tubes (PMTs), operating in anticoincidence with the central crystal. The guard crystal had two main functions: it vetoed incomplete energy depositions by detecting secondary escaping radiation and defined an active collimation system with a 24° full width at half maximum acceptance cone, reducing background from atmospheric and diffuse cosmic sources.

An additional shielding layer consisted of a 0.25-inch-thick plastic scintillator placed over the gamma-ray aperture of each detector, viewed by three PMTs in anticoincidence to reject charged particles from the forward direction while remaining transparent to gamma rays.

Each detector included a pulse-height analysis system that digitized energy depositions into 127 channels, with an overflow channel for the highest energy events. Signals that passed both anticoincidence layers were digitized into eight-bit words, tagged for detector identification, and transmitted to a ground station.

The detectors were mounted in parallel within an equatorial telescope configuration inside an inner gondola capable of azimuthal rotation relative to an outer gondola. The outer gondola, suspended by four cables from the balloon, stabilized along the local vertical, while the inner gondola rotated to track celestial objects. Pointing was managed by a servo system guided by flux-gate magnetometers, which aligned the telescope’s polar axis with the Earth's rotation axis, with pre-launch adjustments for latitude and magnetic declination.

To account for atmospheric backgrounds, the system alternated approximately every ten minutes between source and background observations by rotating the detectors 180° in azimuth while keeping the zenith angle constant, ensuring the source was removed from the field of view under comparable conditions.

The gondola system reduced coupling to balloon rotation for improved pointing stability. A commandable onboard system allowed remote switching between energy analysis modes, operation of a radioactive calibration source, and adjustments during flight. The detectors were periodically calibrated using a manganese-54 source emitting 835 keV gamma rays, deployed by radio command to maintain gain stability.

Balloon launched on: 11/20/1971 at 6:30 local
Launch site: Second Air Brigade, Paraná, Entre Rios, Argentine  
Balloon launched by: National Center for Atmospheric Research (NCAR) / Comision Nacional de Investigaciones Espaciales (CNIE)
Balloon manufacturer/size/composition: Zero Pressure Balloon Winzen 15.000.000 ft3 (0.7 / 0.6 (x2 caps) Stratofilm)
Flight identification number: 67N
End of flight (L for landing time, W for last contact, otherwise termination time): 11/20/1971 at 18:20 local
Balloon flight duration (F: time at float only, otherwise total flight time in d:days / h:hours or m:minutes - ): 9 h 50 m
Landing site: Near Monte Ralo, Cordoba, Argentine
Campaign: GALAXIA 71  
Payload weight: 1795 lbs

• Detection of a Gamma-Ray Spectral Line from the Galactic-Center Region Astrophysical Journal, Vol. 184, pp. 103-126 (1973)
• Gamma-ray lines from the galactic center Proceedings of 9th Texas Symposium on Relativistic Astrophysics, New York Academy of Sciences, 1980, p. 248-266
• Low-energy gamma-radiation from the galactic-center region PhD dissertation by Wiley Neil Johnson, Rice University, 1973
• NCAR Scientific Balloon Facility Annual Report, 1971 National Center for Atmospheric Research, February 1972