Purpose of the flight and payload description
The CESAR GAMMA-RAY SPECTROMETER was an experiment aimed to measure the intensities and widths of nuclear gamma-ray lines originating from the galactic center and other specific celestial objects in the 1970s decade. The instrument was part of a collaboration between the Département de Physique Nucléaire at the Centre d'Études Nucléaires (CEN) de Saclay, the Centre dÉtude Spatiale des Rayonnements (CESR) in Toulouse, France, and the Instituto de Pesquisas Espaciais (INPE) in São José dos Campos, Brazil.
In the figure at left we can see an scheme of the instrument (click for more details). The core of CESAR was a high-resolution gamma-ray spectrometer equipped with a lithium-drifted germanium Ge(Li) detector. The Ge(Li) crystal, manufactured at Saclay, had a coaxial closed-end configuration with a volume of approximately 140 cm³ and was cooled using liquid nitrogen. The cooling system included a cryostat with a cold finger design and was pressurized to maintain proper thermal regulation.
To reduce background noise and enhance the signal-to-noise ratio, the detector was surrounded by an active anti-coincidence shield composed of sodium iodide (NaI) scintillators. These defined the instrument's field of view (about 25° at half maximum) and suppressed Compton scattering contributions. The NaI system functioned also as a pair spectrometer at higher energies. The signals from both the Ge(Li) and NaI detectors were processed through a dual electronic analysis system. One path recorded signals in anti-coincidence mode with 2000 channels over the 60 keV2 MeV range, while another, lower-power system recorded in 4096 channels over a broader energy range.
The instrument was housed in a cubic gondola composed of aluminum frame measuring 1,50 meters for side and weighing approximately 400 kg. The core spectrometer assembly, containing the Ge(Li) detector and NaI(Tl) anti-coincidence shielding, was mounted in a rotating frame with two perpendicular horizontal axes. This design kept the liquid nitrogen container upright during landing and enabled zenith angle adjustments from 0° to 50° via remote commands. Magnetic clutches locked the axes during flight and could be disengaged for safe repositioning.
An autonomous stabilization system maintained fixed azimuthal orientation using a compass-based servo control with inertia wheels, achieving ±10 arcminute pointing accuracy. Ground commands could adjust the azimuthal direction to compensate for magnetic declination variations along the flight path.
Digital encoders measured elevation and azimuth with 0.5° and 1.5° precision respectively, transmitting data as 9-bit words via telemetry. Atmospheric pressure was monitored using two sensors: an Albin-Springer sensor for ascent and a Rosemount sensor for float altitude, with readings digitized as 10-bit words.
Housekeeping data including voltages, temperatures, attitude control parameters, and motor diagnostics were transmitted every 30 seconds through an analog commutator. All telemetry used an FM/FM system with eight IRIG channels, while a telecommand system provided nine control options for real-time adjustments during flight.
Details of the balloon flight
Balloon launched on: 2/8/1977 at 6:35 utc
Launch site: Guaratinguetá Air Base, Sao Paulo, Brazil
Balloon launched by: Centre National d'Etudes Spatiales (CNES)
Balloon manufacturer/size/composition: Zero Pressure Balloon model 350sf Zodiac - 350.000 m3
End of flight (L for landing time, W for last contact, otherwise termination time): 2/8/1977 at L+7
Balloon flight duration (F: time at float only, otherwise total flight time in d:days / h:hours or m:minutes - ): 7 h
Landing site: In Sao Joao Cajua - Paraná
External references
- Gamma-ray lines from the galactic center Proceedings of 9th Texas Symposium on Relativistic Astrophysics, New York Academy of Sciences, 1980, p. 248-266
- Observação da radiação y de baixa energia (0,05-3,0 mevj com telescópio ge(li) na latitude geomagnêtica x = 12°s Thesis para obtener Título de Doutor em Ciência Espacial - José Marques da Costa
- Recent measurements with a balloon borne Ge(Li) spectrometer in gamma-ray astronomy Nuclear Instruments and Methods - Volume 155, Issues 12, 15 September 1978, Pages 171176
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