FIGARO was the acronym of French-Italian GAmma Ray Observatory, which were a series of balloon-borne experiments specifically designed to observe gamma rays in the 0.15-6 MeV energy range from sources with a definite time signature. As the name implies it was a cooperative development between research institutions which included: Istituto di Fisica Cosmica e Informatica, CNR, Palermo, lstituto di Astrofisica Spaziale (IAS) Frascati and Istituto Astronomico, Universitá "La Sapienza", Roma in the Italian side while France participated through the Centre d'Etude Spatiale des Rayonnements (CESR), Toulouse and the Centre d'Etudes Nucleaires (CEN) in Saclay.
Following the total destruction of the first iteration of the instrument in a free-fall from the stratosphere in 1983, a second instrument was built. In the figure at left we can see an scheme of the composition of this second version known as FIGARO II (click to enlarge). It consisted of a principal detector of Nal(TI) surrounded by a system of active shields in order to reduce the background counting rate and to reject charged particles. The main detector was made of 9 identical square crystals of Nal(TI) with a side length of 20 cm and a thickness of 5 cm. The total geometric area was therefore 3.600 cm2. Each crystal was surrounded by thin Millipore paper acting as light diffuser, and was enclosed in an aluminium case with a 1.5 mm thick upper window. In order to save volume no light pipe was used and the scintillation light was collected directly by a 3 inches photomultiplier through a borosilicate glass window in the centre of the tile. Each photomultiplier was enclosed, together with the voltage divider, the high voltage supply and the preamplifier, inside its own pressurized aluminium vessel. The nine vessels were then placed within cavities left empty in the bottom shield.
Along the four sides of the principal detector there were a wall of twelve Nal(TI) blocks; any individual block had dimensions 23 cm X 25 cm X 7 cm, and was viewed by a photomultiplier identical to, and arranged in the same way as, the main detector photomultipliers. The rim of the wall was 20 cm higher than the entrance window, reducing the field of view to about 72º FWHM along the sides and to about 77º along the diagonals.
In the lower part of the experiment four thick blocks of plastic scintillator acted as an anticoicidence system against upward moving atmospheric gamma rays and charged cosmic rays. The shape of each block was irregular because they were specifically designed to house the photomultipliers of the main detector. Each block was viewed by two 5 inches photomultipliers, and the surface was covered with Millipore paper in order to improve the light collection. Downward moving charged particles of low energy were rejected by an upper shield. It was made of the same plastic scintillator as the lower shield, and consisted of two adjacent pieces of 75 cm X 35 cm x 0.5 cm, fixed on top of the lateral wall. The scintillation light from each sheet was collected by two photomultipliers through adiabatic light pipes connected to the short sides. The in-flight control of possible drifts of the gains of the main detector modules was achieved by means of a weak sodium 22 source, plus a BGO crystal scintillator enclosed in a pressurized vessel together with photomultiplier, preamplifier and power supply. This auxiliary detector was placed at the top of a vertical rod about 90 cm above the detector window.
Finally the whole instrument was fixed on a robust metallic frame able to rotate around a horizontal axis up to 50º out of the zenith. The balloon platform which supported the gondola (3 m x 2 m x 0.6 m) could also be directed toward any azimuthal direction, measured with respect to the horizontal component of the geomagnetic field. The accuracy of the pointing was about 1º. The balloon pivot was designed under the responsibility of CNES by the firm STEEL.
Balloon launched on: 7/9/1990 at 4:33 utc
Launch site: Base di Lancio Luigi Broglio, Trapani, Sicily, Italy
Balloon launched by: Centre National d'Etudes Spatiales (CNES)
Balloon manufacturer/size/composition: Zero Pressure Balloon Zodiac - 830.000 m3
End of flight (L for landing time, W for last contact, otherwise termination time): 7/10/1990 at 3:20 utc
Balloon flight duration (F: time at float only, otherwise total flight time in d:days / h:hours or m:minutes - ): 22 h 47 m
Campaign: ODISSEA 90
Payload weight: 2312 kg
Gondola weight: 1760 kg
Succesful payload cut-down command
This was the third flight of the FIGARO II experiment. The main obervational goal was to observe the Crab pulsar (PSR 0531+21) after a first scan that occured during the first flight in 1986 that indicated the possible existence of a feature in the energy spectrum between 0.43 and 0.46 MeV in the phase interval of the second peak. During this flight the 5 hours of observation allowed a more sensitive study of the source. The balloon was launched from the base of Milo, near Trapani, Sicily, on 1990 July 9 at 4:33 UTC. The tracking of the Crab source started at 7:06 UT at a residual pressure of 9 mbar and a zenithal distance of about 36º. The ceiling, at 4.4 mbar, was reached at 7:42 UTC. The instrument was pointed at the source direction until 13:30 UTC, but because of a failure in the telecommand system resulting in the improper operation of the experiment from 11:55 UTC to 13:30 UTC, the useful portion of the data obtained was limited to those acquired before 11:55 UTC, for a total duration of 4 hr and 49 minutes. During this time all the experiment parameters remained at the nominal values and the residual pressure changed very little in the range between 4.36 and 4.47 mbar.
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