The original gamma-ray detector was composed by a 4 inch-diameter by 2-inch-thick NaI(Tl) crystal viewed by an RCA 8054 phototube. Collimation is provided by a 10-inch-diameter by 12-inch-long NaI(Tl) well scintillator that surrounds the central crystal. It is viewed by six 8054 tubes. The summed output of the six tubes is connected in anticoincidence with the central photomultiplier output. Also connected in anticoincidence with the central phototube are three tubes that view a 1/2 inch thick plastic scintillator covering the aperture of the well. The instrument is maintained at a constant 80° F temperature by thermostatically controlled heaters in order that the effects of temperature changes on the detector be minimized. The half-flux angle for a point gamma source is approximately 12° from the axis. The energy resolution of the detector is 9.2 per cent full width half-maximum at 511 keV. A 128 channe| balloon-borne pulse-height analyzer accepts gamma radiation between 35 keV and 560 keV, with all photons in excess of 560 keV counted in one integral channel. The output of the analyzer is digitalIy telemetered to receiving stations on the ground. Housekeeping data (battery voltages, high-voltage monitors, state-of-pressurization indicators, and heater status) are telemetered separately by the FM/FM system. The gamma-ray detector is equatorially mounted with a geomagnetically oriented azimuth servo system. A clock drive-gear arrangement is used for tracking celestial sources in hour ang|e. Pointing-system accuracy has been checked by cameras mounted on the detector which periodically photograph the Sun's position. The system points to an accuracy of 1º.
This instrument was flown previously but for this particular flight the detector itself was modified: an additional active collimator was inserted in the beam in front of the actively collimated detector previously used and the assembly was flown as a narrow-angle device. The thickness of the additional collimator was nine inches of plastic scintillator. It was viewed by four additional photomultipliers connected in anti-coincidencc with the photomultiplier viewing the central Nai(Tl) crystal.
The additional collimator resulted in a half-flux angle that was 3° from the axis, at energies less than 100 keV. The purpose of this improvement in the angular response was to enable M87 to be isolated from 3C273, one of the other candidates for the source of hard radiation previously found from Virgo.
Balloon launched on: 12/11/1968 at 3:05 cst
Launch site: Columbia Scientific Balloon Facility, Palestine, Texas, US
Balloon launched by: National Center for Atmospheric Research (NCAR)
Balloon manufacturer/size/composition: Zero Pressure Balloon Winzen 10.600.000 cuft (0.7/0.7 mil. Stratofilm)
Flight identification number: 444P
End of flight (L for landing time, W for last contact, otherwise termination time): 12/11/1968 at 11:10 cst
Balloon flight duration (F: time at float only, otherwise total flight time in d:days / h:hours or m:minutes - ): 5 h 15 m
Payload weight: 1248 lbs.
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