Purpose of the flight and payload description

The objective of the flight was to detect high-energy gamma-rays and hard X-rays from the supernova SN1987A in the Large Magellanic Cloud.

The balloon-borne payload consisted of two independent instruments mounted on a common frame: a Gamma-Ray detector developed by Case Western Reserve University along with the Imperial College from London, and the the University College, ADFA, from Australia and a Hard X-Ray detector developed by the Istituto di Astrofísica Spaziale, Frascati, Italy and the Astronomisches Institut of Tubingen University from Germany.

The Gamma-Ray detector was composed by a 30 gap optical spark chamber having a geometric area of 400 cm2 which was triggered by the coincidence signal from two plastic scintillators and a directional Cerenkov counter responding only to downward moving particles. A five sided plastic anticoincidence shield provided rejection of charged particles. The spark chamber plates were made of stainless steel (total thickness 18.7 mm) and acted as gamma-ray converters. The 4mm gap between successive plates was filled with a helium-neon gas mixture at 1.2 bar. It defined an acceptance cone of ~30° HWHM about the spark chamber axis with a flat response out to ~ 11°. For each event two orthogonal views of the spark patterns were recorded on 16mm film along with the time according to an on-board crystal clock and the output from a 3 axis fluxgate magnetometer which determined the orientation of the payload with respect to the Earth's magnetic field.

The Hard X-Ray detector consisted of a high-pressure multiwire proportional counter having a sensitive area ~500cm2 with a sandwiched aluminium honeycomb entrance window, and a transparency of 50 per cent for 15 keV photons. The detector was filled with a gas mixture of 80% Xenon + 15% Argon + 5% CH4 at a pressure of 2.0 bar, providing an average detection efficiency of 60% over the entire energy range of 15-180 keV. The field of view was restricted to 10° by a hexagonal passive collimater. The three-sided guard counter and the mutual anti-coincidence system kept low the background rate. Pulse-height information and time of arrival, accurate to 10 microseconds were transmitted to ground with housekeeping information for each good event.

Both detectors were mounted on an azimuthally stabilized orientation platform at a fixed elevation angle of 45° to the zenith.

Details of the balloon flight

Balloon launched on: 4/5/1988 at 20:43 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 SF-3-31966-080-NSCR
Balloon serial number: W12.48-1-01
Flight identification number: 244N
End of flight (L for landing time, W for last contact, otherwise termination time): 4/6/1988 at 9:37 utc
Landing site: 100 miles SSW of Mongrel Downs, Northern Territory, Australia

External references

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