GAMMA RAY BURST DETECTOR

Objective of the flight was to detect cosmic gamma ray bursts using a LARGE-AREA PLASTIC SCINTILLATION COUNTER designed to detect gamma-ray bursts smaller than those previously observed by satellite-based detectors like Vela. The instrument was developed at the Blackett Laboratory of the Imperial College in London.

In the image at left we can see a sectional diagram of the detector. It comprised two large slabs of NE102A plastic scintillator, each measuring 100 cm by 50 cm by 5 cm. Three 5-inch photomultiplier tubes were directly coupled to the surface of each slab using silicon fluid. The outputs from all three tubes were summed and passed through a discriminator that generated a standard 5-volt pulse for pulse sizes falling within a defined range, thereby establishing an energy window between 50 keV and 2 MeV for detectable x-rays. The slabs were mounted side-by-side in the same plane, and their count rates were summed. Power was supplied by rechargeable silver cells producing 30 volts, stepped down via low-voltage converters and stabilized with a corona EHT converter.

Efficient light collection was essential, and two methods were tested: diffusion boxes and direct coupling of photomultipliers to the scintillator. Direct coupling proved more effective, especially with reflective coatings. The best results were achieved using shiny-side Alcan foil.

The timing system incorporated a high-resolution data encoding and transmission system capable of resolving time intervals as small as 63 microseconds. This precision allowed the detector to observe rapid spikes in burst events. Accurate timing was also vital for triangulating the direction of burst origins using data from both the balloon and Vela satellites.

To minimize noise from the photomultipliers, a lower threshold discriminator was implemented. An upper threshold discriminator rejected charged particles while allowing detection of x-rays up to 2 MeV. This threshold was calibrated so that charged particles depositing more than 1.7 MeV -such as minimum ionizing electrons- would be excluded. Because only the region under the phototubes had a high enough light collection efficiency to produce large numbers of photo-electrons from lower-energy x-rays, most of the scintillator retained sensitivity across the desired energy range.

Mechanically, the detector was encased in an aluminum frame lined with polystyrene for thermal insulation and shock-absorbing foam rubber to protect the equipment upon landing. Thermal shielding was especially important during ascent through the tropopause, where external temperatures could fall to –60°C. The frame also provided electromagnetic shielding for the electronics. To prevent coronal discharges at low atmospheric pressure, all high-voltage components were embedded in RTV silicone rubber and vacuum-tested for electrical stability.

Data acquisition involved a digital flagging system that monitored count rates by changing state every eight counts, sampling the flag status 2048 times per second, and transmitting 16 frames per second. Additional telemetry included eight analog channels monitoring temperatures, power lines, high voltage, and a ratemeter. Data were transmitted on a 400 MHz carrier and recorded alongside a Rubidium frequency standard for accurate absolute timing.

Balloon launched on: 9/27/1974 at 6:45 utc
Launch site: Centre de Lancement de Ballons CLBA, Aire Sur L'Adour, Landes, France  
Balloon launched by: Centre National d'Etudes Spatiales (CNES)
Balloon manufacturer/size/composition: Zero Pressure Balloon  
End of flight (L for landing time, W for last contact, otherwise termination time): 9/27/1974 at 13:00 utc
Balloon flight duration (F: time at float only, otherwise total flight time in d:days / h:hours or m:minutes - ): 7 h 15 m

The balloon was launched from the CNES base in Aire Sur L'Adour on September 27, 1974, and achieved a float altitude of 4.6 mbars for 4.25 hours. Data was collected via an encoded transmission system with analog and digital monitoring. One significant event was detected at 10:21 UTC, lasting five seconds with a peak increase of ~2290 counts/sec over background. Statistical analysis confirmed the Gaussian nature of the count distribution during and after the event. Power lines and temperatures remained stable throughout, eliminating instrumental malfunction as a cause.

Alternative explanations such as solar activity and charged particle dumping were ruled out due to lack of associated solar flares or geomagnetic disturbances. Although no satellite confirmed the event, the characteristics closely matched known gamma-ray bursts, making a gamma-burst origin the most likely explanation.

• Balloon-Borne Detector Search for Small Gamma-Ray Bursts 14th International Cosmic Ray Conference, Volume 1, p.208
• Measurements of Hard Cosmic X-rays with special reference to burst phenomena Thesis by John Stewart Mills, University of London, June 1978
• Size distribution of cosmic gamma-ray bursts Nature, Volume 258, Issue 5537, pp. 686