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. This was a modified version of the instrument flown in 1974 from Aire Sur L'Adour.

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. Originally, it was intended to obtain directional information about a burst by comparing the arrival times measured by the balloon detector and one or more satellites. However, during the first flight the event observed was too small to trigger any of the satellite systems in use. Furthermore, the low event rate meant that it was unlikely that a burst large enough to register on both detector types would be observed in a flight of similar duration to the first. The two slabs of scintillator were therefore arranged in such a configuration, that the comparison of their individual outputs was sensitive to the angle of incidence of a burst. The slabs were mounted so that they were both inclined to the horizontal at an angle of 30°. In order to determine the arrival direction of a burst, it was necessary to know the detector's orientation with respect to the celestial sphere. To achieve this, a two axis fluxgate magnetometer was attached to the framework in order to fix the payload's orientation with respect to the Earth's magnetic field, and hence to the universe at large.

Three 5-inch photomultiplier tubes were coupled to each slab using silicon fluid. Outputs from each group were monitored separately and then summed through a discriminator that generated a standard 5-volt pulse for signals within a defined range, establishing an energy window between 50 keV and 2 MeV for detectable x-rays. Power came from rechargeable silver cells producing 30 volts, stepped down via converters and stabilized with a corona EHT converter.

Efficient light collection was essential. Two methods were tested: diffusion boxes and direct coupling of photomultipliers to the scintillator. Direct coupling, especially with reflective coatings, was more effective. Best results came from shiny-side Alcan foil.

The timing system used high-resolution encoding and transmission capable of resolving 63-microsecond intervals, enabling detection of rapid burst spikes. Accurate timing was also key for triangulating burst origins using balloon and Vela satellite data.

To reduce photomultiplier noise, a lower threshold discriminator was used. An upper threshold rejected charged particles while allowing x-rays up to 2 MeV. It was calibrated to exclude particles depositing over 1.7 MeV, such as minimum ionizing electrons. Only the region beneath the phototubes had enough light collection efficiency to produce sufficient photo-electrons from lower-energy x-rays, but most of the scintillator remained sensitive across the energy range.

The detector was housed in an aluminum frame lined with polystyrene for thermal insulation and foam rubber for shock absorption. Thermal shielding was critical during ascent through the –60°C tropopause. The frame also shielded electronics from electromagnetic interference. High-voltage components were embedded in RTV silicone rubber and vacuum-tested to prevent coronal discharges at low pressure.

Data acquisition used a digital flagging system that changed state every eight counts, sampled 2048 times per second, and transmitted 16 frames per second. Additional telemetry included eight analog channels monitoring temperatures, power, high voltage, and a ratemeter. Data were transmitted on a 400 MHz carrier and recorded with a Rubidium frequency standard for precise timing.

Balloon launched on: 6/11/1975 at 5:26 UTC
Launch site: Aérodrome de Gap-Tallard, Haute Alpes, 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): 6/11/1975
Balloon flight duration (F: time at float only, otherwise total flight time in d:days / h:hours or m:minutes - ): 8 h

The balloon was launched from the CNES base in Gap-Tallard, France at 5.26 UTC on 11th June, 1975. The flight duration at float altitude (3-4 mbars) was six hours, but owing to transmitter
failure only three hours of data were received.

• 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