LOW ENERGY GAS CERENKOV DETECTOR

The objective of the flight was to study atmospheric and extraterrestrial gamma radiation in the energy range from 10 to 100 MeV by employing a GAS-CERENKOV DETECTOR designed specifically for high-altitude balloon flights. The primary scientific goals were to measure the atmospheric gamma-ray flux above 15 MeV and to establish upper limits on the gamma-ray flux from celestial sources such as the Crab Nebula and M87. The project, developed by the Smithsonian Astrophysical Observatory and the Harvard College Observatory, aimed to achieve measurements with a detector that combined a large sensitive area, good angular resolution, and effective background rejection, all within a relatively simple structural and operational design.

In the image at left we can see a schematic view of the instrument and a picture of the external structure during test in the laboratory (click to enlarge).

The detector worked by having incident gamma rays interact in a 1-inch-thick polystyrene scintillator slab called the converter-scintillator. This slab converted gamma rays via pair production or Compton scattering, producing electrons which then emitted Cerenkov radiation when entering a gas volume beneath the slab. The design used the scintillator itself as the converter, rather than a separate lead layer, to reduce multiple scattering of low-energy electrons and preserve angular resolution. Photomultiplier tubes detected both the scintillation and Cerenkov light, with events identified through coincidence logic between scintillation signals and Cerenkov light signals, while anticoincidence shielding excluded charged particle events.

The Cerenkov light was focused by a large, metallic, rhodium-coated mirror onto a 5-inch RCA 4522 photomultiplier tube. The mirror had a focal length of 24 inches and was originally a surplus searchlight mirror, selected for practical reasons despite its weight. The Cerenkov radiator was filled with propane gas, chosen for its low electron multiple scattering and sufficient light emission, offering an optimal balance between detection efficiency and angular resolution. The gas pressure was adjustable to vary the energy threshold during flight, enabling the measurement of gamma rays above different energy levels.

The detector was enclosed in a lightweight aluminum canister designed to hold differential pressures up to one atmosphere and to withstand the vacuum of space during preparation. The frame consisted of aluminum support rods connecting the converter-scintillator at the top to the mirror and photomultiplier at the bottom. The structure included three sections to allow assembly and disassembly without tilting the detector. Electronics mounted on a plate near the base controlled data acquisition and signal processing.

The electronics system included commercial fast-timing modules that processed signals from the photomultipliers. It detected gamma rays via coincidence logic, rejected charged particles with anticoincidence logic, performed pulse-height analysis with an in-house four-channel pulse-height analyzer, and monitored photomultiplier rates and housekeeping parameters such as gas pressure, temperature, and battery voltage. Information was transmitted via telemetry to the ground, and the system was designed for low event rates to avoid data pile-up.

A pointing system was implemented based on a design from the University of Southampton, using a motorized azimuth drive, magnetometers, and a sun sensor to achieve azimuth and elevation control. Orientation was corrected by ground command using non-latching relays. Magnetometers provided coarse direction, while a pendulum-driven potentiometer measured elevation. The system allowed the detector to be steered with a few degrees of accuracy despite rotational and oscillatory motions of the balloon platform.

Balloon launched on: 12/5/1971 at 6:55 local
Launch site: Second Air Brigade, Paraná, Entre Rios, Argentine  
Balloon launched by: National Center for Atmospheric Research (NCAR) / Comision Nacional de Investigaciones Espaciales (CNIE)
Balloon manufacturer/size/composition: Zero Pressure Balloon Raven 11.100.000 cuft (0.75 / 0.75 mil. X-124)
Flight identification number: 74N
End of flight (L for landing time, W for last contact, otherwise termination time): 12/5/1971 at 16:55 local
Balloon flight duration (F: time at float only, otherwise total flight time in d:days / h:hours or m:minutes - ): 10 h
Landing site: Near Milagro, La Rioja, Argentine
Campaign: GALAXIA 71  
Payload weight: 950 lbs

• A Gas-Cerenkov Telescope Experiment to Observe Cosmic Gamma Rays SAO Special Report #335 (1971)
• Gamma-ray Observations of the Galactic Centre Nature physical science 243, 6-8
• NCAR Scientific Balloon Facility Annual Report, 1971 National Center for Atmospheric Research, February 1972