GAMMA RAY TELESCOPE

The objective of the flight was to launch a GAMMA-RAY TELESCOPE to measure ultra high energy gamma rays from Crab Nebula or from Quasars. The telescope was developed at the School of Physics and Astronomy of the University of Minnesota, with support from both NASA and the U.S. Office of Naval Research.

The instrument which can be seen in the scheme at left (click to enlarge) consisted of a sophisticated array that combined nuclear emulsions with electronic detectors to achieve high angular resolution while maintaining detection efficiency.

The core component was a photographic nuclear emulsion block that served as the converter material, chosen specifically to achieve the highest possible angular resolution. Below this emulsion block, the instrument incorporated two thin-plate spark chambers, each with four gaps, which were designed to record the passage of electrons produced when gamma rays converted in the emulsion. The positioning of these chambers was critical for tracking the electron pairs with high precision.

The trigger and acceptance geometry of the system was defined by an array of electronic counters placed below the spark chambers. This array was composed of two scintillation counters separated by a specialized gas Cherenkov counter. The Cherenkov counter was 1 meter in length and contained nitrogen gas maintained at a pressure of 1 atmosphere, specifically configured to eliminate any particles with energies below 40 me².

For radiation shielding and background rejection, the emulsion block and spark chambers were completely surrounded by an anticoincidence scintillation counter. This shield featured a strategic opening in its top surface, positioned directly above the emulsion block, which was covered by an additional scintillation counter that was optically isolated from the main shield. This top counter was versatile in its operation, capable of working in either coincidence or anticoincidence mode, and was specifically designed for the detection of very high-energy multiply charged nuclei.

All these components, along with their associated electronics and power supplies, were housed within a pressurized gondola. The gondola was mounted within a frame using a fixed zenith angle configuration to enable orientation control during flight. For precise pointing and orientation, the system employed a flux-gate magnetometer that provided azimuthal orientation to either north or south. The orientation of the gondola was continuously tracked using two independent systems: a pair of moving-film cameras arranged perpendicularly that photographically recorded the position of Polaris, and a galvanometer record of the magnetometer output that was photographed simultaneously with each spark-chamber event.

This dual tracking system achieved remarkable precision, allowing the determination of true azimuthal orientation to within ±0.1° using the Polaris films and ±1° using the magnetometer galvanometer.

Balloon launched on: 3/6/1966 at 21:15 CST
Launch site: Columbia Scientific Balloon Facility, Palestine, Texas, US  
Balloon launched by: NCAR National Scientific Balloon Flight Station
Balloon manufacturer/size/composition: Zero Pressure Balloon Winzen 10.600.000 cuft (0.5 MIL. Stratofilm)
Flight identification number: 207P
End of flight (L for landing time, W for last contact, otherwise termination time): 3/6/1966
Landing site: Aborted flight. Payload destroyed at launch.
Payload weight: 933 kgs

The mission was intended to launch on March 3, 1966 from the NCAR balloon base in Palestine Texas. The balloon was released at 21:15 CST but during a crosswind launching maneuver the NCAR launch vehicle (Tiny Tim) ran off the paved surface and encountered rough terrain which induced oscillations in the boom supporting the gondola. This caused failure of the suspension system, allowing the gondola to drop to the ground where it was crushed by a wheel before the vehicle could be stopped.

After extensive refurbishment, the instrument was flown again succesfully from Palestine, Texas in October 1966.

• Balloon Flight Record Facilities for Atmospheric Research Nº 1, Fall 1966, Pag. 18
• NCAR Scientific Balloon Facility Annual Report, 1966 National Center for Atmospheric Research, January 1967
• An experiment to measure the gamma ray flux of various celestial point sources from high altitude balloons, Semi-Annual progress report to University of Minnesota - Stratocat's private collection