GAMMA RAY TELESCOPE

The objective of the flight was to search for pointed sources of high-energy gamma radiation (>100 MeV) from the galactic center region. The instrument used in the experiment consisted of a system of four counter telescopes, logically combined in various ways to modify the angular acceptance profile. The scientific payload was developed at the Department of Physics of the University of Southampton, United Kingdom.

For the mission, a fan-shaped pattern with a narrow zenith dimension was chosen to minimize the contribution of atmospheric background gamma radiation from the horizon. The primary detection mechanism relied on a spark chamber, which recorded the track of each detected event. The spark chamber system was designed to capture a 90° stereo pair image of each event on 35 mm film, along with a digital display showing the outputs of a two-axis gimbal-mounted magnetometer, the zenith angle, and the event time with a precision of ±1 millisecond.

The instrument used visual classification criteria to sort detected events. It categorized events into pairs and straight single tracks, which were included in the analysis, while events with significant scattering or those attributed to charged particle contamination were excluded. The celestial coordinates of each event's origin were determined based on the track direction and telescope orientation. These coordinates were then stored in a model sky array composed of equal solid angle bins, each covering 1° x 1° at the celestial equator.

To characterize the telescope's angular response, a 24 x 40 array of 1° square bins was generated for two orthogonal views. This array was smoothed and normalized before being used to compute the effective exposure time for each sky bin. Corrections were applied to account for variations in atmospheric background due to different zenith angles. The telescope system recorded data by summing the number of photons detected within an 11° x 11° array, which was then moved in 1° increments across the sky. This method ensured that any detected point sources, having a much smaller angular extent, had minimal influence on the calculated exposure time.

Additionally, a secondary summation into 5° x 5° bins was performed to facilitate the search for point sources. The telescope also incorporated energy measurement capabilities, determining the energy of detected gamma rays by analyzing the scattering of electron tracks in the spark chamber. These measurements were combined with Monte Carlo simulations of telescope efficiency as a function of energy. The resulting values allowed for an estimation of atmospheric background intensity, which was found to be consistent with previous experimental results.

Balloon launched on: 9/20/1971 at 15:17 CDT
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 20.800.000 cuft (0.8/0.9 (2 CapS.) mil. Stratofilm)
Balloon serial number: SF 375.08-080-NSC 02 - Serial Nº 3
Flight identification number: 630P
End of flight (L for landing time, W for last contact, otherwise termination time): 9/21/1971
Balloon flight duration (F: time at float only, otherwise total flight time in d:days / h:hours or m:minutes - ): 11 h
Landing site: In Bastrop, Texas, US
Payload weight: 2947 lbs.

The balloon was launched from the NCAR Scientific Balloon Facility in Palestine, Texas, on September 20, 1971. The balloon ascended and reached its ceiling altitude of approximately 127,000 feet at 22:30 UTC, floating at this altitude until 08:30 UTC on September 21 for landing in Bastrop, Texas.

• High Energy Gamma Radiation from the Galactic Centre Region Nature, 238 (5360), p. 138 (1972)
• NCAR Scientific Balloon Facility Annual Report, 1971 National Center for Atmospheric Research, February 1972
• Flight Summary of balloons made of Stratofilm - 1971 to 1972, Technical report, Winzen Research International. Circa 1973 - Stratocat's private collection
• Review of Recent Performance of Large Balloons and Balloons with Payloads in Excess of 3000 Pounds Proceedings, Seventh AFCRL Scientific Balloon Symposium, 1973