DOUBLE COINCIDENCE TELESCOPE COUNTER

The objective of the flight was to measure the intensity and characteristics of cosmic rays at high altitudes following a major solar flare event that occurred on February 23, 1956. The flight aimed specifically to investigate how the cosmic-ray flux varied with atmospheric depth during this unusual solar event, with a focus on detecting any enhancements in cosmic-ray intensity attributed to particles of solar origin.

The instrument used in the flight which can be seen in the scheme at left was a vertical counter telescope designed for high-altitude measurements. It was constructed with an exceptionally low total stopping power of approximately 0.2 g/cm², including the casing, allowing it to detect low-energy radiation with minimal shielding effects. The telescope was sensitive both to charged particles and, to a lesser degree, to gamma radiation due to Compton scattering events that could produce coincident counts. Its efficiency for detecting vertically incident Co-60 gamma rays was very low, at about 0.05%, while for charged particles passing within its defined acceptance angle, the detection efficiency was assumed to be 100%. The telescope subtended a solid angle of 9.38 steradian cm² for isotropic radiation, determined by its mechanical and electronic design. The instrument assembly was housed inside a lightweight, 0.5 mm thick styrofoam container to minimize additional material and preserve the desired low stopping power. Detection elements inside included Victorine Type IB85 counters, strategically arranged to maximize the probability of capturing throughgoing particles while minimizing the detection of background noise. Prior to the flight, the apparatus underwent thorough testing to ensure operational integrity and was confirmed to be fully functional upon recovery after the mission, lending strong confidence to the data collected.

Balloon launched on: 2/23/1956 at 19:00 utc
Launch site: Anoka County Airport, Minnesota, US  
Balloon launched by: Cosmic Ray Group - University of Minnesota
Balloon manufacturer/size/composition: Zero Pressure Balloon  
End of flight (L for landing time, W for last contact, otherwise termination time): 2/23/1956
Balloon flight duration (F: time at float only, otherwise total flight time in d:days / h:hours or m:minutes - ): 5 h

The balloon was launched from Anoka, Minnesota seventeen hours after the onset of the flare, on February 23, 1956. It ascended to a depth of 10 g/cm² in the atmosphere between 1900 and 2100 GMT. The investigation was motivated by reports of widespread, significant increases in cosmic-ray intensity, suggesting a rare opportunity to study solar particle emissions and their interactions with Earth's magnetic field and atmosphere.

The radiation observed during the flight clearly demonstrated a low energy profile, inferred to involve protons with energies of less than 1 BeV. This conclusion was supported by the absence of a strong transition maximum typically seen at about 80 g/cm² and by the failure of the radiation to produce detectable effects at atmospheric depths greater than 300 g/cm². It was suggested that if the particle acceleration process occurred only during the visual solar outburst observed from 3:34 to 4:14 GMT in Tokyo, then the significant cosmic-ray effects detected seventeen hours later implied that the beam underwent considerable scattering and/or trapping by magnetic fields near the solar system.

• Cosmic-Ray Increase at High Altitude on February 23, 1956 Phys. Rev. 104, 220 - Published 1 October 1956
• Spectrum of Low-Rigidity Cosmic Rays during the Solar Flare of February 23, 1956 Physical Review, Vol 106 Nº 5 p. 1072