ULTRA-HEAVY COSMIC RAY DETECTOR

The objective of the flight was to measure the relative abundances of Fe, Co, Ni, Cu, and Zn in the cosmic rays using a large-area balloon-borne electronic detector system. The system was developed by the McDonnell Center for the Space Sciences and Department of Physics at the Washington University in St. Louis.

The detector was composed of two nearly identical systems, side-by-side inside a single aluminum pressure vessel. A cross section of one system is shown in the figure at left and below a picture of the fully assembled system ready to flight. The detector had a large geometrical factor and was very thin to allow for the collection of a large number of cosmic rays with a small probability of nuclear interaction on it. The charge and velocity of each incident nucleus were determined by the dE/dx-C method wherein dE/dx, the rate of energy loss of the nucleus, was measured by the ionization in the ion chamber gas; and C was the intensity of Cerenkov radiation produced by the nucleus in passing through a Lucite radiator.

The measurement of the rate of energy loss, dE/dX, was made by dual gap, pulse ionization chambers. Four independent ionization measurements were made, two above, and two below the Cherenkov counter. The ionization signal, I, was derived by taking the mean of these four measurements. The light output from the 0.64 cm thick Pilot 425 radiator in the Cherenkov counter was measured by two independent sets of photomultiplier tubes. The Cherenkov signal, C, was derived by taking the mean of the signals from these two sets of tubes. The ionization hodoscopes provided the trajectory information necessary to calculate the path-length in each detector and to correct for areal non-uniformities in the ionization and Cherenkov responses.

The cosmic-ray trajectory defined by the hodoscopes also gave the zenith angle of the incoming particles, which was necessary for calculating the actual atmospheric depth traversed by each nucleus. The signals I and C were both proportional to the square of the charge but proportional to different functions of the velocity of the cosmic ray. This allowed to derive a unique charge and energy determination for every element above magnesium (Z=12) in the energy range from 330 to 1200 MeV measured at the middle of the Cherenkov radiator. For iron, the energy interval for essentially unambiguous element identi?cation could be extended to approximately 20 GeV because of the negligible abundance of cobalt (Z=27). To record the events for each of the two detector blocks was used an incremental magnetic tape recorder, which was capable of recording up to five events per second.

The vessel was covered with a special insulation canvas and mounted in a support structure of aluminum tubing that provided a platform for external power supplies, balloon instrumentation, and supplemental landing shock absorbers. The structure was designed to absorb landing forces without affecting the vessel. Ballast was contained in a centrally located canvas hopper under the experiment.

Balloon launched on: 9/15/1975 at 21:35 local
Launch site: Joe Foss field, Sioux Falls, South Dakota, US  
Balloon launched by: Raven Industries Inc.
Balloon manufacturer/size/composition: Zero Pressure Balloon  
End of flight (L for landing time, W for last contact, otherwise termination time): 9/15/1975
Balloon flight duration (F: time at float only, otherwise total flight time in d:days / h:hours or m:minutes - ): F 1 h 30 m
Landing site: Balloon failure. Landing 60 miles SE of Sioux Falls, South Dakota, US

The balloon was launched from Joe Foss Field airport in Sioux Falls, at 21:35 on Monday, September 15, 1975. The launch proceeded with no major problems, and at 2:00 Tuesday morning the balloon leveled off at its float altitude of between 115,000 and 120,000 feet. Less than two hours into the flight, the balloon ruptured so was transmitted a radio command that cut the detector loose from the balloon and returned it to earth via parachute. The deflated balloon, however, caught in the parachute, which opened only partially in its descent with the detector. The detector landed 60 miles southeast of Sioux Falls in an open field. While there was some damage to the detector from its hard landing, it was returned to Sioux Falls and repaired in time. Finally was launched succesfuly on October 2.

• WU Space Scientists Launch Detector In Search of Ultra-Heavy Cosmic Rays Washington University Record, September 25, 1975