HEAVY COSMIC RAY DETECTOR

The objective of the flight was to measure the flux and composition of cosmic-ray nuclei with atomic number (Z) greater than 15 and rigidity greater than 4.5 GV. These measurements were made with a balloon-borne large-area detector using ionization chambers and a Cerenkov counter. The instrument was developed at Department of Physics of the Washington University, with the collaboration of the McDonnell Douglas Research Laboratories both from St. Louis.

A schematic diagram of the detector system is shown at left. The principal components were the two pulse-type ionization counters and the Lucite Cerenkov counter. The two parallel-plate ionization counters were 1 square meter in area, 5.1 cm thick, and filled with a mixture of 90% argon and 10% methane at a pressure of one atmosphere. The Cerenkov radiator was a square meter sheet of 0.6-cm thick ultraviolet-transmitting (UVT) Lucite, with light collection by total internal reflection. Each of the four edges of the radiator was viewed by a 12.7-cm photomultiplier (RCA 4525) through an adiabatic light pipe constructed of UVT Lucite.

The particle trajectories were given by a hodoscope consisting of two crossed layers of scintillator strips above the other counters, and two crossed layers below. Each layer consisted of ten strips, giving 9.5 cm resolution. A six-fold coincidence among the four hodoscope layers and the two ion chambers triggered the recording of an event. The ion chamber discriminators determined the experiment threshold and were typically set at a signal level corresponding to a vertically incident relativistic nucleus of charge 16. For each event, the signals from each of the ion chambers and from the Cerenkov counter were pulse height analysed, and the trajectory of the particle was indicated by recording which hodoscope-strip discriminators were triggered.

There were four guard counters, one on each side of the detector square. The guard counters were not in active anti-coincidence, but the outputs of the four guard counter discriminators were recorded for each particle triggering the system.

The experiment was enclosed in an aluminum sphere 59-3/8 inches in diameter. Power supplies, photo barographs, and balloon control instruments were positioned outside the sphere on a support frame which served both as a carrying stand and support frame for honeycomb impact cushion. The entire gondola was then covered with a wall of six-inch thick styrofoam.

Balloon launched on: 1/29/1970 at 18:19 cst
Launch site: Hobbs, New Mexico, US  
Balloon launched by: National Center for Atmospheric Research (NCAR)
Balloon manufacturer/size/composition: Zero Pressure Balloon Winzen 10.600.000 cuft (0.7 / 0.7 Mil. Stratofilm)
Flight identification number: 526H
End of flight (L for landing time, W for last contact, otherwise termination time): 1/30/1970
Balloon flight duration (F: time at float only, otherwise total flight time in d:days / h:hours or m:minutes - ): F 4 h 55 m
Landing site: McComb, Mississippi, US
Payload weight: 1805 lbs.

The balloon was launched at 18:19 cst on January 29, 1970 from Hobbs, New Mexico. After a nominal ascent the balloon attained an altitude equivalent to 3.95 mb of residual atmosphere. After 5 hours at float the flight was terminated with landing of the payload in McComb, Mississippi. The flight was succesful from the scientific point of view althought shorter than expected: the trajectory was somewhat more southerly than predicted, leading to cut-down when the balloon approached the Gulf of Mexico.

• NCAR Scientific Balloon Facility Annual Report, 1970 National Center for Atmospheric Research, February 1971
• Relativistic Heavy Cosmic Rays Astrophysics and Space Science, Volume 22, Issue 1, pp.45-65