Radioactivity induced in sodium iodide by trapped protons

The earth satellite Oso 1 contained as one of its prime instruments the University of Minnesota γ-ray experiment, which was designed to search for extraterrestrial γ rays in the 50-kev to 3-Mev range [Peterson and Howard, 1961]. The detection system consisted of an array of NaI scintillation counters arranged in various logic and shielding configurations to provide directional properties and to reject unwanted cosmic-ray-produced background. In this letter we report on radioactivity induced in the NaI crystals during the brief periods when the satellite passed through the lower edge of the inner Van Allen zone over the South Atlantic region. The principal detector with an isotropic response was a phoswich scintillation counter [Peterson and Nitardy, 1961] which consisted of a 5.1-cm diameter by 5.4-cm long NaI crystal surrounded by a 0.32-cm layer of plastic phosphor. The assembly was viewed by a single photomultiplier. Energy losses greater than about 0.5 Mev in the plastic generated an anticoincidence gate, thus rejecting particle-like events. Gamma-ray-like events were divided into two energy ranges, 0.3 to 1.0 Mev and 1.0 to 3.0 Mev, and were accumulated continuously in binary registers which were read out about once every 70 seconds. The counter was surrounded by about 5 g/cm2 of aluminum, magnesium, and electronics and was located about 18 inches from the center of gravity of a 450-pound payload configuration.