Satellite measurements of the decay of the artificial electron belt

Measurements of the radiation environment have been made with dosimetry films and activation materials flown on a series of low-altitude satellites. In orbits with about 400-km apogee, the electrons injected by the high-altitude nuclear burst of July 9, 1962, increased the average daily radiation dose by a factor of 1000 over the normal 0.01 rad from cosmic rays. The apparent half-life for disappearance of the artificial electron belt in the lower regions of the South American anomaly was initially less than 10 days and lengthened to more than 100 days in the first 9 months after the burst. Evidence confirming the identification of the radiation type was provided by depth-dose curves, obtained with Al, Pb, and other absorbers, which were similar to theoretical curves derived for fission electrons. The observation that zenith-side doses were 2 to 5 times greater than earth-side doses is consistent with the expected direction of approach of trapped electrons to the satellite in the region of the South American anomaly. No activation caused by space protons or neutrons was observed in any of the activation materials.