Post-Shot Operations On-site Monitoring. Soon after a detonation, monitors in the Test Director's organization move forward into the shot area to establish and mark fallout lines, such as the line where fallout may be measured at 10 roentgens or above. Guided by this survey, work crews then move into the area to recover instrumentation or materials of various kinds. \ Monitors continue to measure and record the close-in, on-site fallout until its radioactivity decays te the point that it presents no hazard to personnel. Cloud Sampling and Tracking. Soon after the detonation, Air Force cloud sampling crews begin flying through the radioactive cloud to obtain fission products so they may be analyzed in the AEC laboratories. As the cloud moves off the Test Site, Air Force cloud tracking planes follow and trace its path, usually for hun- dreds of miles, until it disperses into a mildly radioactive air mass. Air Closure by CAA. From information supplied by the cloud tracking air crews to a Civil Aeronautics Administration official stationed at the Test Site, the CAA may order the closing of certain areas to air travel for specified times, until the radioactive cloud has dispersed and no longer constitutes a hazard. Establishing the Fallout Pattern: Ground and air monitoring personnel take measurements of radiation off-site to determine the path of the cloud and to establish areas in which the greatest concentrations of off-site fallout are deposited. Distant Monitoring. Several monitoring networks in the United States and abroad measure radioactive fallout from Nevada tests at points distant from the Test Site. Such fallout always is slight, and it has never been found in concentrations that would be significant to the health of any living thing. me -19 - '