An airborne radioactivity survey was made of the area around the National Reactor Testing Station near Idaho Falls, Idaho. The survey area is a square, 100 miles on a side, centered on the Testing Station reservation. Parallel flight lines were flown in an east-west direction with a flight line interval of 1 mile and at an altitude of 500 ft above the ground. The survey was made by the U. S. Geological Survey for the U. S. Atomic Energy Commission as part of its nationwide program of airborne radioactivity surveys of nuclear installations.
The area includes parts of three physiographic subdivisions. They are, from north to south, the Northern Rocky Mountains province, the Snake River Plain section of the Columbia Plateaus province, and the Great Basin section 6f the Basin and Range province. Owing to the rugged topography, only small parts of the Northern Rocky Mountains and the Great Basin sections within the survey area could be flown. In those parts flown, silicic volcanic rocks of Tertiary age have radioactivity levels as high as 2100 cps (counts per second). On the Snake River Plain radioactivity levels ranged from 200 to 1600 cps and averaged about 600 to 800 cps. Delineation of some Recent lava flows by the radioactivity data is excellent. Increase in radioactivity level toward the center of a lava flow may be indicative of successive flows from a differentiating magma source, The radioactivity level of blocky lava flows is 50 to 100 cps higher than that of adjacent ropy flows. This is probably due to a greater emitting surface area per unit volume on the blocky flows rather than to any difference in mineralogical content.