The Los Alamos air fluorescence detection system

A nuclear explosion far from the earth is a strong source of thermal X-rays that deposit their energy in the upper atmosphere and cause air fluorescence, which provides a method of detecting such explosions by ground instrumentation. Energy deposition occurs in the ionosphere over a region several tens of kilometers thick, but because the X-ray flux and the light emitted travel downward with the same velocity, the time smear along the direct line of sight to the explosion depends only on the X-ray source lifetime and on the stopping time of photoelectrons produced by the X-rays. These times are short, and the effective deposition rate is high; along this line of sight, therefore, is observed a bright flash of light whose direction can serve to locate the explosion in space. If the sky is viewed by a wide-angle detector covering π steradians, a pulse rising in < 1 microsecond and falling to half intensity in ¼ millisecond is obtained. A system for detection of this pulse in the presence of daylight has been developed and is described in this paper. The range for detection is estimated to be R = 10
⁵
√Y
x
kilometers in daylight where Y
x
is the thermal X-ray yield of the explosion in kilotons. At night the range is more than one order of magnitude larger.