Fast neutrons in the Earth's atmosphere: 2. Time variations at high altitudes

A series of balloon flights launched during 1963 at λ = 41° N are analyzed. The balloons carried omnidirectional detectors sensitive to fast neutrons and (separately) γ rays to altitudes of about 125,000 feet for periods of up to 24 hours. The observed latitude dependence is in accordance with the theory of Lingenfelter. The observed day-to-night ratios of the fluxes were 1.01±0.02 and 1.00±0.01. An upper limit of 0.02 neutron/cm2 sec for the solar flux at the earth can be determined if we assume that the solar spectrum is similar to the terrestrial spectrum. The explanation for the small fluctuations that were observed probably lies in a slight time variation of the intensity of the charged radiation incident on the top of the earth's atmosphere. No evidence has been found for γ rays (energies between 0.47 and 3.50 Mev) emanating from the sun or from the Crab Nebula.