The majority from star evaporations of atmospheric nuclei and representing about 50 neutrons produced per primary particle.

The observations of Adams (1950) who detected a fivefold increase in neutron flux during a solar flare and of Simpson (1952) who claims a 27 day period associated with the solar activity suggest that increases in neutron activity may represent an increase in primary cosmic particles in the low (> 10 Bev) energy range or even some neutrons from the sun.