Nitrogen gas is irradiated by fast neutrons from a ${\mathrm{U}}^{235}$ fission plate and tritium is produced. The tritium is identified and counted by two different methods: (1) The gas is put into a cloud chamber where the ${\mathrm{H}}^3$ electrons are identified by their range and counted; (2) hydrogen is separated from nitrogen by its passage through palladium and is then counted in a Geiger counter. The average cross section for fission neutrons with energy sufficient to make the reaction proceed [4.4 Mev is (11±2)×${10}^{-27\mathrm{}}$ ${\mathrm{cm}}^2$].

This cross section combined with cosmic-ray neutron data gives an ${\mathrm{H}}^3$ production rate of between 0.10/${\mathrm{cm}}^2$ sec and 0.20/${\mathrm{cm}}^2$ sec averaged over the earth. Cosmic-ray stars eject ${\mathrm{H}}^3$ at a rate estimated between 0.30/${\mathrm{cm}}^2$ sec and 0.70/${\mathrm{cm}}^2$ sec. These two processes maintain a world reservoir of 50 to 110 million curies of ${\mathrm{H}}^3$. This ${\mathrm{H}}^3$ production leads to a mean escape time of ${\mathrm{He}}^3$ from the atmosphere of about 5 million years. This is consistent with a temperature at the base of the exosphere of 1500°K.

• Received 13 March 1953

DOI:https://doi.org/10.1103/PhysRev.91.922