Measurement of the (n,H3) Cross Section in Nitrogen and Its Relationship to the Tritium Production in the Atmosphere

Abstract

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\ifmmode\pm\else\textpm\fi{}2)\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}27}$ ${\mathrm{cm}}^{2}$].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\ifmmode^\circ\else\textdegree\fi{}K.