Depth Variation of Tritium and Argon-37 Produced by High-Energy Protons in Iron

Abstract

The ${\mathrm{H}}^{3}$ produced by 0.16-, 1.0-, 3.0-, and 6.2-Bev protons and the ${\mathrm{A}}^{37}$ by 0.16-, 1.0-, and 6.2-Bev protons in long iron targets is measured as a function of depth. The maximum effective ${\mathrm{H}}^{3}$ cross section is 7.2 mb at 0.16 Bev, 60 mb at 1.0 Bev, 100 mb at 3.0 Bev, and 130 mb at 6.2 Bev. The maximum effective ${\mathrm{A}}^{37}$ cross section is 0.19 mb at 0.16 Bev, 4.7 mb at 1.0 Bev, and 6.7 mb at 6.2 Bev. The depth variation of these isotopes is very energy-dependent. At the higher energies a slight transition effect occurs. At the lower energies there is an exponential decrease with depth. The comparison of these measurements with the depth variation of ${\mathrm{He}}^{3}$ in the Carbo meteorite shows that the cosmic rays that produced the ${\mathrm{He}}^{3}$ in this material were very similar to the protons with 6-Bev energy. The ${\mathrm{H}}^{3}$ to ${\mathrm{A}}^{37}$ ratio is not very energy- or depth-dependent. The ratio is 15 at 1 Bev and 20 at 6.2 Bev. However, for 0.16-Bev protons the ${\mathrm{H}}^{3}$ to ${\mathrm{A}}^{37}$ ratio increases from 40 to 1000 in 4-cm depth. The ${\mathrm{H}}^{3}$ to ${\mathrm{A}}^{37}$ ratio is compared with the ${\mathrm{He}}^{3}$ to ${\mathrm{A}}^{36}$ ratio in four iron meteorites. These are comparable to a remarkable extent with the Bev proton results. The small variation of the ${\mathrm{He}}^{3}$ to ${\mathrm{A}}^{36}$ ratio in meteorites is related to the fact that the ${\mathrm{H}}^{3}$ to ${\mathrm{A}}^{37}$ ratio is not very energy- or depth-sensitive for protons in the Bev range; however, the ${\mathrm{He}}^{3}$ to ${\mathrm{A}}^{36}$ ratio also favors the 6-Bev energy.