Cross-Section Measurement near 50 Bev

An experiment to measure the cross section for high-energy cosmic-ray protons and neutrons to interact with the Fe nucleus has been carried out at 3250 m elevation. The detector had a relatively good energy resolution, and was designed to select nucleons in the vicinity of 50 Bev. The purpose was to obtain a single interaction cross section with good accuracy in order to compare it with results obtained with accelerators in the 1- to 5-Bev region. This comparison yields a value for the elementary meson-production cross section [the average of $\sigma \mathrm{}\left(\mathrm{pp}\right)\mathrm{}$ and $\sigma \mathrm{}\left(\mathrm{np}\right)\mathrm{}$] near 50 Bev. The Fe results are: neutrons, ${\sigma }_{\mathrm{Fe}}=0.61\mathrm{}\pm 0.03$ barn; protons, ${\sigma }_{\mathrm{Fe}}=0.61\mathrm{}\pm 0.04$ barn. The corresponding average nucleon-nucleon "inelastic" (presumably meson production) cross section is ${\sigma }_{\mathrm{nucleon}}=21\mathrm{}\pm 4$ millibarns.