Cosmic-ray production rates of Be⁷ in oxygen, and P³², P³³, S³⁵ in argon at mountain altitudes

Abstract

The production rates of radioisotopes P³², P³³, and S³⁵ in argon, and of Be⁷ in oxygen have been measured by exposing argon and water to cosmic rays at mountain altitudes for periods of two to four months during 1959. The measured values at λ=51° N, atmospheric depth 685 g cm^-2 are 7.6×10^-6, 6.2×10^-6, 1.4×10^-5 atoms (g argon )^-1 sec^-1 of P³², P³³, and S³⁵, respectively, and 9.0×10^-6 Be⁷ atoms (g oxygen )^-1 sec^-1. Isotope production rates for all regions in the atmosphere have recently been calculated by Lal et al. The measured production rates are higher than the calculated rates by factors of 1.1, 1.8, 1.4, and 1.8 in the case of radioisotopes Be⁷, P³², P³³, and S³⁵, respectively. In this comparison, account has been taken of the fact that cosmic-ray intensity has decreased by about 15% since 1948, the time period to which the calculations apply. The measured production rates in oxygen are assumed to apply to air since cross sections for Be⁷ formation have been found to be the same in nitrogen and oxygen over most of the energy region of interest. The procedure used by Lal et al. yields a fairly accurate picture of the variation in production rates with altitude and latitude in the atmosphere. Isotope production rates in all regions of the atmosphere can, therefore, be obtained by normalizing their calculations at the points where our measurements have been made. The calculated production rates of these isotopes in the troposphere, and in the stratosphere corresponding to observed cosmic-ray intensity during 1948-49, are given. The available data on the concentration of the isotopes in rain-water and their averaged yearly deposition rates are compared with their revised production rates. From such a comparison, more definite conclusions can be drawn than hitherto possible in view of the more accurate knowledge of isotope production rates.