Nuclide production by primary cosmic‐ray protons

Abstract

The production rates of cosmogenic nuclides in the solar system and in interstellar space were calculated for the primary protons in the galactic and solar cosmic rays. Only thin‐target production rates, which are directly applicable to small objects in space, were calculated; however, these results also are used to examine the relative variations in production rates that are possible for large objects. In small objects at 1 AU from the sun, the long‐term average fluxes of solar protons usually produce many more atoms of a cosmogenic nuclide than do the primary protons in the galactic cosmic rays (GCR)—the exceptions being nuclides made only by high‐energy reactions (such as 10Be). Because the particle fluxes inside meteorites and other large objects in space include many secondary neutrons, the production rates are much higher, and ratios inside large objects are often very different from those produced by just the primary GCR protons in small objects. The production rates of cosmogenic nuclides are calculated to vary by factors of about 2.5 between the extremes for a typical 11‐year solar cycle; this variation is in agreement with measurements of short‐lived radionuclides in recently fallen meteorites. The production of cosmogenic nuclides by GCR particles outside the heliosphere is higher by factors of ∼4 than that by the modulated GCR primaries normally in the solar system. However, there is considerable uncertainty about the fluxes of interstellar protons and, therefore, in the production rates of cosmogenic nuclides in interstellar space. Production rates and ratios for cosmogenic nuclides could be used to identify particles that were from small bodies in space or that were exposed to an unmodulated spectrum of GCR particles.