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.

Noble gas space exposure ages of individual interplanetary dust particles

“…Dust particles expelled from freshly injected comets may be particularly ancient, having resided near the surface of the cometary body for time periods much longer than the predicted ages of asteroidal regoliths. Oort cloud objects, in particular, could harbor material with long apparent GCR exposure ages, since spallation production rates by GCR primaries are expected to be a factor of four higher in interstellar space where these objects spent most of their lifetimes (Reedy 1987). The possible connection with comets is consistent with the fact that both I8 and B19 are heated particles, having probably entered the Earth's atmosphere at relatively high velocities.…”

“…Chondritic abundances of relevant target elements are assumed. Calculated by Reedy (1987) and Reedy (1999). c Assumes 4π exposure and CI chemistry.…”

“…Relevant SCR and GCR production rates are listed in Table 3. Reedy (1987Reedy ( , 1999 calculated production rates using measured proton cross sections and assuming reasonable shapes for the energy spectra of GCR and SCR protons. Eugster's (1988) shieldingdepth dependent values were determined by comparing measured abundances of cosmogenic nuclides to the exposure ages inferred from the presence of short-lived cosmogenic 81 Kr.…”

Radiation Doses for Human Exposed to Galactic Cosmic Rays and Their Secondary Products on the Lunar Surface

“…The differential energy spectra of protons with three modulation parameters of 375, 550 and 950 MeV are shown in Fig.2. These parameters correspond to the solar minimum, average and maximum phases, respectively (Reedy, 1987), though they depend on the phase of solar cycle. The flux of proton less than 10 GeV is modulated by the solar activity, so-called 11-year cycle, as shown in Fig.…”

Complexities of gamma-ray line intensities from the lunar surface

“…φ is the modulation parameter affected by solar activity and takes approximate values of φ = 375 MeV at solar minimum, 550 MeV at solar average, or 950 MeV at solar maximum (Reedy, 1987). The GCR proton has a maximum intensity at 400-800 MeV and is modulated only at lower energies, as seen from Fig.…”