Records of ancient cosmic radiation in extraterrestrial rocks

Abstract

Recent results on cosmic ray interactions in lunar samples and meteorites resulting in production of stable and radionuclides, particle tracks and thermoluminescence are reviewed. A critical examination of ~SA1 depth profiles in lunar rocks and soil cores, together with particle track data, enables us to determine the long term average fluxes of energetic solar protons (> 10 Mey) which can be represented by (.Is, Ro) = (125, 125). The lunar rock data indicate that this flux has remained constant for 5 x 105 to 2 x 100 years~ Production rates of stable and radionuclides produced by galactic cosmic rays is given as a function of size and depth of the meteoroid. Radionuclide (SSMn, seA1) depth profiles in meteorite cores, whose preatmospheric depths are deduced from track density profiles are used to develop a general procedure for calculating isotope production rates as a function of meteoroid size. Based on the track density and lSNe/2XNe production rates, a criterion is developed to identify meteorites with multiple exposure history, l~Ne/SXNe ratio < 1.06 is usually indicative of deep shielded exposure. An examination of the available data suggests that the frequency of meteorites with multiple exposure history is high, at least 15 ~o for LL, 27 ~ofor L and 31 ~o for H chondrites. The epi-therrnal and the thermal neutron density profiles in different meteorites are deduced from e~ and track density data in Dhajala, Kirin and Allende chondrites. The data show that the production profile depends sensitively on the size and the chemical composition of the meteoroid. Cosmic ray-induced thermoluminescence in meteorites of known preatmospheric sizes has been measured which indicates that its production profile is nearly fiat and insensitive to the size of the meteoroid. Some new possibilities in studying cosmic ray implanted radionuclides in meteorites and lunar samples using resonance ionisation spectroscopy are discussed.

Nuclide production by primary cosmic‐ray protons

“…fluxes and spectral shapes used for solar maximum, solar minimum, and the average over an l 1-year solar cycl• (including the slightly higher value for the solarcycle average) are similar to those ofMichel and Stiick [1984],Bhandari et al [1979], andBhandari [1981].Figure Ishows the solar-proton spectrum, the GCR-proton spectra for solar maximum, the solar-cycle average, and solar minimum discussed above, and an interstellar (IS) proton spectrum based on (1) with M = 0.The spectra of GCR particles in IS space has not been measured directly and is hard to model or estimate. The protons with energies below --•100 MeV in IS space never enter the inner solar system; hence it is hard to extrapolate back from measured spectra to IS space.…”

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“…The track, neon isotope and radionuclide levels in Gujargaon indicate a small preatmospheric size (RE = 9-10 em) and small amount of ablation (1-3 em) and are consistent with the production rates given in Bhandari (1981). The high 22Nef2'Ne ratio suggests that some solar contribution may be present in surface samples.…”

“…The production profiles of all these isotopes were calculated from an empirical cascade model for meteoroids of various sizes (Bhandari, 1981). Gujargaon meteoroid is similar to Bansur, which had a preatmospheric radius of 9 em and an oriented entry into the earth's atmosphere (Bhattacharya et al, 1980).…”

“…respectively. The calculated production rates appropriate to size, shielding depth and composition of Gujargaon samples (Bhandari, 1981) are given in Table 3. For long-lived isotopes, the observed values agree to within 20% of calculated values.…”

The Torino, H6, meteorite shower

“…13N -5.845 J o was calculated from the neutron monitor counting rates (N) from January 1965 to May 1988 (the date of fall of Torino). These fluxes were fed into the model developed for calculating isotope production in meteorites described earlier (Bhandari, 1981). There are several other models available for calculation of isotope production rates (e.g., Reedy, 1985;and references therein).…”