Production of nucleogenic neon in the Earth from natural radioactive decay

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Abstract
Nucleogenic neon isotope production resulting from α decay of naturally occurring U and Th was calculated for typical compositions of the mantle and crust using a simple non‐Monte Carlo code for neutron transport. The (α,n), (α,p), and (n, α) reactions, leading to the production of neon isotopes in the outgoing reaction channel, are taken into account in the calculations. In the first stage of calculation, neon production from (α,n) and (α,p)reactions on 17O, 18O, and 19F is considered, and the neutron energy spectrum from (α,n) reactions on the major elements is derived. This spectrum is further used for calculation of the neon isotope yield from the reactions 24Mg (n,α)21Ne, 25Mg(n,α)22Ne, and 23Na(n,α)20Ne. The calculated production of nucleogenic neon is dominated by (α,n) reactions with oxygen in the Earth's mantle and crust. The present calculations give 21Ne/4He production ratios that are approximately half those of the previous estimates by Rison [1980] and Kyser and Rison [1982]. The present study also has shown that the nucleogenic 21Ne production from the 24Mg(n,α) reaction is only about one‐fifth the previous estimate. The contribution of nucleogenic 21Ne from the 24Mg(n,α) reaction is insignificant compared with nucleogenic 21Ne production from 18O(α,n). Our estimate of the nucleogenic 21Ne/radiogenic 4He production ratio both in the mantle and in the crust is 4.5×10−8. This production ratio is quite consistent with nucleogenic 21Ne/radiogenic 4He ratios actually observed in crustal fluid samples ((4.6±0.8) ×10−8). The error in calculation of the nucleogenic neon production, resulting from a random statistical variation of the entry neutron yields and cross sections, is estimated to be less than 5%.