Abstract
U-rich minerals were investigated for the products of radioactive decay, fission and specific nuclear reactions induced thereby. Geologically undisturbed uranium minerals with concordant U-Pb ages were used to determine the product of the 238U spontaneous fission decay constant λ sf and the yield iY sf of a specific Xe or Kr isotope, the isotopic ratios of spontaneous fission xenon and krypton and the 136Xe 86Kr spontaneous fission ratio. Values of (5.7 ±0.4) · 10 −18/ a and 6.1 ± 0.1 were obtained for λ sf · 136Y sf and ( 136Xe 86Kr ) sf , respectively. With our result for λ sf · 136Y sf and literature data on 238U spontaneous fission yields, we propose that the proper value for λ sf is around 8.6 · 10 −17/a rather than the 6.4 · 10 −17/ a as mainly proposed by fission track investigators. Isotopic ratios of neon and argon differ significantly from atmospheric composition. No evidence for nucleogenic neon and argon from strongly asymmetric fission or from ( n, α) or ( n, γ) reactions was found. These gases are produced by the nuclear reactions 18 O( α, n) 21 Ne, 19 F( α, n) 22 Ne, 35 Cl( α, p) 38 Ar and 35Cl( n, γ) 36Ar as already proposed in 1954 by Wetherill. For these reactions the production ratios P(21) norm, P(22) norm and P(38) norm of 21Ne nuc, 22Ne nuc and 38Ar nuc relative to the α-dose and the target element concentrations in the minerals were deduced. The problem of diffusional 4He losses was circumvented by using fissiogenic Xe as a measure of the α-dose. The contributions of 22Ne nuc and 38Ar nuc to the Earth's atmosphere are negligible, whereas up to 2.4% of 21Ne in the atmosphere may be nucleogenic.
Published Version
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call