Thermal neutron capture effects in radioactive and stable nuclide systems

Abstract

Abstract-Neutron capture effects in meteorites and lunar surface samples have been successfully used in the past to study exposure histories and shielding conditions. In recent years, however, it turned out that neutron capture effects produce a nuisance for some of the short-lived radionuclide systems. The most prominent example is the 182 Hf-182 W system in iron meteorites, for which neutron capture effects lower the 182 W/ 184 W ratio, thereby producing too old apparent ages. Here, we present a thorough study of neutron capture effects in iron meteorites, ordinary chondrites, and carbonaceous chondrites, whereas the focus is on iron meteorites. We study in detail the effects responsible for neutron production, neutron transport, and neutron slowing down and find that neutron capture in all studied meteorite types is not, as usually expected, exclusively via thermal neutrons. In contrast, most of the neutron capture in iron meteorites is in the epithermal energy range and there is a significant contribution from epithermal neutron capture even in stony meteorites. Using sophisticated particle spectra and evaluated cross section data files for neutron capture reactions we calculate the neutron capture effects for Sm, Gd, Cd, Pd, Pt, and Os isotopes, which all can serve as neutron-dose proxies, either in stony or in iron meteorites. In addition, we model neutron capture effects in W and Ag isotopes. For W isotopes, the GCR-induced shifts perfectly correlate with Os and Pt isotope shifts, which therefore can be used as neutron-dose proxies and permit a reliable correction. We also found that GCR-induced effects for the 107 Pd-107 Ag system can be significant and need to be corrected, a result that is in contrast to earlier studies.

Pd–Ag chronometry of iron meteorites: Correction of neutron capture-effects and application to the cooling history of differentiated protoplanets

“…Moreover, the most heavily irradiated IVB iron, Tlacotepec, shows no clearly resolved excess 107 Ag, in spite of a very high 108 Pd/ 109 Ag ratio of $43,500 (Chen and Wasserburg, 1996). Leya and Masarik (2013) argued that the low 107 Ag/ 109 Ag of Tlacotepec most likely is due to neutron capture, consistent with the long cosmic ray-exposure of this sample. The Pd-Ag systematics of the IVB irons may be reassessed using the correction method for GCR-induced effects presented in this study.…”

“…The model calculations used here are essentially the same as those presented earlier (Leya and Masarik, 2013). In brief, the model is based on the same particle spectra as used by Ammon et al (2008), and the cross sections for thermal and epithermal neutron capture reactions were taken from the JEFF-3.0A database.…”

“…A first quantitative assessment of these cosmogenic effects on Ag isotopes modeled spallation reactions on Pd isotopes induced by fast protons and neutrons, and showed that the resulting positive shifts in 107 Ag/ 109 Ag are much smaller than the large radiogenic 107 Ag excesses observed for iron meteorites (Reedy, 1980). However, more recently Leya and Masarik (2013) found that thermal neutron capture on Pd isotopes, which was not considered in the earlier study by Reedy (1980), can lower the 107 Ag/ 109 Ag ratio significantly. The cosmic ray-induced shifts not only depend on the neutron fluence, but also increase with increasing Pd/Ag ratio.…”

“…Here we will show that these features are best explained by cosmic ray-induced reactions modifying 107 Ag/ 109 Ag. Model calculations demonstrate that secondary neutron capture on Pd isotopes leads to a decrease of e 107 Ag, predominantly through the production of 109 Ag via neutron capture on 108 Pd and subsequent b À -decay of 109 Pd to 109 Ag (Leya and Masarik, 2013). The overall cosmic ray-induced effect on e 107 Ag depends on the 108 Pd/ 109 Ag ratio and the neutron dose.…”

“…Quantifying cosmic ray-induced effects on Ag isotopes requires an independent neutron-dose proxy for iron meteorites, and a model linking this dosimeter to the effects on the 107 Ag/ 109 Ag ratio. To this end we have extended the model calculations of cosmogenic effects on Pd-Ag systematics from Leya and Masarik (2013) with calculations of neutron capture effects on Pt isotopes, which provide a powerful neutron-dose proxy for iron meteorites (Kruijer et al, 2013;Wittig et al, 2013). The updated model makes it possible to correct cosmogenic effects on 107 Ag/ 109 Ag using measured Pt isotope compositions.…”