2:30 PM - 2:45 PM
[PPS07-16] Depth and compositional dependences of epithermal neutron spectra in planetary materials studied by Hf isotpes
Keywords:neutron capture, hafnium isotope, cosmic ray, meteorite
At upper several meters of planetary surfaces, spallation reactions induced by galactic cosmic rays generate neutrons with energies of >1 MeV. As a consequence of absorptions of these neutrons by surronding atomic nuclei, significant isotopic variations occur in certain elements (e.g., Sm and Gd; Lingenfelter et al. 1972). Since neutron capture is one of the fundamental processes contributing to isotopic variations in planetary materials like radioactive decay and nucleosynthetic heterogeneity, its understanding is crucial (e.g., Leya & Masarik 2013). Moreover, isotopic variations resulting from neutron capture reactions provide information about the neutron flux and energy spectrum, which can be used to estimate the cosmic-ray exposure durations and conditions of planetary materials (e.g., Hidaka et al. 2000). Previously, studies on neutrons in planetary materials based on isotopic variations have been limited to investigations of thermal neutrons (E < 0.1 eV) using Sm and Gd isotopes. However, advancements in analytical instruments and techniques have recently enabled the analysis of elements sensitive to epithermal neutrons (0.1 eV < E < 0.1 MeV), such as Er, Yb, and Hf (e.g., Hidaka et al. 2020; Sprung et al. 2010). Despite these preliminary efforts in epithermal neutron studies, the available isotope datasets remain limited, and detailed methodologies for data analysis have not been fully established. As a result, discussions on the neutron energy spectrum in the epithermal energy region have not yet been fully developed. In this study, we conducted Hf isotope analysis in lunar meteorites and Apollo samples, which is characterized by pronounced neutron capture resonances of 177Hf and 178Hf at 1–3 eV and 8 eV, respectively. Using these Hf isotopic datasets, we analyzed epithermal neutron energy spectra of these samples in the energy range of 1–10 eV to investigate their variations and contributing factors.
After HF–HClO4 decomposition, Hf was chemically separated from the samples using a two-step column-chromatography procedure using cation-exchange resin and Ln resin, respectively. Isotopic analyses were conducted using MC-ICP-MS (Neptune Plus) at the Korea Institute of Geoscience and Mineral Resources. To characterize the epithermal neutron spectrum, we assumed a spectral distribution of the form Ψepi(E) = A/Ep (where E is neutron energy, and A and p are constants; e.g., Ryves 1969). We then computed the relationship between the spectral shape parameter p and neutron capture-induced isotopic variations using neutron capture cross sections of Hf isotopes (ENDF/B-VIII.0; Brown et al. 2018).
The p-values (0.93–1.06) derived from Hf isotopic data in Apollo core samples (depth: 40–240 cm; 15001–15006) exhibited a clear depth-dependent variation, having a peak minimum at a depth of 80–120 cm. A lower p-value indicates an enrichment in high-energy epithermal neutrons. This depth, where the p-value reaches its minimum, coincides with the depth at which epithermal neutron fluence is maximized. This suggests that at depths with higher neutron fluence, fast neutrons (E > 1 MeV) generated by spallation reactions contribute to the increase in high-energy epithermal neutrons. By compiling our Hf isotopic data with previous studies (Sprung et al. 2010; 2013; Dauphas et al. 2025), we examined the relationship between the parameter representing epithermal neutron capture efficiency (ΣRI/ξΣs), calculated from the chemical composition of each sample, and the p-value. A negative correlation was observed between these two parameters. Since neutron capture cross sections generally follow a 1/v dependence, meaning they are larger for lower-energy neutrons, lower-energy neutrons should be removed effectively in samples with higher ΣRI/ξΣs ratios. Consequently, p-values decrease in such samples, indicating an enrichment in high-energy epithermal neutrons.
After HF–HClO4 decomposition, Hf was chemically separated from the samples using a two-step column-chromatography procedure using cation-exchange resin and Ln resin, respectively. Isotopic analyses were conducted using MC-ICP-MS (Neptune Plus) at the Korea Institute of Geoscience and Mineral Resources. To characterize the epithermal neutron spectrum, we assumed a spectral distribution of the form Ψepi(E) = A/Ep (where E is neutron energy, and A and p are constants; e.g., Ryves 1969). We then computed the relationship between the spectral shape parameter p and neutron capture-induced isotopic variations using neutron capture cross sections of Hf isotopes (ENDF/B-VIII.0; Brown et al. 2018).
The p-values (0.93–1.06) derived from Hf isotopic data in Apollo core samples (depth: 40–240 cm; 15001–15006) exhibited a clear depth-dependent variation, having a peak minimum at a depth of 80–120 cm. A lower p-value indicates an enrichment in high-energy epithermal neutrons. This depth, where the p-value reaches its minimum, coincides with the depth at which epithermal neutron fluence is maximized. This suggests that at depths with higher neutron fluence, fast neutrons (E > 1 MeV) generated by spallation reactions contribute to the increase in high-energy epithermal neutrons. By compiling our Hf isotopic data with previous studies (Sprung et al. 2010; 2013; Dauphas et al. 2025), we examined the relationship between the parameter representing epithermal neutron capture efficiency (ΣRI/ξΣs), calculated from the chemical composition of each sample, and the p-value. A negative correlation was observed between these two parameters. Since neutron capture cross sections generally follow a 1/v dependence, meaning they are larger for lower-energy neutrons, lower-energy neutrons should be removed effectively in samples with higher ΣRI/ξΣs ratios. Consequently, p-values decrease in such samples, indicating an enrichment in high-energy epithermal neutrons.