14:15 〜 14:30
[MZZ42-02] Neutron capture effects on Er isotopes in meteorites
キーワード：中性子捕獲、エルビウム、ユークライト、メソシデライト、ベスタ、熱外中性子
Some elements heavier than iron in the solar system are synthesised by the capture of secondary neutrons produced by cosmic ray radiation. When cosmic rays collide with the surface material of a celestial body, secondary neutrons are produced by a nuclear spallation reaction. Then the neutrons lose energy by colliding with near-surface material and shift from fast neutron to epithermal and thermal neutrons. Finally they get captured by the constituent atoms of the material via reaction of (AX(n,γ)A+1X). The quantitative evaluation of the effect of this neutron capture reaction on the isotopic compositions of meteorites and returned samples is of great importance for radiometric dating of these samples. Furthermore, since the abundance ratio of secondary neutrons between thermal and epithermal neutrons highly depends on the hydrogen content in the vicinity of the sample, neutron-capture-induced isotopic variations can be used to evaluate the presence of water on the surface of the object.
The probability of neutron capture (neutron capture cross section) differs for each isotope of the element and is called thermal neutron-capture cross section (σth) and epithermal neutron-resonance integral (RI), respectively. The isotopes 149Sm and 157Gd are known as isotopes with large σth , whereas 167Er and 177Hf are known as isotopes with large RI. Hence, the thermal and epithermal neutron fluences of a geological sample can be estimated by measuring these isotope ratios. However, for samples from bodies other than the Moon, only thermal neutron fluence has been estimated mainly using Sm isotopic compositions, and studies estimating epithermal neutron fluence are very limited. The basaltic meteorite eucrite is the most abundant achondrite found on Earth, and its parent body is considered as the asteroid Vesta. The stony-iron meteorite mesosiderite is thought to have been formed by collisional mixing of eucritic crust and iron-rich impactors on the eucrite parent body. The epithermal neutron fluence on these meteorites has not been reported so far.
In this study, we measured Er isotope ratios in six eucrite meteorites and a mesosiderite meteorite Vaca Muerta. Our results reveal Er isotopic anomalies due to neutron capture effects, with the maximum value of ε167Er = -0.41±0.04 (2σ) for eucrite and ε167Er = -1.31±0.28 (2σ) for mesosiderite. The thermal neutron fluence, epithermal neutron fluence and their ratio (ep/th) were calculated from these results and ε149Sm from Saito et al.(2023). We obtained ep/th=1~3 for the eucrite meteorites and ep/th=10.1~19 for the mesosiderite meteorite Vaca Muerta.
In this study, we also attempted to reproduce the calculated ep/th using the Monte Carlo code Phits (Ver. 3.34). Simulating cosmic-ray irradiation on the parent body of eucrite showed that the observed ep/th=1~3 could not be reproduced if secondary neutrons captured by these meteorite samples were generated within eucritic crust. This suggests that there were light elements such as hydrogen near the surface of the parent body that more effectively generate thermal neutrons. This interpretation is consistent with the observational results of the dawn mission (McCord et al., 2012). Similar simulations were performed for Vaca Muerta assuming a parent body surface with mesosiderite meteorite composition, indicating that the observed ep/th ratio cannot be reproduced the ep/th determined from the measurements. This suggests that the observed ep/th may be the result of mixing of iron with an eucritic surface with a small ep/th, followed by an increase in epithermal neutron fluence due to subsequent cosmic ray irradiation.
Finally, the effect of the neutron capture on the previously reported Hf-W age of the Vaca Muerta meteorite (Schönbächle et al., 2002) is evaluated based on the results of this study. We found that the neutron capture results in ~1.4 Myr older apparent Hf–W age, calling for the revision of the Hf–W age to be 4.4 Myr after CAI formation.
The probability of neutron capture (neutron capture cross section) differs for each isotope of the element and is called thermal neutron-capture cross section (σth) and epithermal neutron-resonance integral (RI), respectively. The isotopes 149Sm and 157Gd are known as isotopes with large σth , whereas 167Er and 177Hf are known as isotopes with large RI. Hence, the thermal and epithermal neutron fluences of a geological sample can be estimated by measuring these isotope ratios. However, for samples from bodies other than the Moon, only thermal neutron fluence has been estimated mainly using Sm isotopic compositions, and studies estimating epithermal neutron fluence are very limited. The basaltic meteorite eucrite is the most abundant achondrite found on Earth, and its parent body is considered as the asteroid Vesta. The stony-iron meteorite mesosiderite is thought to have been formed by collisional mixing of eucritic crust and iron-rich impactors on the eucrite parent body. The epithermal neutron fluence on these meteorites has not been reported so far.
In this study, we measured Er isotope ratios in six eucrite meteorites and a mesosiderite meteorite Vaca Muerta. Our results reveal Er isotopic anomalies due to neutron capture effects, with the maximum value of ε167Er = -0.41±0.04 (2σ) for eucrite and ε167Er = -1.31±0.28 (2σ) for mesosiderite. The thermal neutron fluence, epithermal neutron fluence and their ratio (ep/th) were calculated from these results and ε149Sm from Saito et al.(2023). We obtained ep/th=1~3 for the eucrite meteorites and ep/th=10.1~19 for the mesosiderite meteorite Vaca Muerta.
In this study, we also attempted to reproduce the calculated ep/th using the Monte Carlo code Phits (Ver. 3.34). Simulating cosmic-ray irradiation on the parent body of eucrite showed that the observed ep/th=1~3 could not be reproduced if secondary neutrons captured by these meteorite samples were generated within eucritic crust. This suggests that there were light elements such as hydrogen near the surface of the parent body that more effectively generate thermal neutrons. This interpretation is consistent with the observational results of the dawn mission (McCord et al., 2012). Similar simulations were performed for Vaca Muerta assuming a parent body surface with mesosiderite meteorite composition, indicating that the observed ep/th ratio cannot be reproduced the ep/th determined from the measurements. This suggests that the observed ep/th may be the result of mixing of iron with an eucritic surface with a small ep/th, followed by an increase in epithermal neutron fluence due to subsequent cosmic ray irradiation.
Finally, the effect of the neutron capture on the previously reported Hf-W age of the Vaca Muerta meteorite (Schönbächle et al., 2002) is evaluated based on the results of this study. We found that the neutron capture results in ~1.4 Myr older apparent Hf–W age, calling for the revision of the Hf–W age to be 4.4 Myr after CAI formation.