講演情報
[PPS12-22]176Lu-176Hfシステムで測定された隕石形成年代と宇宙線中性子による176Luの加速された崩壊
*早川 岳人1、飯塚 毅2、吉原 慧2、梶野 敏貴3、千葉 敏4 (1.量子科学技術研究開発機構、2.東京大学、3.Beihang University、4.東京科学大学)
キーワード：
同位体組成、隕石、宇宙線、長寿命放射性同位体
A meta-stable isotope 176Lu decays to 176Hf with a half-life of (3.719+-0.007)x1010 yr [1]. The 176Lu-176Hf system could be used as a nuclear chronometer for the ages of formation of parent bodies of meteorites. It has been also used for the study of crust mantle evolution of various planetary bodies such as Earth, Moon, Mars, and the asteroid Vesta. However, the formation ages of some meteorites such as eucrites and angrites evaluated by the 176Lu-176Hf system are older than the age of the oldest solid grains, Calcium-aluminium-rich inclusion (CAI), in the solar system. These results clearly show that there is unresolved mechanism leading excess of 176Hf.
To explain the older ages, several models have been proposed [2]. Recently, Iizuka et al. found that some Ryugu samples show excess of 176Hf and this is the evidence for removement of Lu in water flow [3]. The parent body of Ryugu was formed behind the H2O snow line with ice, whereas there is no evidence that Vesta and the parent body of angrites have rich water. Another possible mechanism is the accelerated decay of 176Lu with cosmic gamma-ray irradiation [4]. There is an isomer at 123 keV in 176Lu which decays to 176Hf with a half-life of approximately 3.7 h. When 176Lu is irradiated by high energy gamma-rays, intermediate states in 176Lu are excited by absorption of gamma-rays and subsequently a part of the intermediate states decay to the isomer decaying to 176Hf. When the decay of 176Lu is accelerated, the isotopic abundance of 176Lu relative to 175Lu should decrease because the isotopic abundance of 175Lu is kept under the gamma-ray irradiation. However, the measured 176Lu/175Lu ratios in meteorites do not show decreased ratios [5].
Hayakawa et al. [1] have proposed another accelerated decay with cosmic-ray neutron irradiation. The isotopic abundance anomalies caused by cosmic neutrons irradiation have been widely reported. In this model, neutrons are produced by nuclear reactions including spallation reactions with high-energy cosmic rays such as protons and alpha particles on the surface of parent bodies of meteorites. The energies of the neutrons decrease through multiple scattering on other atomic nuclei. Excess of the isotopic abundance of 176Hf is made by two nuclear reaction paths. First, the isomer in 176Lu is produced by neutron capture reactions on the stable isotope 175Lu, and the isomer decays to 176Hf. Second, inelastic scattering with high-energy neutrons on the ground state of 176Lu forms a compound nucleus of 177Lu, and a part of 177Lu decay to the isomer in 176Lu through neutron inelastic scattering; the isomer decays to 176Hf. We have calculated the detailed isotopic abundance change of 176Hf using the equation presented in Ref. [3]. In this process, the meta-stable ground state of 176Lu is produced by neutron capture reaction on 175Lu and the calculated result shows that the 176Lu/175Lu ratio does not decrease in typical neutron fluences, which were obtained from the isotopic abundance analyses of Sm and Er in eucrites. This result indicates that even if the measured 176Lu/175Lu ratio is not lower than the chondritic value it does not indicate that accelerated decay did not occur. We also calculated the expected isotopic abundances of Hf with assumed neutron fluences and evaluated the ages that can be obtained from Hf and Lu isotopic abundance without modification of cosmic-neutron irradiations. We found that the evaluated older ages for the eucrites can be explained by this mechanism.
[1] Hayakawa, T., Shizuma, T. & Iizuka T. Communications Physics, 6, 299 (2023).
[2] Iizuka, T. Yet al. Lithos 274-275, 304 (2017).
[3] Iizuka, T. et al. Nature, 646, 62 (2025).
[4] Albarede, F. et al. Geochim. Cosmochim. Acta 70, 1261 (2006).
[5] Wimpenny, J. et al. Astrophys. J. 812, L3 (2015).
To explain the older ages, several models have been proposed [2]. Recently, Iizuka et al. found that some Ryugu samples show excess of 176Hf and this is the evidence for removement of Lu in water flow [3]. The parent body of Ryugu was formed behind the H2O snow line with ice, whereas there is no evidence that Vesta and the parent body of angrites have rich water. Another possible mechanism is the accelerated decay of 176Lu with cosmic gamma-ray irradiation [4]. There is an isomer at 123 keV in 176Lu which decays to 176Hf with a half-life of approximately 3.7 h. When 176Lu is irradiated by high energy gamma-rays, intermediate states in 176Lu are excited by absorption of gamma-rays and subsequently a part of the intermediate states decay to the isomer decaying to 176Hf. When the decay of 176Lu is accelerated, the isotopic abundance of 176Lu relative to 175Lu should decrease because the isotopic abundance of 175Lu is kept under the gamma-ray irradiation. However, the measured 176Lu/175Lu ratios in meteorites do not show decreased ratios [5].
Hayakawa et al. [1] have proposed another accelerated decay with cosmic-ray neutron irradiation. The isotopic abundance anomalies caused by cosmic neutrons irradiation have been widely reported. In this model, neutrons are produced by nuclear reactions including spallation reactions with high-energy cosmic rays such as protons and alpha particles on the surface of parent bodies of meteorites. The energies of the neutrons decrease through multiple scattering on other atomic nuclei. Excess of the isotopic abundance of 176Hf is made by two nuclear reaction paths. First, the isomer in 176Lu is produced by neutron capture reactions on the stable isotope 175Lu, and the isomer decays to 176Hf. Second, inelastic scattering with high-energy neutrons on the ground state of 176Lu forms a compound nucleus of 177Lu, and a part of 177Lu decay to the isomer in 176Lu through neutron inelastic scattering; the isomer decays to 176Hf. We have calculated the detailed isotopic abundance change of 176Hf using the equation presented in Ref. [3]. In this process, the meta-stable ground state of 176Lu is produced by neutron capture reaction on 175Lu and the calculated result shows that the 176Lu/175Lu ratio does not decrease in typical neutron fluences, which were obtained from the isotopic abundance analyses of Sm and Er in eucrites. This result indicates that even if the measured 176Lu/175Lu ratio is not lower than the chondritic value it does not indicate that accelerated decay did not occur. We also calculated the expected isotopic abundances of Hf with assumed neutron fluences and evaluated the ages that can be obtained from Hf and Lu isotopic abundance without modification of cosmic-neutron irradiations. We found that the evaluated older ages for the eucrites can be explained by this mechanism.
[1] Hayakawa, T., Shizuma, T. & Iizuka T. Communications Physics, 6, 299 (2023).
[2] Iizuka, T. Yet al. Lithos 274-275, 304 (2017).
[3] Iizuka, T. et al. Nature, 646, 62 (2025).
[4] Albarede, F. et al. Geochim. Cosmochim. Acta 70, 1261 (2006).
[5] Wimpenny, J. et al. Astrophys. J. 812, L3 (2015).