11:05 〜 11:30
[MZZ45-02] 長寿命放射性核種176Luの信頼できる半減期
★招待講演
キーワード：地質時計、宇宙時計、長寿命放射性核種、マントル地殻進化
The long-lived radionuclide 176Lu decays to its daughter nuclide 176Hf with a half-life of approximately 1010 yr. Thus, the 176Lu-176Hf system has a large potential for measurement of the age of astrophysical and geological events. Both Lu and Hf are refractory and lithophile elements. Furthermore, Lu is more compatible than Hf during melting of mantle, producing a new crust with low Lu/Hf ratios and the high Lu/Hf-ratio residual mantle. Therefore, the Lu-Hf system is a powerful tool for the study of crust-mantle evolution in various celestial bodies such as planets, satellites, and asteroids. For example, the Lu-Hf system has been used for the study of the crust-mantle evolution of Mars [Bouvier, L. C. Nature 558, 586 (2018)]. However, there are two critical problems in the half-life of 176Lu. First, the half-life values of 176Lu have been measured with various experimental techniques in nuclear physics and isochron methods in cosmochemistry, but their values differ significantly. Furthermore, the half-life values evaluated from Lu-Hf isochrons in meteorites and terrestrial rocks whose formation ages were well evaluated using the U-Pb method show two different values. Therefore, a measurement of an accurate half-life independent of the known uncertainty is expected.
Here we report half-life measurements using a method that is almost independent of various uncertainties. The previous experiments in nuclear physics can be classified into the three groups. We have carefully evaluated the reason for the systematical uncertainties for each group. In general, in the previous experiments, the energies of gamma-rays or beta-rays radiated from the decay of 176Lu using radiation detectors. In these methods, there are uncertainties of detection efficacy and calculation using standard radioisotope source. We have designed a method completely different from these known methods. We measure the total energy released from 176Lu decay using a windowless 4π solid angle detector based upon bismuth germanate (BGO) scintillation crystals, where a natural Lu sample is located inside of the detector. This method is almost free from the uncertainties of various uncertainties such as detection efficiency and calculation sources. To the best of our knowledge this is the most accurate value of 176Lu half-life. The measured half-life of (3.719 ± 0.007) × 1010 yr corresponding to a decay constant of (1.864 ± 0.003) × 10−11 yr−1. In future, it is expected that the use of the 176Lu-176Hf system in fide fields using the presently obtained value.
The presently obtained value is consistent with the longer value obtained from the analysis of terrestrial rocks within the uncertainty, and the other value is shorter than the true value. To explain the shorter value, we have proposed a new hypothesis that the decay of 176Lu is accelerated by nuclear reactions induced by neutrons that are generated by nuclear reactions with high-energy cosmic-rays. This hypothesis is not inconsistent with the measured 176Lu/175Lu ratio. To examine this hypothesis, measurement of Hf isotopes in various meteorites are required.
Here we report half-life measurements using a method that is almost independent of various uncertainties. The previous experiments in nuclear physics can be classified into the three groups. We have carefully evaluated the reason for the systematical uncertainties for each group. In general, in the previous experiments, the energies of gamma-rays or beta-rays radiated from the decay of 176Lu using radiation detectors. In these methods, there are uncertainties of detection efficacy and calculation using standard radioisotope source. We have designed a method completely different from these known methods. We measure the total energy released from 176Lu decay using a windowless 4π solid angle detector based upon bismuth germanate (BGO) scintillation crystals, where a natural Lu sample is located inside of the detector. This method is almost free from the uncertainties of various uncertainties such as detection efficiency and calculation sources. To the best of our knowledge this is the most accurate value of 176Lu half-life. The measured half-life of (3.719 ± 0.007) × 1010 yr corresponding to a decay constant of (1.864 ± 0.003) × 10−11 yr−1. In future, it is expected that the use of the 176Lu-176Hf system in fide fields using the presently obtained value.
The presently obtained value is consistent with the longer value obtained from the analysis of terrestrial rocks within the uncertainty, and the other value is shorter than the true value. To explain the shorter value, we have proposed a new hypothesis that the decay of 176Lu is accelerated by nuclear reactions induced by neutrons that are generated by nuclear reactions with high-energy cosmic-rays. This hypothesis is not inconsistent with the measured 176Lu/175Lu ratio. To examine this hypothesis, measurement of Hf isotopes in various meteorites are required.