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内容記述 |
The half-life, t1/2, of electron-capture decay of 7Be is inversely proportional to the electron density, ρ(0), at the nucleus. We experimentally measured t1/2 of 7Be implanted in Nb crystals and found that t1/2 = 53.18 ± 0.03 day at room temperature (T ≈ 293 K) and t1/2 = 53.27 ± 0.03 day at liquid helium temperature (T ≈ 5 K). Using density functional theory calculations at T = 0 K, we found that nearly all 7Be atoms occupy the most stable hexahedral interstitial sites [ρ(0) = 35.509 a.u.], due to the high activation energy required for substitutional incorporation. In contrast, at T ≈ 293 K, approximately 73% of 7Be atoms migrate to the second-most stable octahedral interstitial sites [ρ(0)= 35.576 a.u., which is 0.19% higher than at the hexahedral sites], because the energy difference between the hexahedral and octahedral sites is only 0.008 eV. This satisfactorily explains the reason for the experimental observation that t1/2 changes approximately 0.17% from T ≈ 5 K to T ≈ 293 K. Although this temperature dependence appears similar to that of Be metal, the underlying mechanisms are quite different in Nb and Be metal, where in the latter all 7Be stay at substitutional sites at T ≈ 5 K and about half migrate to basal octahedral interstitial sites at T ≈ 293 K [Phys. Rev. C 109, 024609 (2024)]. |
