The half-life, $t_{1/2}$, of electron-capture decay of ${}^7\mathrm{Be}$ is inversely proportional to the electron density, $\rho \left(0\right)$, at the nucleus. We experimentally measured $t_{1/2}$ of ${}^7\mathrm{Be}$ implanted in Nb crystals and found that $t_{1/2}=53.18\pm 0.03$ days at room temperature ($T\approx 293\mathrm{K}$) and $t_{1/2}=53.27\pm 0.03$ days at liquid helium temperature ($T\approx 5\mathrm{K}$). Using density functional theory calculations at $T=0\mathrm{K}$, we found that nearly all ${}^7\mathrm{Be}$ atoms occupy the most stable hexahedral interstitial sites [$\rho \left(0\right)=35.509$ a.u.], due to the high activation energy required for substitutional incorporation. In contrast, at $T\approx 293\mathrm{K}$, approximately 73% of ${}^7\mathrm{Be}$ atoms migrate to the second-most stable octahedral interstitial sites [$\rho \left(0\right)=35.576$ 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 $t_{1/2}$ changes approximately 0.17% from $T\approx 5\mathrm{K}$ to $T\approx 293\mathrm{K}$. 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 ${}^7\mathrm{Be}$ stay at substitutional sites at $T\approx 5\mathrm{K}$ and about half migrate to basal octahedral interstitial sites at $T\approx 293\mathrm{K}$ [Kuwahara et al.,  Phys. Rev. C 109, 024609 (2024)].

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• Electron & muon capture
• Interstitials
• Ion impact & scattering
• Lifetimes & widths
• Nuclear decay
• Nuclear reaction rates
• Room temperature RF
• Temperature
• Transition metals
• 39 ≤ A ≤ 58
• Density functional theory
• First-principles calculations
• GGA
• Ion implantation
• Liquid helium cooling
• Single impurity model
• Thermal & statistical models